Factors compromising antibiotic activity against biofilms of Staphylococcus epidermidis

Several factors associated with bacterial biofilms were studied for their r ole in phenotypic resistance to antibiotics. These factors included bacteri al slime extracted from biofilms, reduced growth rates of biofilm-embedded bacteria and high bacterial inocula. Antibiotic activity against suspended bacteria in the presence of these factors, either alone or combined, was co mpared with activity against adherent biofilms. All MICs, determined by sta ndard susceptibility tests, were below the sensitivity breakpoints for Stap hylococcus epidermidis strain V2. The addition of bacterial slims to suspen ded bacteria reduced the bactericidal activity of glycopeptides but had les s or no effect on the activity of the other antibiotics tested. High bacter ial inocula affected the activity of flucloxacillin and quinolones only mod erately or not at all, though a more pronounced effect on glycopeptides was observed. In contrast, the bactericidal activity of most antibiotics was s everely compromised when adherent bacterial biofilms were used as inocula. In conclusion, the presence of slime, slow growth rates and high bacterial counts may explain the poor activity of glycopeptides against biofilm-embed ded organisms, but these factors, either alone or in combination, do not ex plain the lack of bactericidal activity of other drugs against biofilms. Th us, additional factors need to be identified.