Effect of subinhibitory concentration of antibiotics on Rhodococcus erythropolis and Pseudomonas fluorescens biofilm formation

The occurrence of antimicrobial agents in any environment in subinhibitory concentrations can have a substantial effect on the indigenous microbial population. The release of such substances into the environment even at low concentrations should be closely monitored. In this work, ubiquitous soil bacteria, Gram-positive Rhodococcus erythropolis and Gram-negative Pseudomonas fluorescens, were studied for their susceptibility to subinhibitory concentrations of bacitracin, erythromycin, chlorhexidine, and polymyxin B. A time-dependent adherence of cells to a glass or plastic carrier in the presence of antimicrobials and antibiotics was observed. A significant influence of all antimicrobial compounds on the colonized area and microcolony size distribution was found. The highest increase of colonized area was observed in the presence of chlorhexidine for both model microorganisms. Polymyxin B significantly inhibited the cell adherence of both R. erythropolis and P. fluorescens, and tentative corresponding changes in hydrophobicity and zeta potential of cells cultured in the presence of polymyxin B were found.