Abstract:
Compositions and methods are provided for storing prokaryotic cells including competent prokaryotic cells at −20° C. in a buffer so that the cells are suitable for transformation at 0° C. with a foreign molecule.

Description:
BACKGROUND 
       [0001]    Competent cells are available for cloning vectors of interest for molecular biology research. A variety of competent cells are provided commercially for various types of laboratory procedures such as routine cloning, protein expression, and library production. Bacterial cells are rendered competent in the presence of high salt and are then washed and resuspended in a high salt solution for storage at −80° C. Storage at −80° C. is at best a costly inconvenience. 
       SUMMARY 
       [0002]    In general, a method is provided that includes removing salts from prokaryotic cells optionally induced to competency by a first salt containing buffer; adding a storage buffer comprising glucose or an oligosaccharide containing glucose; storing cells at −20° C. for a period of time; and adding a salt containing buffer to the cells at a time proximate to transformation of the cells with a foreign molecule. 
         [0003]    In one aspect, the foreign molecule is a DNA, an RNA or a protein. In another aspect, the prokaryotic cells are  E.coli . In another aspect, the storage buffer further comprises glycerol and may further include DMSO. In one aspect, where glucose or an oligosaccharide is added to the storage buffer, its concentration is within a range of 10 mM-500 mM. In one aspect the period of time may be greater than 5 days. 
         [0004]    In general, a composition is provided having stabilized competent bacterial cells in a buffer including a monosaccharide and/or an oligosaccharide such as for example trehalose or sucrose or glucose. 
         [0005]    In various aspects, the concentration of the one or more monosaccharides or oligosaccharides may be the range of 50 mM-500 mM. The competent bacterial cells may have a concentration of OD600=2-500. 
         [0006]    In general, a method is provided that includes storing a composition described above at about −20° C. for at least 5 days; adding a salt containing buffer to the cells in the composition at a time proximate to transformation of the cells with a foreign molecule; and transforming the cells. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0007]      FIG. 1A-1B  shows  E.coli  cells which were stored at 0, 28, 42, 56, 77 and 105 days at −20° C. in a storage buffer containing oligosaccharides and no salt without any significant loss of transformation efficiency compared with a control sample stored at −20° C. in the standard competent cell storage buffer containing salt used for routine storage at −80° C. 
           [0008]      FIG. 1A  shows the effect of storage at −20° C. of competent cells
       (•••) Competent  E.coli  cells which were previously stored at −80° C. with substantially diminished activity after being stored at −20° C. for 29 days. At the end of the selected time period, the sample was moved to 0° C. and transformed with pUC19.   (—) Competent  E.coli  which was made by removing the buffer used to prepare cells at time zero followed by storage of the cells in a novel buffer containing 200 mM trehalose, as well as standard DMSO and glycerol and lacking salt did not lose significant activity after being stored up to 105 days at −20° C.       
 
           [0011]      FIG. 1B  shows a comparison of competency of  E.coli  cells which were stored at −20° C. in the novel storage buffer formulation that lacked salt and contained various concentrations of different oligosaccharides in addition to glycerol and DMSO in a standard buffer over various time periods—1, 28,42, 56, 77 and 105 days showing remarkable stability. 
       
    
    
       [0012]    At the end of the selected time period (0, 28, 42, 56, 77 and 105 days), the sample was moved to 0° C. and transformed with pUC19. The data shows that cells could be stored at −20° C. for as long as 105 days with little or no change in transformation efficiency. Several examples of oligosaccharides were shown to have similar preservative activities. A=trehalose at 200 mM, B=sucrose at 250 mM, C=sucrose at 200 mM and D=sucrose at 150 mM. Data is provided for transformation efficiency after storage. 
       DESCRIPTION OF EMBODIMENTS 
       [0013]    The term “competent” bacteria refers to bacteria having altered cell walls so that DNA can pass through more easily. Generally competent cells are fragile and may lose viability rapidly after thawing from −80° C. so that they are no longer available to take up DNA. 
         [0014]    The term “stable” or “stabilized” refers to a preparation of bacterial cells that are capable of retaining a transformation efficiency of at least 1.00E+08 cfu/ug pUC19 when stored for at least 5 days at −20° C. 
         [0015]    The term “−20° C.” is intended to refer to a temperature suitable for storage that is preferable at −20° C. but may vary according to various factors so that the temperature may represent a range of −15° C. to −30° C. 
         [0016]    In embodiments of the invention, bacterial cells can optionally be rendered competent when prepared under standard conditions in a salt containing buffer using standard protocols for preparing competent cells after which the salt is preferably removed (See for example, Hanahan, et al.,  Methods in Enzymology , 204:63 (1991)). The competent cells can then be stored in the novel storage buffer described herein at −20° C. and after thawing can remain competent for an extended period of time when a salt containing buffer is added to the thawed cells. Such competent bacterial cells are suitable for transformation by biological macromolecules such as DNA, RNA and protein. The examples provided herein measure transformation efficiency with pUC19 DNA. 
         [0017]    Competent bacterial cells (for example those tested in  FIG. 1B ), can be washed (for example, with water) to remove salt which may optionally be utilized prior to placing cells at −20° C. in a storage buffer containing glycerol, DMSO and sugars. Glycerol may be used at a concentration of 1%-50%, and DMSO may be used at a concentration of 1%-25%. By way of an example,  FIG. 1A  shows loss of competency of cells stored in a conventional storage buffer at −20° C. where the conventional storage buffer includes 10% glycerol and 7% DMSO suitable for standard conditions of storage at −80° C. In contrast,  FIG. 1B  shows sustained levels of competency at −20° C. in the novel storage buffer which contains 20% glycerol, 14% DMSO, one or more sugars and optionally ethylene glycol and/or propylene glycol. In  FIG. 1B , the assay for competency included moving the cells to 0° C. and adding an equal volume of a salt buffer to the storage buffer. The cells were then transformed with biological macromolecules. 
         [0018]    The sugars in the storage buffer comprise one or more glucose monosaccharides or oligosaccharides comprising glucose and having a size of 2-10 monosaccharides. In one embodiment , the monosaccharide or oligosaccharide in an amount of 10 mM-500 mM, for example, 50 mM-500 mM for example 50 mM-200 mM of the monosaccharide or oligosaccharide was added to cells having a concentration in the range of OD 600 =1-500, for example, OD 600 =2-250, OD 600 =3-100, OD 600 =4-75, or OD 600 =5-50. It should be understood by a person of ordinary skill in the art that a monosaccharide or oligosaccharide for use herein might include something other than glucose such as xylose or ribose. While not intended to be limiting,  FIG. 1B  shows the advantageous effect of a single concentration of a disaccharide, trehelose that is composed of two alpha glucose units, and sucrose at 3 different concentrations. In addition to the above, ethylene glycol and/or propylene glycol to the storage buffer may be added at concentrations in the range of 0.1 mM-1000 mM. 
         [0019]    Storage preferably occurs in the absence of salts of the amount and type used to generate the competent cells. 
         [0020]    The storage conditions used herein are suitable for any competent  E.coli  strain including  E.coli  K-12,  E.coli  B,  E.coli  W and  E.coli  C. 
         [0021]    Cells stored in the manner described herein were found to be capable of retaining competency for at least 5, 10, 15, 20, 25, 30, 35, 40, 42 days.