[Effect of subinhibitory doses of some antibiotics on bacteria in vitro]

The study was aimed at observation of influence of subinhibitory doses of selected beta-lactam antibiotics (cefuroxime, ceftazidime, thienamycin) and of ciprofloxacin on morphology and physiology of S. aureus and E. coli. The bacteria were subjected to action of these antibiotics in various subinhibitory concentrations in fluid Mueller-Hinton medium at 37 degrees C for 18 hours, and morphological changes in bacterial cells were observed in light and electronic microscope. In case of E. coli changes appeared as presence of elongated forms, while S. aureus formed giant cells with changes in staining. It was found that intensity of these morphological changes was proportional to concentrations of antibiotics. For determination of influence of tested antibiotics on growth of bacteria they were cultured in the fluid Mueller-Hinton medium containing antibiotics in concentrations of 4, 1 1/4 MIC. The culture lasted for 6 hours, and number of bacterial cells in 1 ml of the medium was periodically determined. Distinct inhibition of growth of bacterial cells was proportional to the used concentrations of antibiotics. Described in this paper morphological changes of bacteria found confirmation in change of their physiology expressed as inhibition of bacterial cell growth.