Effect of different antibiotics against in vitro Staphylococcus aureus biofilm grown on chitin flakes

Abstract

MIC determination is the standard assay for testing the susceptibility of planktonic bacteria to antibiotics. It has been observed that biofilm grown cells express properties distinct from planktonic cells, for which antibiotics are generally not effective against biofilm forming organisms. The current study aims at comparison of the susceptibilities of biofilm grown cells to sin-gle antibiotic and in combination with others to identify those that were effective against Staphylococcus aureus biofilms. S. aureus ATCC 25923 were used for the purpose. They were grown in Tryptic Soya Broth (TSB) with chitin flakes as the inert surface to which the organisms adhered to produce the biofilm. Growth pattern of both biofilm producing and planktonic cells were studied. Viable counts were determined on Tryptic Soya Agar (TSA) plates. Different antibiotics viz. gentamicin, vancomycin, ciprofloxacin were used to determine the sensitivity of the bacterial strain. There was a marked differ-ence in antibiotic susceptibility between the planktonic and biofilm popula-tion of the organism. It was found that the biofilm colonies were more resistant to the antibiotics like ciprofloxacin, vancomycin and gentamicin than the planktonic cells. The reduction in growth of bacteria was much more for gentamicin compared to that of ciprofloxacin and vancomycin and when antibiotic combination gentamicin - vancomycin it is much more reduced. It is thus clear from the test that the antibiotic susceptibilities of planktonic populations are not necessarily applicable to the effective treatment of the same organism once a biofilm has been established.