Antimicrobial dressings such as those containing silver are now being used widely to control wound bioburden, and tests to demonstrate their efficacy predominantly involve in vitro models using free-living or planktonic bacteria. In this present study a wide range of antibiotic-sensitive and resistant bacteria were tested in their quasi-sessile state using a standard agar assay and a second method used a poloxamer gel (true biofilm state - poloxamer encourages microorganisms to exhibit a more clinically relevant biofilm phenotype) technique. The antimicrobial activity of two silver dressings, a silver-containing Hydrofiber (SCH) dressing and a nanocrystalline silver-containing dressing (NCS), were evaluated on a variety of microorganisms, using a zone-of-inhibition (ZOI) test. When grown on agar (presenting a quasi-sessile state of each organism), the antibiotic-susceptible microorganisms were generally more susceptible to the SCH dressing compared with the NCS. ZOIs associated with SCH dressing ranged between 5.7 and 17.5 mm; those for the NCS against the same group of organisms ranged between 1.9 and 8.6 mm. When grown on poloxamer gel, (presenting the biofilm state of each organism) the same group of microorganisms were less susceptible to both dressings. The SCH dressing was most effective against strains of Pseudomonas aeruginosa, Candida albicans and Staphylococcus aureus (ZOI range: 2.6-6 mm); the NCS was most effective against strains of Klebsiella pneumoniae, Enterococcus faecalis and Escherichia coli (i.e. ZOI range: 1-2.8 mm). Similarly to the antibiotic-susceptible microorganisms, nine of ten antibiotic-resistant bacterial strains when grown on agar were more susceptible to the SCH dressing compared with the NCS. Although the microorganisms tested were universally less susceptible to the silver dressings when in their biofilm state, in the majority of cases, the SCH dressing demonstrated greater biofilm-inhibiting activity than the NCS.

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BackgroundWound infections, due to biofilms, are a constant problem because of their recalcitrant nature towards antibiotics. Appropriate antibiotic selection for the treatment of these biofilm infections is important. The traditional in vitro disc diffusion method for antibiotic selection uses bacterial cultures grown on agar plates. However, the form of bacterial growth on agar is not representative of how bacteria grow in wounds and other tissue sites as here bacteria grow naturally in a biofilm. The aim of this research was to test a more appropriate method for testing antimicrobial efficacy on biofilms and compare with the standard methods used for antibiotic sensitivity testing.MethodsOuter Membrane Protein analysis was performed on E.coli, Staphylococcus aureus, Pseudomonas aeruginosa, Proteus mirabilis and Acinetobacter juni when grown on Mueller Hinton agar ('quasi-biofilm state') and 30% Poloxamer hydrogel ('true- biofilm state). Susceptibility to antibiotics on 28 clinical isolates was determined using the modified Kirby Bauer disc diffusion method, on agar and 30% Poloxamer.ResultsSimilar outer membrane proteins [OMPs] were identified in bacteria grown in a biofilm state and on a 30% poloxamer hydrogel, which were very different to the OMPs identified in bacteria grown on Mueller-Hinton agar and broth. There was a significant difference between the means of the clearance zones around the antibiotic discs on standard agar and poloxamer gels [P < 0.05]. The zones of clearance were generally smaller for poloxamer-grown bacteria than those grown on standard agar. Diffusion distances of various antibiotics through agar and 30% poloxamer showed no significant difference [P > 0.05].ConclusionThe findings of this experiment suggest that poloxamer gel could be used as an appropriate medium on which to conduct biofilm antibiotic susceptibility tests as it enables bacteria to be grown in a state representative of the infected surface from which the culture was taken.

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SUMMARYThe inner membrane of the air cell of hens' eggs was inoculated with Pseudomonas putida, Staphylococcus xylosus, Enterococcus faecalis, Escherichia coli and Salmonella enteritidis. The first mentioned eventually dominated the contamination of the albumen of eggs stored at 4, 15, and 20 0C. The last mentioned did so in eggs stored at 37 0C. The interval between inoculation of the membrane and gross contamination of the albumen was markedly influenced by site of contamination relative to yolk movement.