Patent Abstract:
the invention provides vesicles that may be used in diagnosing infection , especially with infection by bacteria such as staphylococcus aureus and pseudomonas aeruginosa . the vesicles can be used in wound dressings .

Detailed Description:
the invention will now be described in detail by way of example only , with reference to the figures , in which : fig1 shows the fluorescence response of vesicle type 1 upon inoculation of vesicles with e . coli dh5α , pao1 and mssa476 bacteria in respective growth media — vesicles in hepes buffer is the negative control and vesicles + triton x - 100 is the positive control ; fig2 shows the fluorescence response of vesicle type 2 upon inoculation of vesicles with e . coli dh5α , pao1 and mssa476 bacteria in respective growth media ; fig3 shows the fluorescence response of vesicle type 3 upon inoculation of vesicles with e . coli dh5α , pao1 and mssa476 bacteria in respective growth media ; fig4 shows the fluorescence response of vesicles type 1 , 2 and 3 after 18 hours incubation of vesicles with bacteria containing growth media ; and fig5 shows the fluorescence switch on of vesicle type 1 , 2 and 3 in 96 wells plate after 18 hours incubation of vesicles with bacteria containing growth media — vesicle type 1 shows fluorescence colour change to both pao1 and mssa476 but not to e . coli dh5α while vesicle type 2 and 3 show selective fluorescence colour change to pao1 only and mssa476 only respectively — negative control in hepes is not shown in this image ( inset on top right corner — 96 wells plate at the beginning of incubation ). fig6 shows the response of vesicle types a ( fig6 a ), b ( fig6 b ), c ( fig6 c ), e ( fig6 d ) to hepes , pao1 and mssa476 . fig6 a shows type a vesicles ( 1 , 2 , 3 ): dppc with varying cholesterol concentration . fig6 b type b vesicles ( 5 , 6 , 7 , 8 , 9 ): dppc ( tcda 25 mol %) with varying cholesterol concentration . fig6 c shows type c vesicles ( 10 , 11 , 12 ): dspc with varying cholesterol concentrations . fig6 d shows type e vesicles ( 17 , 18 , 19 ): dspc ( tcda 25 mol %) with varying cholesterol concentration . fig7 shows the response of type d vesicles ( 12 - 17 ): dspc ( cholesterol 20 mol %) with varying tcda concentration , to hepes , pao1 and mssa476 . fig8 shows the response of vesicles 6 , 9 and 11 to hepes , pao1 , mssa476 and e . coli . fig8 a shows that vesicle 6 responds to both p . aeruginosa pao1 and s . aureus mssa 476 fig8 b shows that vesicle 11 only responds to p . aeruginosa pao1 . fig8 c shows that vesicle 9 only responds to s . aureus mssa476 and fig8 d shows that none of vesicles 6 , 9 and 11 responds to non - pathogenic e . coli dh5α . fig9 shows colorimetric selectivity of vesicles 6 , 9 and 11 between non - pathogenic e . coli dh5α and pathogenic strains of p . aeruginosa pao1 and s . aureus mssa 476 in 96 wells plate assay . fig1 shows the results of a comparison of the selectivity of vesicles 9 and 11 for various strains of s . aureus and p . aeruginosa . lipids used in making the vesicles are 1 , 2 - dipalmitoyl - sn - glycero - 3 - phosphocholine ( dppc ), 1 , 2 - dipalmitoyl - sn - glycero - 3 - phosphoethanolamine ( dppe ), 1 , 2 - distearoyl - sn - glycero - 3 - phosphocholine ( dspc ) and 1 , 2 - distearoyl - sn - glycero - 3 - phosphoethanolamine ( dspe ), all purchased from avanti polar lipids , usa . 99 . 99 % purity biological grade cholesterol , 10 , 12 - tricosadiynoic acid ( tcda ) and 5 ( 6 )- carboxyfluorescein were directly purchased from sigma - aldrich , uk . cholesterol is a key component in cell membranes of eukaryotic cells . cholesterol not only has a profound effect on regulation of cell membrane fluidity [ t . m . koyama et al ( 1999 )] but also plays an important role in interaction with bacterial toxins [ m . plainer ( 2004 )]. tcda ( a synthetic polymerizable lipid ) is used to stabilize the vesicles , by lateral cross - linking within the lipid bilayer after uv exposure , in a partially dehydrated condition . 5 ( 6 )- carboxyfluorescein is a fluorescent dye with excitation and emission wavelengths of 495 and 520 nm respectively . it is encapsulated inside the lipid vesicles . 