Abstract:
A filtration assembly can comprise a chamber for holding a fluid sample to be filtered and a cover assembly defining a petri dish into which a filter element can be placed for cultivating microorganisms present on the filter element. A filtration assembly can also comprise a sample reservoir for holding a fluid sample and a base for supporting the sample reservoir detachably connected to the sample reservoir. One of the sample reservoir and the base can have a projection extending around its periphery and the other of the sample reservoir and the base can have a groove extending around its periphery and detachably engaging the projection in a fluid-type manner around its periphery. A filtration assembly can also comprise a sample reservoir for holding a fluid sample to be filtered and a base for supporting the sample reservoir. The base can include a fluid port and communication with an interior of the sample reservoir and a skirt surrounding the fluid port for contact with a vacuum manifold. A method of filtering a fluid may comprise disposing a filter element on a support surface formed on one of a sample reservoir and a base. The method can further comprise detachably connecting the sample reservoir to the base in a fluid type manner without using the ceiling member by engagement between a projection formed on one of the sample reservoir and the base and a groove formed in the other of the sample reservoir and the base. A method of using a filtration assembly can comprise placing a base of a filtration assembly on a vacuum manifold with a skirt of the base contacting an inlet tube of the manifold around the periphery of the skirt. A method of culturing microorganisms can comprise passing the fluid sample through a filter element and placing the filter element in a petri dish defined by a cover assembly mountable on a sample reservoir.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/036,310 filed Jan. 29, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a filtration assembly which can be used for culturing microorganisms. 
     2. Description of the Related Art 
     A common method of investigating for the presence of microorganisms in a fluid is to pass the fluid through a filter element capable of capturing microorganisms larger than a certain size present in the fluid. After the completion of filtration, the filter element and any microorganisms captured by it are placed in a petri dish containing a nutrient solution. The nutrient solution permeates through the filter element to reach the microorganisms, enabling the microorganisms to be cultured atop the filter element. 
     The filtration of the fluid containing the microorganisms is typically performed using a filtration assembly including a fluid reservoir connected to a base on which a filter element can be removably disposed. A petri dish for receiving the filter element after filtration forms no part of the filtration assembly, so a separate petri dish is required in order for culturing of microorganisms to take place. 
     SUMMARY OF THE INVENTION 
     The present invention provides a filtration assembly which can be used both for filtering a fluid containing microorganisms and for culturing microorganisms removed from the fluid by the filtration. 
     The present invention also provides a method of culturing microorganisms. 
     According to one form of the present invention, a filtration assembly includes a chamber for holding a fluid sample to be filtered, a fluid port for filtrate in fluid communication with the chamber, a filter support arranged to support a filter element on a flow path between the chamber and the fluid port, and a cover assembly including a lower cover detachably mounted on the chamber and an upper cover detachably mounted on the lower cover. The cover assembly defines a petri dish into which a filter element can be placed for cultivating microorganisms present on the filter element. The ability of the cover assembly to be used as a petri dish makes the filtration assembly highly convenient to use and renders a separate petri dish unnecessary. 
     In one preferred embodiment, the assembly includes a sample reservoir which defines the chamber, and a base which includes a fluid port and the support surface. The sample reservoir and the base may be permanently connected to each other, or they may be detachable from each other to permit the base to be used separately from the sample reservoir with one of the covers as a petri dish. 
     In some embodiments, the filtration assembly includes a sample reservoir for holding a fluid sample, and a base for supporting the sample reservoir. The base may be detachably connected to the sample reservoir in a fluid-fight manner without use of a sealing member between the sample reservoir and the base. Because no sealing member is required between the sample and the base, the manufacturing costs of the filtration assembly can be reduced. 
     In some embodiments, the filtration assembly includes a sample reservoir for holding a fluid sample to be filtered and a base for supporting the sample reservoir. The base includes a fluid port and a skirt surrounding the fluid port for contact with a vacuum manifold of a vacuum filtration assembly. The skirt makes it unnecessary to provide a stopper or an adapter for connecting the base to a vacuum manifold, so the filtration assembly is easy to use. 
     According to another form of the present invention, a method of culturing microorganisms comprises introducing a fluid sample into a sample reservoir, passing the fluid sample through a filter element communicating with an interior of the sample reservoir to filter the fluid; after filtering the fluid, placing the filter element in a petri dish defined by a cover assembly mountable on the sample reservoir and comprising first and second covers; and incubating microorganisms in the petri dish. 
     In some embodiments, the method includes disposing a filter element on a filter support surface of a sample reservoir or a base, detachably connecting the sample reservoir to the base in a fluid-tight manner without using a sealing member, introducing a fluid sample into the sample reservoir, and removing fluid which has passed through the filter element from a fluid port of the base. 
     In some embodiments, the method includes placing a base of a filtration assembly on a vacuum manifold with a skirt of the base contacting an inlet tube of the manifold, and applying suction to an interior of the inlet tube to draw a fluid through a filter element within the filtration assembly and into the manifold. 
