Effect of antibiotics at subMIC concentration on biofilm formation by Streptococcus pyogenes

Abstract number: 1733_108

Baldassarri L., Recchia S., Creti R., Imperi M., Pataracchia M., Orefici G.

Objectives: To determine the effect of subMIC concentrations of antibiotics on biofilm formation by Streptococcus pyogenes.

Methods: A small collection of strains were chosen on the basis of the genetic determinants for antibiotic resistance they carried (i.e. susceptible, mefA+, or erm(A)-erm(B)+ strains). Four strains for each category were selected. SubMIC concentrations were determined for penicillin, clyndamycin and erythromycin by the microbroth dilution methods. Ninety six wells microtiter plates were filled with serial dilutions (1:2) of a given antibiotic; each strain was inoculated in triplicate for each antibiotic concentration, and allowed to grow for 24 hrs. At the end of the incubation period, absorbance at 630 nm was determined along with CFU counts. Plates were then emptied, dried, and the biofilm deposited on the well's bottom stained with crystal violet. Optical density at 570 nm was measured and a biofilm index generated in consideration of different growth rates.

Results: We had already determined that a majority of S. pyogenes strains from a variety of sources are able to form biofilm. In particular, susceptible strains formed thicker biofilm compared to macrolide-resistant strains. In this study we found that subMIC concentration of penicillin, but not erythromycin or clindamycin, were able to stimulate biofilm formation only in susceptible strains. Increase in biofilm index in three of the four susceptible strains examined reached up to 150%, 176% and 585% of the control, respectively. Biofilm formation was not affected in those strains carrying either genetic determinants of antibiotic resistance.

Conclusion: Data obtained in this study confirm that biofilm may represent a way for S. pyogenes to escape antimicrobial treatment even when strains lack the genetic determinants for antibiotic resistance. Penicillin treatment represents the drug of choice for eradication of S. pyogenes in patients with recurrent infections and/or carriers. It should however be considered that a percentage of treatment failures might still be possible in view of this secondary effect of penicillin.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Location: ICC, Munich, Germany
Presentation type:
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