5 ( 6 )- carboxyfluorescein has been widely used as a tracer agent in molecular biology and to track the cell division , migration and proliferation in both in vitro and in vivo studies [ c . r . parish , 1999 ]. all the lipids , cholesterol and tcda were used without further purifications . triton x - 100 , available from sigma - aldrich , uk is a non - ionic detergent that disintegrates and reduces the lipid vesicles into micelles with simultaneous release of encapsulated carboxyfluoresceins . it was diluted 10 times in milliq water and used as a positive control during plate reading assay experiments . hepes buffer was prepared according to standard protocol and used as a negative control for the stability of vesicles in the absence of bacteria and triton x - 100 . in vesicle preparation , lipids , cholesterol and tcda were individually mixed in chloroform to 100 mmol dm − 3 . they are then mixed together to a desired composition , utilising a 100 ul volume of each component , and dried under nitrogen before being placed in a vacuum chamber at 10 − 3 bar for an hour , to remove solvent residues . thoroughly dried lipid mixture was then rehydrated in 5 ml hepes buffer , at ph 7 . 3 , containing 50 mmol dm − 3 carboxyfluorescein . after thorough remixing in hepes , the lipid solution was heated in a hot water bath at 75 ° c . for 10 minutes before being subjected to three repeated cycles of rapid freeze ( in liquid nitrogen ) and thaw ( in the hot water bath ). the lipid solution was then extruded three times through a polycarbonate membrane , of 100 nm diameter pore size , using a lipofast extruder ( avestin , usa ). finally , the extruded vesicles were purified through a dna grade sephadex g - 25 column ( ge healthcare , uk ) in hepes buffer , to separate vesicles from un - encapsulated carboxyfluorescein dye . lipid vesicles containing tcda lipids ( 1 ml , at a time in a quartz vial ) were exposed to uv - light ( 254 nm ) for a total of 12 seconds in a commercial flood exposure uv source ( hamamatsu , japan ), having been stored at 4 ° c . for at least two hours prior to uv exposure . all the vesicles were fabricated from 100 mmol dm − 3 of freshly prepared lipid stocks in 99 . 99 % purity chloroform . the mixing ratio of lipids , cholesterol and tcda in each type of vesicle is given in % volume . vesicle type 1 is composed of 53 % dppc , 2 % dppe , 20 % cholesterol and 25 % tcda . vesicle type 2 contained 78 % dspc , 2 % dspe and 20 % cholesterol , while vesicle type 3 was prepared with 23 % dppc , 2 % dppe , 50 % cholesterol and 25 % tcda . vesicle types 1 and 3 required uv cross - linking as they contained tcda lipids . after extrusion and uv cross - linking , all the vesicles were stored at 4 ° c . until such time as they were utilised in experiments . 1 ) clinically isolated , gram - positive strain methicillin - sensitive s . aureus ( mssa476 ) [ m . t . g . holden et al ( 2004 ]; 2 ) clinically significant , gram - negative strain of p . aeruginosa ( pao1 ) [ g . l . winsor et al ( 2009 )]; and 3 ) a lab strain ( non - pathogenic ) e . coli ( dh5α ), used as a control , with most virulence factors removed . bacterial cultured lb and tsb media were prepared in milliq water according to the standard ratio and procedure , followed by immediate autoclaving prior to use . dh5α and pao1 were grown in lb media . mssa476 was cultured in tsb medium . bacteria were cultured in tubes containing 10 ml of broth held in a shaking incubator at 37 ° c . for 16 hours . the optical density ( od ) of cultured bacteria was measured , at 600 nm in absorbance mode , before and after dilution . od of bacteria was then related back to the colony - forming unit per mil ( cfu / ml ) by a conventional plating and counting method . initial concentration of bacteria to be tested with vesicles was in the range of 10 2 and 10 3 cfuml − 1 . a bmg labtech fluorostar plate reading fluorimeter with a 96 well plate , in fluorescence mode with excitation and emission wavelengths of 495 nm and 520 nm respectively , was used to measure the time - dependent fluorescence response of vesicles during incubation with growing bacteria . in an individual well , 50 ul of each vesicle solution was inoculated with 100 ul of diluted broth - containing bacteria . experiments were repeated 6 times for each vesicle type and bacteria combination . 