     These and other various aspects of the present invention will be explained in farther detail by the following description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of an embodiment of a filtration assembly according to the present invention. 
     FIG. 2 is a vertical cross-sectional view of the embodiment of FIG.  1 . 
     FIG. 3 is a vertical cross-sectional view of the sample reservoir of the embodiment of FIG.  1 . 
     FIG. 4 is a vertical cross-sectional view of the base of the embodiment of FIG.  1 . 
     FIG. 5 is a top isometric view of the base. 
     FIG. 6 is a bottom isometric view of the base. 
     FIG. 7 is a vertical cross-sectional view of the lower cover of the embodiment of FIG.  1 . 
     FIG. 8 is vertical cross-sectional view of the upper cover of the embodiment of FIG.  1 . 
     FIG. 9 shows two petri dishes stacked atop each other, each petri dish comprising a cover assembly like that of the embodiment of FIG.  1 . 
     FIG. 10 is a vertical cross-sectional view of two petri dishes stacked atop each other, each petri dish comprising a base and an upper cover like those of the embodiment of FIG.  1 . 
     FIG. 11 illustrates a vacuum filtering arrangement with which the embodiment of FIG. 1 can be used. 
     FIG. 12 is a vertical cross-sectional view of the base of the embodiment of FIG. 1 installed on a vacuum manifold using an adapter and a stopper. 
     FIG. 13 is a vertical cross-sectional view of the base of the embodiment of FIG. 1 directly engaging a vacuum manifold for vacuum filtration. 
     FIG. 14 is a bottom isometric view of the base illustrating a method of introducing a nutrient solution through the fluid port of the base. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1-8 illustrate an embodiment of a filtration assembly  10  according to the present invention. As shown in these figures, the assembly  10  includes a sample reservoir  20 , a base  30  which is detachably engageable with the lower end of the sample reservoir  20 , and a cover assembly  50  which is detachably mounted atop the sample reservoir  20 . The sample reservoir  20  defines a chamber  22  which can hold a fluid sample which is to be filtered, while the base  30  serves to support the sample reservoir  20  as well as a filter element  45  through which the fluid sample is to be passed. In the present embodiment, the cover assembly  50  is designed to function as a petri dish by itself, or a portion of the cover assembly  50  may be combined with the base  30  to form a petri dish. 
     The sample reservoir  20  may have any structure which enables it to hold a desired volume of a sample fluid which is to be filtered. In the present embodiment, the sample reservoir  20  is a generally cylindrical member, i.e., a body of revolution, which is open at its upper and lower ends. It has an outer wall  21  which defines the outer periphery of the chamber  22  for the sample fluid. The outer wall  21  has a circular transverse cross-sectional shape and an inner diameter which linearly decreases from its upper to its lower end, but the shape of the outer wall  21  is not critical, and its diameter need not vary over its height. For example, the transverse cross-sectional shape may be polygonal or of a non-circular curved shape, and the inner diameter or other dimensions of the sample reservoir  20  may be constant or vary in any desired manner over the height of the sample reservoir  20 . The sample reservoir  20  may be equipped with gradations on its inner or outer surface to assist a user in measuring the amount of sample fluid being introduced into the sample reservoir  20 . 
     As best shown in FIGS. 4 and 5, which are respectively a vertical cross-sectional view and a top isometric view of the base  30 , the base  30  includes a filter support surface  31  atop which a filter element  45  can be supported during filtration and a fluid port  38  through which filtrate which has passed through the filter element  45  can be discharged from the filtration assembly  10 . The filter support surface  31  is defined by the upper surfaces of a plurality of projections  32  which extend upwards from a bottom inner surface  33  of the base  30 . The projections  32  are spaced from each other to enable filtrate which has passed through the filter element  45  to flow between the projections  32  into the fluid port  38 . One or more drainage openings  39  for filtrate are formed in the projections  32  at the center of the base  30  to connect the interior of the fluid port  38  with the region of the base  30  containing the projections  32 . 
     In the present embodiment, the base  30  is a unitary member formed by injection molding, for example, with the filter support surface  31  being integrally formed with other portions of the base  30 . However, it is also possible for the base  30  to comprise a plurality of separately formed components. For example, the filter support surface  31  may comprise a perforated plate, a porous plate, or a mesh which is removably installed within the interior of the base  30  and has an upper surface which can support the filter element  45 . 
     The filter support surface  31  in the present embodiment is planar, but it may have any shape which enables to support the filter element  45  for filtration. For example, it may be dished, arched, or wave-like in shape 
     The filter support surface  31  is surrounded by a circular wall  34  extending upwards from the outer periphery of the filter support surface  31 , and a plurality of radial projections  35  extend upwards from a ledge formed atop the wall  34 , with the vertical, radially inner surface of each projection  35  being flush with the wall  34 . The wall  34  and the projections  35  serve to surround and position a filter element  45  disposed on the filter support surface  31 . 
