title: Effect of subinhibitory concentrations of fluoroquinolones on biofilm production by clinical isolates of Streptococcus pyogenes.
abstract: OBJECTIVE
  Subinhibitory concentrations (sub-MICs) of antibiotics, although not able to kill bacteria, but influence bacterial virulence significantly. Fluoroquinolones (FQs) which are used against other bacterial pathogens creates resistance in non-targeted Streptococcus pyogenes. This study was undertaken to characterize the effect of sub-MICs of FQs on S. pyogenes biofilm formation.
  
  
  METHODS
  Biofilm forming six M serotypes M56, st38, M89, M65, M100 and M74 of S. pyogenes clinical isolates were challenged against four FQs namely, ciprofloxacin, ofloxacin, levofloxacin and norfloxacin. The antibiofilm potential of these FQs was analysed at their subinhibitory concentrations (1/2 to 1/64 MIC) using biofilm assay, XTT reduction assay, scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM).
  
  
  RESULTS
  Among the four FQs tested, ofloxacin and levofloxacin at 1/2 MIC showed the maximum inhibition (92%) of biofilm formation against M56 and M74 serotypes. FQs effectively interfered in the microcolony formation of S. pyogenes isolates at 1/2 to 1/8 sub-MICs. Inhibition of biofilm formation was greatly reduced beyond 1/16 MICs and allowed biofilm formation. XTT reduction assay revealed the increase in metabolic activity of S. pyogenes biofilm against the decrease in FQs concentration. SEM and CLSM validated the potential of sub-MICs of FQs against the six S. pyogenes.
  
  
  CONCLUSIONS
  Our results showed that the inhibitory effect all four FQs on S. pyogenes biofilm formation was concentration dependent. FQs at proper dosage can be effective against S. pyogenes and lower concentrations may allow the bacteria to form barriers against the antibiotic in the form of biofilm.
authors.affiliation: Department of Biotechnology, Alagappa University, Karaikudi, India.
authors.forename: Kannan
authors.initials: K
authors.lastname: Balaji
authors.forename: Ramalingam
authors.initials: R
authors.lastname: Thenmozhi
authors.forename: Shunmugiah Karutha
authors.initials: SK
authors.lastname: Pandian
mesh_terms: D000900:Anti-Bacterial Agents / Q000627:therapeutic use; D018441:Biofilms / Q000187:drug effects* / Q000254:growth & development; D002939:Ciprofloxacin / Q000494:pharmacology; D024841:Fluoroquinolones / Q000494:pharmacology*; D006801:Humans; D064704:Levofloxacin / Q000494:pharmacology; D008826:Microbial Sensitivity Tests; D018613:Microscopy, Confocal; D008855:Microscopy, Electron, Scanning; D009643:Norfloxacin / Q000494:pharmacology; D015242:Ofloxacin / Q000494:pharmacology; D013290:Streptococcal Infections / Q000188:drug therapy / Q000382:microbiology*; D013297:Streptococcus pyogenes / Q000187:drug effects* / Q000254:growth & development / Q000472:pathogenicity / Q000648:ultrastructure; D014774:Virulence / Q000187:drug effects
chemical_list: D000900:Anti-Bacterial Agents; D024841:Fluoroquinolones; D002939:Ciprofloxacin; D064704:Levofloxacin; D015242:Ofloxacin; D009643:Norfloxacin