100 μl of hepes and diluted triton x - 100 were added into each type of vesicle , as negative and positive controls respectively . fluorescence responses of lipid vesicles inoculated with bacteria , hepes and triton x - 100 were continuously measured at 37 ° c . for 18 hours . for each type of vesicle and bacterial combination , the average and standard deviation of fluorescence release of 6 repeated measurements were used to calculate the fluorescence response , using the equation below : f final = average fluorescence with bacteria / hepes after 18 hours f initial = average fluorescence with bacteria / hepes at the beginning f triton = average fluorescence with triton x - 100 after 18 hours with an initial bacterial concentration in broth of 10 2 ˜ 10 3 cfu ml − 1 , it took an average of 6 hours before a fluorescence response was observed . as a positive control , triton x - 100 was utilised to lyse all vesicles , giving a 100 % response with maximum release of fluorescence molecules ( fig4 ). vesicle type 1 gave a clear response to both pao1 and mssa476 with visible fluorescence switch - on when observed under uv light ( fig5 ). upon completion of the experiment , a fluorescence response of approximately 50 % to pao1 and 80 % to mssa476 was observed with vesicle type 1 ( fig1 ). the response to pao1 was slightly lower than that to mssa476 . in comparison , the response to the negative control , hepes , and the non - pathogenic e . coli dh5α that gave almost no fluorescence release , the responses to both pao1 and mssa476 were clearly visible and distinguishable ( fig5 ). this demonstrated the selective response of vesicle type 1 to pathogenic pao1 and mssa476 but not to non - pathogenic e . coli dh5α ( fig1 ). vesicle type 2 gave an approximately 65 % response to pao1 with a less than 10 % response to mssa476 , thus exhibiting 6 . 5 fold sensitivity to pao1 with respect to mssa476 ( fig2 ). a fluorescence response of less than 10 % was not really detectable and noticeable with respect to the above 60 % response , hence there was a clear difference in fluorescence visibility between the wells containing pao1 and those containing mssa476 ( fig4 ). the majority of toxins produced by p . aeruginosa are lipid - degrading extracellular enzymes such as phospholipases in addition to some endotoxins mainly associated with the outer cell membrane of gram - negative bacteria [ j . g . songer ( 1997 )]. bacterial phospholipases are very site specific and known to target the major lipids of a eukaryotic cell membrane including phosphatidylcholine ( pc ) and phosphatidylethanolamine ( pe ) [ r . w . titball ( 1993 )]. different bacterial phospholipases specifically recognize the target lipids and are capable of hydrolysing lipids by catalysis action . phospholipids hydrolyzed by phospholipases are first reduced to lysophospholipids , followed by the subsequent removal of the fatty acid head groups from lipids . overall , this results in the gradual lipid loss out of the cell membrane and the cell membrane disintegrates upon persistent hydrolysis of lipids by phospholipases . vesicle type 2 was designed to contain 80 % of dspc and dspe and these lipids most likely became primary targets of extracellular enzymatic toxins of pao1 . this mechanism is the likely basis of the sensitivity to pao1 demonstrated by vesicle type 2 sensitive . gram - positive s . aureus pathogen is known to secrete various virulence factors , including extracellular diffusible protein toxins such as alpha , gamma and delta haemolysins , enterotoxins , leukocidins [ m . w . parker and s . c . feil ( 2005 )] and toxic shock syndrome toxins ( tsst ) [ m . m . dinges et al ( 2000 )]. haemolysins are pore forming toxins ( pfts ) and able to form heptameric pores in cell membrane which are hydrophilic and non - specific to ions and charged molecular transports [ l . song et al ( 1996 ) and a . hildebrand et al ( 1991 )]. in the haemolysis of cell membranes in vivo , haemolysin pores interrupt the electrochemical potential gradient across the cell membrane , leading to uncontrolled permeation of ions in and out of the cell , and the premature death of cells occurs by lysis such as apoptosis or necrosis [ b . geny and m . r . popoff ( 2006 )]. primary requirements for haemolysin monomers to bind onto cell membranes are : free availability of lipid head groups and sufficient fluidity of lipid membrane [ j . verdon et al ( 2009 )]. a lipid bilayer made of lipids with more than 14 carbon atoms in the hydrocarbon chains is in the gel phase at 37 ° c ., thus not fluid enough to allow the monomer insertion , and can also not provide the free lipid head groups for successful binding of monomers [ t . m . koyama et al ( 1999 )]. vesicle type 2 is primarily made of dspc and dspe lipids with 18 carbon atoms in their hydrophobic chains , and hence the lipid bilayer is almost