     It is convenient if the filtration assembly  10  is capable of standing upright on a level surface without being supported. In the present embodiment, the base  30  includes an outer wall  41  extending around its entire outer periphery for supporting the base  30  on a table or other level surface. The outer wall  41  does not need to perform a sealing function, so it need not be continuous around the periphery of the base  30  and it need not be fluid tight. Members other than a wall can also be used to support the base, such as a plurality of legs. Furthermore, it is not necessary for the base  30  to be self supporting, and it may have a shape which does not stand upright by itself. For example, the bottom of the base  30  may be shaped like a funnel. 
     The sample reservoir  20  and the base  30  may be separately formed but permanently connected to each other, or they may be formed as a single member. However, in the present embodiment, the sample reservoir  20  is detachably engaged with the base  30  so that the base  30  can be used separately from the sample reservoir  20  as part of a petri dish. The manner of engagement between the sample reservoir  20  and the base  30  is preferably such that the engagement creates a fluid-tight seal without the need for a sealing member, such as an O-ring or a gasket, yet such that the sample reservoir  20  and the base  30  can be readily disengaged from each other by hand. The lower end of the sample reservoir  20  is also preferably shaped so that a fluid-tight seal is formed between the sample reservoir  20  and the upper surface of a filter element  45  disposed on the filter support surface  31  to prevent fluid from the sample reservoir  20  from bypassing the filter element  45  by flowing between the sample reservoir  20  and the filter element  45 . 
     In general, any type of detachable engagement providing intimate, sealing contact between the sample reservoir  20  and the base  30  around the entire inner periphery of the base  30  can be employed to detachably engage the two members. For example, there may be an interference fit between the sample reservoir  20  and the base  30  so that a radial force presses a peripheral surface of the sample reservoir  20  into sealing contact with an opposing peripheral surface of the base  30 , or opposing surfaces of the sample reservoir  20  and the base  30  may be pressed into sealing contact with each other by a compressive force acting in the axial direction of the filtration assembly. In the present embodiment, the sample reservoir  20  and the base  30  are engaged with each other by an interference fit which produces a fluid-tight seal between the outer peripheral surface of the sample reservoir  20  and the inner peripheral surface of the base  30 . The sample reservoir  20  and the base  30  may be structured so as to provide resistance to an axial force tending to pull them apart so as not to be inadvertently disconnected from each other during use. In the present embodiment, resistance to disengagement is provided by a snap fit in which the lower end of the sample reservoir  20  is received inside the upper end of the base  30 . As shown in the cross-sectional elevation of FIG. 3, the lower end of the sample reservoir  20  has a groove  24  and a radially outward projection  25  which extend continuously around its entire outer periphery. Similarly, as shown in FIG. 4, the base  30  has a groove  36  and a radial inward projection  37  extending continuously around its entire inner periphery at its upper end. The outer diameter of the lower end of the sample reservoir  20  and the inner diameter of the base  30  are preferably selected so that the projections  25  and  37  will snap into and fit snugly inside the grooves  36  and  24 , respectively, with an interference fit so that there is intimate contact, such as line contact or surface contact, between each projection and the corresponding groove around the entire circumference of the sample reservoir  20 . The sample reservoir  20  on be disconnected from the base  30  simply by flexing the two members with respect to each other, for example, to disengage the projections from the grooves. It is generally easier to disengage the two members if the groove  36  and the projection  37  are formed as close to the upper end of the base  30  as possible. For example, in the present embodiment, projection  37  immediately adjoins the upper end of the base  30 . The location of the sealing contact between the sample reservoir  20  and the base  30  is not critical as long as the contact can prevent fluid from leaking to the exterior of the filtration assembly  10  during normal use. For example, the sealing contact may be between the mating surfaces of the grooves  24 ,  36  and the projections  25 ,  38 , or it could be formed in a different location, with engagement between the grooves and the projections serving primarily to resist inadvertent disengagement of the sample reservoir  20  and the base  30  or to maintain an axial compressive force between the sample reservoir  20  and the filter element  45  to form a fluid-tight seal against the filter element  45 . In the latter case, the grooves and the projections need not be continuous members. 
     In the present embodiment, each groove is complementary in shape with the corresponding projection, i.e., it has substantially the same radius of curvature as the corresponding projection so that each groove and the corresponding projection are in surface contact, but the curvatures of the groove and the projection may be such that they are in line contact, for example. It is possible to form a seal between the sample reservoir  20  and the base  30  with a single projection formed on the surface of one of the two members and a single groove for engagement with the projection formed on the surface of the other two members, but a plurality of grooves and projections may create a seal of greater integrity. 
     Many other arrangements besides a snap fit can be used to resist disengagement between the sample reservoir  20  and the base  30 , such as a bayonet fit or threaded engagement. It is also possible to dispose tape around the joint between the sample reservoir  20  and the base  30  or to lightly weld or bond the two members to each other (such as by ultrasonic welding) around their peripheries to secure the members together while enabling them to be easily disconnected from each other when desired. Such a manner of connection can be employed instead of or in addition to the interference fit provided by the grooves and projections on the sample reservoir  20  and the base  30 . 
