Singh R, Ray P, Das A, Sharma M
Role of persisters and small colony variants in antibiotic resistance of planktonic and biofilm-associated Staphylococcus aureus: an in vitro study. [JOURNAL ARTICLE]
J Med Microbiol 2009 Jun 15.
The presence of persister cells and small colony variants (SCVs) has been associated with enhanced antibiotic resistance of many organisms in biofilms. We investigated whether persisters and/or SCVs contribute to antibiotic resistance of Staphylococcus aureus biofilms. A detailed dose-dependent killing of biofilms and planktonic cells with five antibiotics (oxacillin, cefotaxime, amikacin, ciprofloxacin and vancomycin) was analyzed by treating them with each antibiotic at a concentration of 0-100 mug ml-1 at 37 degrees C for 48 h. The killing of biofilm cells by all the antibiotics indicated the presence of persister cells - most cells in the population died, leaving a fraction that persisted even at higher concentrations of the antibiotics. These persisters represented a transient resistant phenotype and reverted to a killing curve resembling the wild type parent upon re-exposure to the antibiotics. SCVs were observed in biofilms after treatment with ciprofloxacin only and these SCVs were of a transient nature. The treatment of planktonic cells with oxacillin, cefotaxime, ciprofloxacin and vancomycin killed the entire population and no persisters were detected. Transient SCVs, observed in planktonic cells following exposure to these antibiotics, were killed at higher antibiotic concentrations. The treatment of planktonic cells with amikacin yielded a small sub-population of survivors that included persisters (at numbers significantly lower than the biofilms) and the highly resistant, stable, small colony variants with an increased biofilm forming capacity in comparison to the wild type parent. Thus, the high resistance of S. aureus biofilms to multiple unrelated antibiotics is largely dependent on the presence of persister cells. Biofilms harbor a large number of persisters in comparison to planktonic cultures that either do not harbor persisters or harbor only a small number. Small colony variants, though not specifically associated with S. aureus biofilms, have an increased biofilm forming capacity and this may explain the frequent isolation of SCVs from biofilm-associated infections. The intrinsic resistance of these variants may in turn contribute to the enhanced antibiotic resistance of the biofilms so formed.
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