in gel phase with little or limited fluidity at the experimental temperature of 37 ° c . this made the binding of exotoxins of mssa476 to the lipid bilayer ineffective . consequently type 2 vesicles were relatively stable in the presence of mssa476 ( fig2 ). hence , vesicle type 2 exhibited much higher sensitivity to pao1 than to mssa476 ( fig4 ). vesicle type 3 showed an approximate 60 % response to mssa476 with less than 10 % response to pao1 , and visible switch on only for mssa476 , as shown in fig4 . vesicle type 3 was composed of dppc , dppe , cholesterol and tcda . phospholipids dppc and dppe made up 25 % of the membrane composition , along with 50 % cholesterol and 25 % tcda . this composition ensures the maintenance of sufficient membrane fluidity , primarily due to the high cholesterol concentration . the fluidity of the membrane enabled the exotoxins of mssa476 to insert and exert toxin activities such as pore formation . additionally , cholesterol - rich lipid bilayer membranes are more susceptible to interactions with bacterial toxins such as cbts [ m . palmer ( 2004 ) and r . j . c . gilbert ( 2002 )]. consequently the lipid bilayer of vesicle type 3 was substantially more vulnerable to the toxins of mssa476 than those of pao1 and thus exhibited relatively higher sensitivity to mssa476 ( fig3 ). pao1 , on the other hand , primarily requires lipids to activate most of its lipid - degrading enzymes and endotoxins . with the minimal lipid composition of dppc and dppe in vesicle type 3 , activation of pao1 toxins was mostly ineffective in a lipid membrane dominated by cholesterol and tcda lipids . as a result , vesicle type 3 was much less sensitive to pao1 than mssa476 ( fig4 ). it was observed that all types of vesicles inoculated in hepes and e . coli dh5α showed no fluorescence response , indicating the effective selectivity of all vesicles between pathogenic and non - pathogenic bacteria tested in this experiment ( fig5 ). the inventors have demonstrated the selective detection between non - pathogenic bacteria e . coli dh5α and pathogenic bacteria of s . aureus mssa476 and p . aeruginosa pao1 using fluorescent dye - encapsulated lipid vesicles . there is a clear visual distinction between pathogenic and non - pathogenic strains of bacteria , in the form of fluorescence colour switch - on to bright green , after inoculation of bacteria with lipid vesicles and incubated at 37 ° c . by formulating the appropriate compositions of lipids , cholesterol and polymer lipids , the vesicles have exhibited selective sensitivity between two pathogenic strains of bacteria , pao1 and mssa476 . by integration of self - responsive lipid vesicles into a wound dressing , the current selective and discriminative sensing system has potential for future use as a rapid indicator of possible burn wound infections , without requiring the removal of the wound dressing throughout the wound healing process . vesicles employed in this study were prepared from the following lipids : 1 , 2 - dipalmitoyl - sn - glycero - 3 - phosphocholine ( dppc ), 1 , 2 - dipalmitoyl - sn - glycero - 3 - phosphoethanolamine ( dppe ), 1 , 2 - distearoyl - sn - glycero - 3 - phosphocholine ( dspc ) and 1 , 2 - distearoyl - sn - glycero - 3 - phosphoethanolamine ( dspe ) supplied by avanti polar lipids , usa . cholesterol , 10 , 12 - tricosadiynoic acid ( tcda ) and 5 ( 6 )- carboxyfluorescein were obtained from sigma - aldrich , uk . tcda is a synthetic polymerizable lipid , used to stabilize the vesicles by lateral cross - linking within the lipid bilayer after exposure to uv light . the lipids , cholesterol and tcda were used without further purification . triton x - 100 ( sigma - aldrich , uk ), a non - ionic detergent which solubilises , disintegrates and reduces the lipid vesicles into micelles with simultaneous release of encapsulated carboxyfluoresceins , was diluted 10 fold in milliq water ( 107 mg / ml ) and used as a positive control during plate reading assay experiments . hepes buffer was prepared according to standard protocol and used as a negative control for the stability of vesicles in the absence of bacteria and triton x - 100 . in vesicle preparation , lipids , cholesterol and tcda were individually made up in chloroform to a volume of 100 mmol dm − 3 . they were then combined to the desired composition in 100 μl and dried under nitrogen before being placed in a vacuum chamber at 10 − 3 bar for an hour , to remove residual solvent . the dried lipid mixture was then rehydrated in 5 ml of hepes buffer , at ph 7 . 