     The lower end of the sample reservoir  20  is formed with an annular sealing rim  26  which extends in generally the axial direction of the sample reservoir  20  around the entire periphery of the sample reservoir  20 . When the grooves and the projections of the sample reservoir  20  and the base  30  are engaged with each other, the sealing rim  26  is pressed downwards into sealing contact with the upper surface of the filter element  45  disposed atop the filter support surface  31  of the base  30 . The compressive force between the sealing rim  26  and the filter element  45  is maintained by the engagement between the grooves and the projections of the sample reservoir  20  and the base  30 . In the present embodiment, the sealing rim  26  is positioned on the sample reservoir  20  such that an annular air space is present between the outer periphery of the sealing rim  26  and the inner periphery of the base  30  around the entire circumference of the sealing rim  26 . It is thought that the air space may improve the integrity of the seal between the sample reservoir  20  and the base  30  by forming an air lock which prevents creeping of fluid by capillary action between the two members. However, the air space is not essential, and the sealing rim  26  may closely contact the inner periphery of the base  30 . 
     While the filter support surface  31  is part of the base  30  in the present embodiment, it is also possible for the filter support surface to be part of the sample reservoir  20 . For example, instead of the sample reservoir  20  being completely open at its lower end, it may have a perforated bottom surface for supporting a filter element  45 , and the base  30  may function as a funnel located beneath the sample reservoir  20  to collect filtrate which has passed through the bottom surface of the sample reservoir  20 . 
     The filter element  45  comprises a filter medium capable of removing microorganisms of interest from the fluid being filtered. The filter medium may be of any desired type, such as a microporous membrane of various materials, or filter paper, for example. A wide variety of filter media for microbiological studies are commercially available, and any such filter media can be employed with the present invention as the filter element  45 . Filter media for use in microbiological studies are frequently flat membrane discs, but the filter element  45  need not have any particular shape. For example, instead of being flat, it may have pleats to increase its surface area. 
     The filter element  45  may directly contact the filter support surface  31  of the base  30 , or it may rest upon an intermediate support member, such as a layer of mesh, paper, or fabric which is more porous than the filter element  45  and which provides mechanical support to the filter element  45 . When the filter element  45  is to be left on the base  30  during incubation, it may be convenient if an absorbent pad  46  for use in holding a nutrient solution during incubation is placed beneath the filter element  45  prior to filtration rather than afterwards to reduce the amount of handling of the filter element  45  after filtration. Furthermore, the absorbent pad  46  can provide support for the filter element  45  during filtration. It is also possible to place a prefilter, a protective sheet, or other member atop the filter element  45 . 
     It may be advantageous to place a resilient, compressible member between the lower surface of the filter element  45  and the filter support surface  31  in the region beneath where the sealing rim  26  contacts the filter element  45 . Such a member can compensate for variations in the axial length of the sealing rim  26  or in the smoothness of the opposing surfaces of the sealing rim  26  and the filter support surface  31  to maintain the sealing rim  26  in intimate, sealing contact with the filter element  45 , thereby enabling the manufacturing tolerances of the sample reservoir  20  and the base  30  to be less precise. The resilient member may be either permeable or impermeable to the fluid being filtered. For example, it may comprise a porous sheet or pad, and in the present embodiment, the absorbent pad  46  serves as the resilient member. Alternatively, the resilient member may comprise an impermeable gasket disposed beneath the filter element  45 . It is also possible to place a resilient sealing member, such as a gasket, between the top surface of the filter element  45  and the sealing rim  26  so that the sealing rim  26  does not directly contact the filter element  45  but is pressed into sealing contact with the sealing member, which in turn is pressed into sealing contact with the filter element  45 . Such a sealing member may be separate from or joined to the filter element  45 . 
     In the present embodiment, the wall  34  surrounding the filter support surface  31  preferably has a height such that when an absorbent pad  46  and a filter element  45  are mounted on the filter support surface  31 , the absorbent pad  46  will be surrounded by the wall  34  and disposed at least partially below the upper end of the wall  34 , while the filter element  45  disposed atop the filter element  45  will be positioned at or above the upper end of the wall  34  and will be surrounded by the radial projections  35 . For example, the wall  34  may have a height substantially the same as the thickness of the absorbent pad  46 . With the absorbent pad  46  located partially or entirely below the upper end of the wall  34 , when a user of the filtration assembly  10  wishes to transfer the filter element  45  from atop the absorbent pad  46  to a different location, it is easy for the user to pick up the filter element  45  using forceps without picking up the absorbent pad  46  as well. The spaces between the radial projections  35  provide easy access to the filter element  45  and facilitate its removal from the base  30 . 
     From the standpoint of ease of manufacture, it is preferable if the axial length of the sealing rim  26  of the sample reservoir  20  and the axial height of the radial projections  35  on the base  30  are such that when the sample reservoir  20  sealingly engages the base  30  and the sealing rim  26  of the sample reservoir  20  is pressed into sealing contact with the filter element  45  as shown in FIG. 2, there is an axial gap between the top surface of the radial projections  35  and the bottom surface of the sample reservoir  20 . If such a gap is present, the radial projections  35  and the sealing rim  26  do not need to be manufactured to as precise tolerances as when the upper surfaces of the radial projections  35  contact the bottom surface of the sample reservoir  20 . 