3 , containing 50 mmol dm − 3 carboxyfluorescein . after rehydration , the lipid solution was heated in a hot water bath at 75 ° c . for 10 minutes before being subjected to three freeze - thaw cycles . the lipid solution was then extruded three times through a 100 nm diameter pore size polycarbonate membrane using a lf - 50 lipofast extruder ( avestin , usa ). finally , the extruded vesicles were purified through a dna grade sephadex g - 25 column ( ge healthcare , uk ) in hepes buffer to separate the vesicles from any un - encapsulated carboxyfluorescein dye . after storage at 4 ° c . for at least two hours the tcda - containing lipid vesicles were placed in quartz vial and exposed to uv - light ( 254 nm ) for a total of 12 seconds in a commercial flood exposure uv source ( hamamatsu , japan ). in order to elucidate the role of the various components with respect to vesicle sensitivity towards different bacteria , five classes of vesicles were studied ( a - e in table 1 ), each with one component being varied . type a vesicles were primarily shorter chain dppc , with cholesterol varied between 0 - 30 mol %; type b were similar to type a with inclusion of a photopolymerizable component tcda ; type c were longer chain dspc lipids with varying cholesterol concentrations ; type d were dspc based , had fixed cholesterol concentrations and varying tcda concentrations ; type e included dspc with fixed tcda concentrations and varying cholesterol concentrations . the rationale of this study design being that variation in cholesterol , lipid chain length and tcda concentrations profoundly affect membrane properties such as fluidity and phase transition temperature , which in turn have significant effects on membrane interaction with bacteria and toxins ( potrich et al ., 2009 ). a total of 21 clinical strains of pathogenic s . aureus were provided by the john radcliffe hospital , oxford , uk ; 19 strains of these were methicillin sensitive s . aureus ( mssa ) and the remaining 2 were methicillin resistant s . aureus ( mrsa ). all mssa and mrsa candidates studied in this work were community acquired virulent strains with suspected virulence factors that were susceptible to cell lysis ( peacock et at , 2002 ). as a counterpart to the s . aureus strains with respect to selective sensitivity , we examined 21 clinical strains of p . aeruginosa provided by ampliphi biosciences corporation , uk and southmead hospital , bristol , uk . eight strains of p . aeruginosa acquired from southmead hospital were extracted from either acute wounds or from blood agar of infected patients . the remaining 13 strains were clinical extracts from chronic wounds with known pathogenic effects on human hosts . all the strains studied in this work were tabulated in table 2 . the dh5α strain of e . coli , a laboratory strain with most of virulence factors removed , was employed as a non - pathogenic control ( hanahan , 1985 ). lb and tsb broth were prepared in milliq water according to the standard protocol and immediately autoclaved after preparation . e . coli dh5α and p . aeruginosa were grown in lb while s . aureus was cultured in tsb . bacteria were cultured in 10 ml of broth contained in falcon tubes placed in a shaking incubator at 37 ° c . for 16 hours . the optical density ( od ) of cultured bacteria before and after dilution was taken at 600 nm in absorbance mode . the od of diluted bacteria was then related back to the colony - forming unit per mil ( cfu / ml ) by means of conventional plating and colony counting methods . the initial concentration of bacteria inoculated with vesicles was approximately 10 4 cfu ml − 1 . a bmg labtech fluorostar plate reading fluorimeter with a 96 well plate was used to measure the time - dependent fluorescent response of vesicles during incubation with growing bacteria at 37 ° c . the fluorescent response was measured with excitation and emission wavelengths of 495 nm and 520 nm respectively . a 50 μl volume of each vesicle solution was inoculated with 100 μl of diluted broth containing bacteria in each well , with six replica wells for each type of vesicle - bacteria combination in each experiment . additionally , 100 μl of hepes buffer and diluted triton x - 100 were also added to each vesicle type as a negative and positive control , respectively . the fluorescent response of lipid vesicles inoculated with bacteria , hepes buffer and triton x - 100 was continuously measured for 18 hours at 37 ° c . for each type of vesicle - bacteria combination , the fluorescence average and standard deviation of six parallel wells was used to calculate the response with respect to the fluorescence increase at the end of each measurement . one of the principle differences in membrane components between eukaryotic and prokaryotic cells is the presence or absence of cholesterol . cholesterol is the essential eukaryotic cell membrane component , comprising as much as 25 % of lipids in human erythrocytes and as little as a mere trace amount in disk membrane ( bovine ) ( gennis , 1989 ). apart from its physiological role , cholesterol in general maintains the integrity and lateral fluidity of cell membranes for the normal functioning of the cell ( henna et al ., 2002 , koyama et al ., 1999 ). artificial lipid bilayer membranes containing cholesterol exhibit similar changes in membrane fluidity , line tension and formation of rafts , especially if the membrane contains sphingolipids ( veatch and keller , 2005 ). significantly , the cholesterol plays an important role in interactions of cell membranes with bacterial toxins , either directly or indirectly ( palmer , 2004 ), and its presence or absence in cell membranes influences the susceptibility of the cells to the bacterial toxins . this hypothesis was verified by designing lipid vesicles with varying cholesterol percentages , and examining the effect of cholesterol on sensitivity of vesicles to tested pathogenic bacteria . furthermore , polymerizable lipid tcda was incorporated in the lipid bilayer to create laterally cross - linked polymer networks within the membrane , thus enhancing the thermodynamic stability of the lipid membrane at elevated temperatures ( zhou et al ., 2010 ). tcda had 21 carbon atoms in a hydrophobic tail , which was shortened to 17 after lateral cross - linking by uv - light , minimizing the mismatch in chain length with dppc and dspc lipids . individual and combined effects of cholesterol and tcda on lipid bilayer stability and sensitivity were also explored . studies on type a vesicles indicated that increasing the cholesterol concentration decreased the passive leakage in hepes buffer , but also decreased the response on addition of both p . aeruginosa and s . aureus ( see fig6 a ). the maximum response for type a vesicles , taking minimal passive leakage into account , was exhibited by vesicles containing 20 mol % cholesterol , with around twice the response for p . aeruginosa than for s . aureus . type b vesicles , where the photopolymerizable cross - linker tcda was added at a fixed 25 mol % and the cholesterol concentration was varied , showed increasing stability at high cholesterol concentrations , but a notable , and almost exclusive , response to s . aureus at cholesterol concentrations of 30 mol % and above ( see fig6 b ). type c vesicles , utilising the longer chain lipid dspc , showed a greater stability than the dppc vesicles in general , and a strong effect with respect to cholesterol concentration on the response to the different bacterial toxins , with 20 mol % cholesterol having around a 4 fold greater response to p . aeruginosa than for s . aureus ( see fig6 c ). the type e vesicles ( dspc with 25 mol % tcda ) showed a marginally greater response to s . aureus over p . aeruginosa , which became more marked at higher cholesterol concentrations ( see fig6 d ). type d vesicles , composed of dspc with 20 % cholesterol and varying tcda mol % ( 0 - 25 %) showed minimal passive leakage in hepes buffer and a significant increase in response to s . aureus with respect to increasing tcda percentages , while the response to p . aeruginosa remained almost the same regardless of tcda mol % ( see fig7 ). these results indicated that the dppc vesicles were more sensitive to bacteria than dspc vesicles , with an optimum observed at a 20 % cholesterol concentration . in the absence of tcda , all vesicle types showed higher response to p . aeruginosa than to s . aureus . this was not the case , however , if tcda was included , where a relatively higher response was observed to s . aureus with increasing tcda mol %. the reduced sensitivity of dspc vesicles in general could be partly explained in terms of differences in the gel - liquid phase transition temperature ( t m ): at the relatively higher t m of dspc ( 55 ° c .) the lipid bilayer would still be in a gel phase with