     The cover assembly  50  comprises a lower cover  60  and an upper cover  70 , which are best illustrated in FIGS. 7 and 8, respectively. The lower cover  60  is shaped so as to detachably fit atop the upper end of the sample reservoir  20 , and the upper cover  70  is shaped so as to detachably fit atop the lower cover  60  or to detachably fit atop the upper end of the base  30 , thereby enabling the covers  60 ,  70  to together form a petri dish and enabling the upper cover  70  and the base  30  to together form another petri dish. The upper cover  70  may also be shaped so as to detachably fit directly atop the upper end of the sample reservoir  20  with the lower cover  60  removed. 
     The lower cover  60  may engage with the upper end of the sample reservoir  20  in various manners. For example, they may engage each other with a snap fit, a bayonet fit, threaded engagement, a press fit, or a loose fit. Preferably, the engagement is such as to provide some resistance to disengagement of the lower cover  60  from the sample reservoir  20  so as to enable the filtration assembly  10  to be handled and transported without the cover assembly  50  falling off the sample reservoir  20 , while still permitting the lower cover  60  to be readily detached from the sample reservoir  20 . In the present embodiment, the lower cover  60  comprises a disc-shaped plate  61  having a continuous annular projection  62  extending upwards from its upper surface. When the cover assembly  50  is used as a petri dish, the projection  62  serves as an outer wall of the petri dish. The plate  61  also has a continuous annular projection  63  extending from its lower surface. A snap fit is formed between the annular projection  63  and a radially outward lip  23  formed around the entire outer periphery of the upper end of the sample reservoir  20 . The projection  63  on the lower cover  60  has a radially inward bulge  64 . The minimum inner diameter of the lower cover  60  measured at the bulge  64  in a relaxed (unstressed) state is smaller than the outer diameter of the sample reservoir  20  at the lip  23  in a relaxed state so that when the lip  23  is urged upwards past the bulge  64 , the bulge  64  will resist disengagement of the sample reservoir  20  and the lower cover  60 . The engagement between the lower cover  60  and the sample reservoir  20  may be of varying degrees of tightness. For example, the engagement may be sufficient to provide some resistance to disengagement without forming a seal, or the engagement may provide a fluid-tight seal between the two members. A fluid-tight seal between the lower cover  60  and the sample reservoir  20  is convenient when the sample reservoir  20  is to be used for temporary storage of a fluid sample prior to filtration. For example, in factories, it is common to collect a fluid sample in one part of the factory and then to carry the sample to a laboratory for analysis in a different part of the factory. In such cases, the provision of a fluid-tight seal between the cover assembly  50  and the sample reservoir  20  enables a fluid sample within the sample reservoir  20  to be transported from one location to another without fear of spilling or contamination, A fluid-tight seal can be formed by any suitable means, but preferably by one which does not require the use of a separate sealing member, such as an O-ring or a gasket. In the present embodiment, a fluid-tight seal is achieved between the lower cover  60  and the sample reservoir  20  with the assistance of an annular projection  65  which extends downwards from the lower surface of the lower cover  60 . The outer diameter of the projection  63  in a relaxed state is larger than the inner diameter of the upper end of the sample reservoir  20  in a relaxed state so that when the lip  23  of the sample reservoir  20  is placed into the space between the two projections  63  and  65 , the upper end of the sample reservoir  20  will be urged radially outwards by the inner projection  65  towards projection  63 . The upper end of the sample reservoir  20  is thereby pressed into intimate contact with at least projection  65  and possibly both projections  63  and  65 , resulting in the formation of a fluid-tight seal between the lower cover  60  and the sample reservoir  20  around the entire periphery of the sample reservoir  20  somewhere in the space between the two projections  63  and  65 . 
     The upper cover  70  likewise comprises a disc-shaped plate  71 . The plate  71  has a plurality of annular projections  73 ,  74  extending downwards from its lower surface. A first projection  73  has an outer diameter so as to engage with the inner periphery of the projection  62  on the top surface of the lower cover  60 . 
     Like the fit between the lower cover  60  and the upper end of the sample reservoir  20 , the fit between the lower and upper covers  60  and  70  where they engage at projections  62  and  73  may have varying degrees of tightness, varying from a fluid-tight fit to a loose fit. In the present embodiment, projection  73  on the upper cover  70  snugly engages the inner surface of projection  62  of the lower cover  60  to prevent the upper cover  70  from being inadvertently dislodged from the lower cover  60  during handling but permitting the upper cover  70  to be easily removed from the lower cover  60  by hand when desired. Projection  73  need not extend continuously around the upper cover  70 , particularly when it does not need to seal against projection  62  on the lower cover  60 . 