minimal fluidity at 37 ° c . this may reduce the passive leakage and most likely hinder the successful binding , and subsequent activation , of bacterial toxins onto vesicles . tcda - dependent response to s . aureus was not fully explained , as there was no evidence of interaction between polymerizable lipids and bacterial toxins ( jelinek and kolusheva , 2007 ). it is possible that the formation of a tcda polymer network could create local domains , rich in lipids and cholesterols , which would provide pre - defined targets for local concentrations of bacterial toxins ( verdon et al ., 2009 ). the experimental results in fig6 and 7 implied that the sensitivity of vesicles to either p . aeruginosa or s . aureus , or both , could be achieved by adjusting the percentage composition of dppc , dspc , cholesterol and tcda . three types of vesicles were chosen for the selective response tests ; vesicle 6 for the detection of both bacteria , while vesicles 11 and 9 were utilised for selective detection of p . aeruginosa and s . aureus , respectively . colorimetric selection of bacteria was demonstrated using non - pathogenic e . coli dh5α and pathogenic strains of p . aeruginosa pao1 ( winsor et al ., 2009 ) and s . aureus mssa 476 ( holden et al ., 2004 ). it took on average 6 hours to observe a notable change in fluorescence with an initial bacterial mass of 10 4 cfu ml − 1 ( fig8 ). vesicle 6 produced a 5 - 10 fold higher fluorescent response to both p . aeruginosa and s . aureus relative to negative controls in hepes buffer and e . coli dh5α ( fig8 a ). further selection between two pathogenic strains was illustrated in fig8 b and 8 c ; with vesicle 11 only responding to pao1 strain of p . aeruginosa while vesicle 9 only selected for the mssa 476 strain of s . aureus . this selectivity can be explained with respect to the lipid formulation of vesicles : lipid availability and membrane fluidity as well as the toxins involved with pathogenic bacteria . it has been shown that the activities of pfts of s . aureus increased with an increase in membrane fluidity , most especially in membranes containing lipids of shorter acyl chains ( potrich et al ., 2009 ). vesicle 6 , comprised of dppc , cholesterol and tcda , maintained a higher membrane fluidity , especially at 37 ° c ., to promote nonspecific binding of toxins with lipids in the fluid membrane , thus providing sensitivity to both tested pathogens . using only dspc and cholesterol , the membrane fluidity of vesicle 11 was shifted partly into a gel - like phase at 37 ° c ., making toxins of s . aureus incompatible with the relatively stiff membrane . p . aeruginosa , however , seemed unaffected by the low membrane fluidity , as evidenced in fig8 b , most likely due to the associated toxins and their modes of action . p . aeruginosa secrete lipid degradable enzymes , such as phospholipase c , which targets and hydrolyses the phosphate head group of phospholipids ( songer 1997 ; titball , 1993 ). such toxins require only the lipid head group availability , without being dependent on membrane fluidity for their activation . thus vesicle 11 , which was rich in dspc lipids , possibly became the target for , and thus relatively sensitive to , p . aeruginosa . vesicle 9 , composed of 50 % cholesterol and 25 % tcda , with the remaining 25 % made up of dppc and dppe ( see in table 1 ), could form the raft - like domains coexisting alongside with the lipid - rich domains within the membrane ( garcia - saez and schwille , 2010 ; mateo et al ., 1995 ). the liquid - ordered state of cholesterol - rich domains could possibly give rise to higher concentration of s . aureus toxins and their activities in remaining dppc lipid - rich liquid - disordered domains at 37 ° c . p . aeruginosa , however , primarily required lipids to activate most of its lipid - degrading enzymes . with the minimal lipid composition in vesicle 9 , activation of p . aeruginosa toxins in lipid membranes mostly dominated by cholesterol and tcda was expected to be minimal or ineffective , resulting in the increased selectivity of vesicle 9 to s . aureus ( fig8 c ). the relative fluorescence response of the vesicles to the bacteria and hepes buffer was plotted in fig8 d , and the visible colorimetric selection with respect to e . coli dh5 α , p . aeruginosa pao1 and s . aureus mssa 476 , using three types of vesicles , was illustrated in fig9 . selective sensitivity between 40 strains of p . aeruginosa and s . aureus