     The lower cover  60  has another annular projection  74  which extends downwards from its lower surface concentric with and surrounding projection  73  As shown in FIG. 10, the upper cover  70  can be placed atop the upper end of the base  30  to serve as a cover for the base  30 , with the upper cover  70  and the base  30  together forming a petri dish. The inner diameter of projection  74  is selected so that the inner surface of projection  74  can snugly engage the outer peripheral surface of the base  30  to prevent the upper cover  70  from falling off the base  30  during handling or when the base  30  and the upper cover  70  are inverted. In this embodiment, the engagement between projection  73  and the outer periphery of the base  30  does not form a seal. However, a looser or tighter fit between the upper cover  70  and the base  30  (including a fit forming a fluid-tight seal) is possible. 
     When the cover assembly  50  is used as a petri dish, a filter element  45  and an absorbent pad  46  are typically placed in the space between the two covers  60  and  70 , and the covers are placed in an incubator to culture microorganisms present on the filter element  45 . The absorbent pad  46  placed between the covers is usually a different absorbent pad from the one which may be placed beneath the filter element  45  during filtration (although they may be identical to each other) so that the user does not need to transfer a wet absorbent pad from one location to another. In accordance with one method of culturing which may be employed, the petri dish defined by the cover assembly  50  is stored right-side up during incubation with the filter element  45  and absorbent pad  46  resting on the top interior surface of the lower cover  60 . However, in accordance with another method of culturing which may be employed, the petri dish is stored upside down during incubation with the lower cover  60  positioned atop the upper cover  70  and with the filter element  45  and absorbent pad  46  pressed against the interior surface of the lower cover  60 . To facilitate the use of the cover assembly  50  with this second culturing method, the upper cover  70  may be equipped with a retaining member on its lower surface for retaining a filter element  45  and absorbent pad  46  against the top surface of the lower cover  60 , with the weight of the filter element  45  and the absorbent pad  46  supported by the retaining member, when the cover assembly  50  is inverted. In the present embodiment, the retaining member comprises a projection  72  in the shape of an annular wall which extends downwards from the lower surface of the upper cover  70  towards the lower cover  60 . When the upper surface of projection  62  of the lower cover  60  abuts against the bottom surface of the upper cover  70 , the distance between the bottom surface of projection  72  and the top surface of the lower cover  60  is such that a filter element  45  and an absorbent pad  46 , if present, can be pressed against the top surface of the lower cover  60  by projection  72  and be prevented from falling down when the cover assembly  50  is inverted. Projection  72  does not need to form a seal against the filter element  45 , so it does not need to extend continuously around the entire periphery of the filter element  45 . Furthermore, a retaining member need not be in the shape of a wall. For example, it could be in the form of a plurality of pins or other projections extending downwards from the upper cover  70  towards the lower cover  60 . Preferably, the retaining member contacts the filter element  45  near the outer periphery of the filter element  45  so as to minimize interference with the growth of microorganisms on the filter element  45 , but if the filter element  45  is particularly heavy and needs support at locations other than around its periphery, the retaining member may contact the filter element  45  in locations other than the periphery. When the upper cover  70  is mounted atop the base  30 , the retaining member functions in a similar manner to retain a filter element and absorbent pad  46  against the filter support surface  31  of the base  30  when the upper cover  70  and base  30  are inverted. In situations in which the cover assembly  50  or the upper cover  70  and the base  30  are not expected to be inverted during culturing, the retaining member may be omitted. It is also possible to employ a retaining member which is formed separately from the upper cover  70 , such as a ring which can be inserted between the two covers  60  and  70  where projection  72  is formed in the present embodiment so as to be pressed against the top surface of a filter element  45 . 
     In order to save space, a plurality of petri dishes are typically stacked atop each other during incubation of microorganisms in the petri dishes. The present embodiment is arranged so that a plurality of petri dishes (each comprising one of the cover assemblies or else comprising an upper cover  70  and a base  30 ) can be stacked atop each other. FIG. 9 is a vertical cross-sectional view of two petri dishes, each comprising a cover assembly  50 , stacked atop each other. In this figure, projections  63  and  65  on the bottom surface of each lower cover  60  rest atop the top surface of the upper cover  70  of the cover assembly  50  located below it. If the stack of petri dishes is inverted, the top surface of each upper cover  70  rests atop projections  63  and  65  on the bottom surface of the lower cover  60  of the cover assembly  50  located below it. FIG. 10 is a vertical cross-sectional view of two petri dishes, each comprising a base  30  and an upper cover  70 , stacked atop each other. The outer wall  41  of each base  30  rests on the upper surface of the upper cover  70  of another petri dish located beneath it. Alternatively, if the petri dishes are inverted, the upper surface of each upper cover  70  rests atop the outer wall  41  of the base  30  of the petri dish located beneath it. Any number of petri dishes can be stacked atop each other in the manner shown in FIGS. 9 and 10. Furthermore, a stack of petri dishes can contain one or more petri dishes like those shown in FIG. 9 along with one or more petri dishes like those shown in FIG.  10 . In order to give a stack of petri dishes greater stability, each upper cover  70  may be equipped with a stabilizing structure which can resist lateral movement of an adjoining petri dish to prevent one petri dish from inadvertently being knocked off the petri dish located below it. In the present embodiment, the stabilizing structure comprises two annular ridges  75  and  76  which extend upwards from the top surface of the upper cover  70 . When the lower cover  60  of one petri dish sits on the upper cover  70  of another petri dish, the outer annular ridge  75  of the upper cover  70  is located between the two projections  63  and  65  on the lower surface of the lower cover  60 . When a lateral force is applied to one of the covers, the outer annular ridge  75  on the upper cover  70  contacts one or both of projections  63  and  65  on the lower cover  60  to resist relative lateral movement of the two covers. As shown in FIG. 10, when the base  30  of one petri dish sits on the upper cover  70  of another petri dish, the outer wall  41  of the base  30  contacts the upper cover  70  between the two annular ridges  75  and  76 , and lateral movement of the outer wall  41  relative to the upper cover  70  is resisted by one or both of the ridges. It is not necessary for the ridges  75 ,  76  to form a seal against the portion of another petri dish which they contact, so they need not be continuous and they need not tightly engage the adjoining petri dish. Furthermore, a stabilizing structure is not restricted to the form of ridges. For example, a stabilizing structure could be in the form of pins, bumps, tabs, or other projections on the top surface of the upper cover  70 , or it could be in the form of a recess formed in the top surface of the upper cover  70  for receiving one or both of the projections  63  and  65  on the lower cover  60  or the outer wall  41  of the base  30 . 
     The filtration assembly  10  can be made from a wide variety of materials, including those conventionally used for funnels, reservoirs, petri dishes, and other laboratory equipment, such as metals, plastics, and glass, depending upon factors such as the desired strength, flexibility, heat resistance, and corrosion resistance and upon whether the filtration assembly  10  is intended to be reusable or discarded at the completion of use. Different portions of the filtration assembly  10  may be formed of different materials. For economy of manufacture, plastics which can be shaped by molding are particularly suitable for the filtration assembly  10 . Some examples of suitable plastics are polypropylene, nylon, and polyacrylate. In some instances, it is convenient if portions of the assembly  10 , such as one or both of the lower and upper covers  60  and  70 , are translucent or transparent to permit substances within the assembly  10  to be readily observed. 
     Filtration of a fluid sample in the sample reservoir  20  can be performed by a variety of conventional methods, including gravity filtration and vacuum filtration. In vacuum filtration, the filtration assembly  10  is mounted on a vacuum manifold, a filtration flask, or other device through which suction can be applied to the fluid port  38  to suck fluid in the sample reservoir  20  through the filter element  45  and out of the fluid port  38 . FIG. 11 is a schematic view of a vacuum filtration arrangement with which a filtration assembly  10  according to the present invention can be employed. The illustrated arrangement includes a vacuum filtration manifold  80  having a plurality of inlet tubes  81 , each of which can support a filtration assembly  10 . Any one of the inlet tubes  81  can be fluidly connected through the interior of the manifold  80  to a vacuum port  82  of the manifold  80  by a stopcock  83 . Suction can be applied to the vacuum port  82  by a vacuum pump  84  connected to it by a hose  85 . Depending on the structure of the pump  84 , a vacuum filtration flask  86  and a filter  87  for removing aerosols from air may be installed between the manifold  80  and the pump  84  to prevent the fluid being filtered from being sucked into the pump  84 . In order to perform filtration with this arrangement, a filtration assembly  10  containing a filter element  45  and possibly an absorbent pad  46  disposed on the filter support surface  31  of the base  30  is mounted on one of the inlet tubes  81  with the fluid port  38  of the base  30  fluidly communicating with the inlet tube  81 . The fluid port  38  may be connected to one of the inlet tubes  81  in a variety of manners. One way, schematically shown in FIG. 12, is to insert the fluid port  38  into the upper end of a hollow adapter  88  and to insert the lower end of the adapter  88  into the bore of a hollow rubber stopper  89  sized to fit into the upper end of one of the inlet tubes  81 . The adapter  88 , which may be either a rigid or flexible member, is sized so as to form line or surface contact with the outer surface of the fluid port  38  when the fluid port  38  is inserted into the adapter  88  with a sufficiently tight fit between the fluid port  19  and the adapter  88  to