in order to evaluate the sensitivity and the selectivity over a range of pathogenic strains , three selected vesicles were tested against 40 additional strains ( 20 each ) of s . aureus and p . aeruginosa , extracted from wounds with known infections . experimental procedure was as described above . the fluorescence and absorbance of the three replicates of each vesicle - bacterium combinations were measured at the start and the end of each experiment , and the results were plotted in fig1 a and 10 b for p . aeruginosa and s . aureus , respectively . vesicle 11 showed positive responses to all the p . aeruginosa strains , with the exception of strains 856 and 887 ( fig1 ). absorbance data indicated that strains 856 and 887 did not grow during the experiment ; consequently the positive responses were not shown , in contrast to the remaining tested strains . the reason for suppressed growth of strains 856 and 887 was not clear , despite the fact that all the strains of p . aeruginosa grew well in falcon tubes . it is possible that the growth condition of these particular strains in 96 wells plate may not be optimum to support the toxin expression , and subsequently no lysis of vesicles was observed . selectivity of vesicle ii over vesicle 9 in terms of the increment in fluorescence intensity was observed as a 2 - 6 fold increase ( fig1 a ), which was clearly distinguishable when observed under uv light . interestingly , all 20 strains of s . aureus showed higher response to vesicle 9 than to vesicle 11 ( fig1 b ). a two to ten - fold increase in fluorescence with respect to vesicle 9 against vesicle 11 was clearly observed , and all tested strains of s . aureus grew well in the experiment , according to absorbance data . in terms of fluorescence signal strength , responses of vesicle 9 to s . aureus were slightly less than those of vesicle 11 to p . aeruginosa . this relatively low fluorescence “ switch on ” could be due to the particular involvement of toxins secreted by the strains of s . aureus , which may not necessarily lyse the vesicles completely . if this was the case , it implies that the toxins associated with p . aeruginosa in general have a relatively stronger lipolytic action than those produced by s . aureus . nevertheless , visible and selective responses of vesicle 9 against vesicle 11 to all strains of s . aureus were clearly observed . it was evidenced according to the experiments that 90 % of all p . aeruginosa strains and 100 % of all s . aureus strains were selectively distinguished one from the other . we demonstrated the selective detection between non - pathogenic bacteria e . coli dh5α and pathogenic strains of s . aureus mssa 476 and p . aeruginosa pao1 using lipid vesicles containing carboxyfluorescein dyes . there was a clear , visual selection between pathogenic and non - pathogenic strains of bacteria in the form of fluorescence colour change ; switching from relatively pale to a bright green colour , after inoculation of bacteria with lipid vesicles and incubation at 37 ° c . more importantly , by formulating the appropriate compositions of lipids , cholesterol and polymerizable lipids , the vesicles showed further selective discrimination between two pathogenic strains of p . aeruginosa and s . aureus . additionally , selectivity over a wide range of two common pathogenic species was demonstrated using 40 clinical strains of p . aeruginosa and s . aureus . 90 % of p . aeruginosa strains and all of s . aureus strains were selectively distinguished , with visible fluorescence colour change , with two to ten - fold responses relative to each other . by integrating self - responsive lipid vesicles within a wound dressing , we believe that the current selective and discriminative sensing system has the potential for future use as an indicator of wound infection , without requiring the removal of the wound dressing throughout the wound healing process . we would like to thank professor mark c . enright ( ampliphi biosciences corporation , bedfordshire , uk ), southmead hospital ( bristol , uk ) and john radcliffe hospital ( oxford , uk ) for the supply of bacteria . we also acknowledge the european commission &# 39 ; s 7 th framework programme for funding via the ec - fp7 consortium project no . 245500 bacteriosafe . bailey , c . j ., redpath , m . b ., 1992 . biochem . j . 284 , 177 - 180 branski , l . k . et al ., 2009 . surg . infec . 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