obtain a desired suction in the fluid port  38  when the vacuum pump  84  is operated. Alternatively, as schematically shown in FIG. 13, the base  30  of the filtration assembly  10  may also be shaped so as to directly engage with the inlet tube  81  of the manifold  80  without the need for an adapter  88  or a stopper  89 . In this embodiment, the base  30  includes an annular skirt  42  disposed between the fluid port  38  and the outer wall  41  and extending downwards from the lower surface of the base  30 . The outer periphery of the skirt  42  is shaped so as to be in line contact or surface contact with the inner surface of the inlet tube  81  around its entire periphery when the skirt  42  is inserted into the inlet tube  81 . The skirt  42  may but need not form a fluid-tight seal against the inlet tube  81 . The skirt  42  preferably engages the inlet tube  81  sufficiently tightly that the vacuum pump  84  can generate sufficient suction in the inlet tube  81  to suck fluid contained in the sample reservoir  20  through the filter element  45 . It may be easier to obtain a desired fit between the skirt  42  and the inlet tube  81  if the skirt  42  is somewhat flexible. The skirt  42  may also be shaped to directly contact filtration equipment other than an inlet tube of a vacuum filtration manifold, such as the mouth of a filtration flask. Either before or after the assembly  10  is mounted on the inlet tube  81 , a desired quantity of a fluid sample to be filtered is placed into the sample reservoir  20 . With the filtration assembly  10  mounted on one of the inlet tubes  81 , the vacuum pump  84  is operated to suck the fluid sample through the filter element  45  and into the filtration flask  86 . During operation of the pump  84 , the cover assembly  50  is usually removed from the sample reservoir  20  so that the interior of the sample reservoir  20  above the fluid being filtered will be at atmospheric pressure, thereby making filtration easier and preventing suction generated by the pump  84  from causing the sample reservoir  20  to collapse. When the fluid sample has been sucked out of the sample reservoir  20  and through the filter element  45 , the pump  84  is turned off. At this time, the filtration assembly  10  may be removed from or left mounted on the vacuum manifold  80 . When the cover assembly  50  is to be used as a petri dish, an absorbent pad  46  is placed atop the lower cover  60  within the region surrounded by annular projection  62 , and a suitable nutrient solution for culturing microorganisms is applied to the absorbent pad  46  in a conventional manner. The sample reservoir  20  is then detached from the base  30  by hand and the filter element  45  is removed from atop the base  30  with forceps, for example, and placed atop the absorbent pad  46  on the lower cover  60 . The upper cover  70  is then placed atop the lower cover  60  to form a petri dish, and the microorganisms in the petri dish are incubated in a suitable manner, such as by being placed into a conventional incubator. Incubation of a single petri dish may be performed, or a plurality of petri dishes can be stacked atop each other during incubation as shown in FIG.  10 . 
     If the base  30  and the upper cover  70  are instead to be used as a petri dish, after the completion of filtration, the sample reservoir  20  is detached by hand from the base  30  by releasing the snap fit between them, and the filter element  45  is left atop the base  30  while a suitable nutrient solution is applied to the absorbent pad  46  located beneath the filter element  45 , the absorbent pad  46  typically having been placed beneath the filter element  45  prior to filtration. The nutrient solution can be applied to the absorbent pad  46  either from above, through the filter element  45 , or from below via the fluid port  38 . A method of introducing the solution through the fluid port  38  is shown in FIG.  14 . The nutrient solution is usually contained in an ampule  90  having a tapered snout  91  which can be inserted into the fluid port  38  and from which the nutrient solution can be dispensed. Since the fit between the outer surface of the snout  91  of the ampule  90  and the inner surface of the fluid port  38  may be fairly tight, one or more air vents  40  may be formed in the fluid port  38  to enable air to escape from the fluid port  38  when the outer surface of the snout  91  of the ampule  90  is pressed tightly against the inner surface of the fluid port  38  to prevent the formation of an air lock which could impede the introduction of the nutrient solution into the fluid port  38 . In the present embodiment, the fluid port  38  has three air vents  40 , each comprising an elongated groove formed in the inner periphery of the fluid port  38  between the openings  39  in the fluid port  38  and its outer end. When the nutrient solution is being applied to the absorbent pad  46  through the fluid port  38 , the sample reservoir  20  or the upper cover  70  may be mounted on the base  30  to prevent the filter element  45  and absorbent pad  46  from falling off. Once the nutrient solution has been applied to the absorbent pad  46  and the upper cover  70  is mounted on the base  30 , the petri dish comprising the base  30  and the upper cover  70  are ready to be incubated. If desired, a closure, such as a cap or a plug, may be mounted on the lower end of the fluid port  38  to prevent fluid from leaking out of it during incubation. If the base  30  is disposed upside down during incubation with the fluid port  38  facing upwards, a closure may be unnecessary. 
     In general, a petri dish comprising the cover assembly  50  and a petri dish comprising the base  30  and the upper cover  70  are both highly satisfactory. However, in some situations, one type of petri dish may have advantages over the other. For example, it may be more convenient to use a petri dish comprising the base  30  and the upper cover  70  because it is not necessary to remove the filter element  45  from the base  30  at the completion of filtration, resulting in fewer steps to be performed and less wear on the filter element  45 . On the other hand, at the completion of filtration, the absorbent pad  46  beneath the filter element  45  on the base  30  may be saturated with filtrate. If the presence of the filtrate in the absorbent pad  46  is objectionable or if the filtrate excessively dilutes the nutrient solution which is added to the absorbent pad  46  in order for culturing to take place, it may be desirable to instead use the cover assembly  50  as a petri dish, since an absorbent pad  46  within the cover assembly  50  will not have been exposed to fluid during filtration.