DIRECT VIABLE COUNT OF GRAM-POSITIVE AND GRAM-NEGATIVE BACTERIA USINGCIPROFLOXACIN AS INHIBITOR OF CELLULAR DIVISION

Following the method described by Kogure et al. (Can. J. Microbiol. 25 (1979) 415-420), we used ciprofloxacin, a quinolone which inhibits ce ll division of Gram-positive and Gram-negative bacteria, to quantify t he number of viable bacteria in a series of cultures. As test bacteria we used three Escherichia coli strains, two of them sensitive to nali dixic acid and a third resistant to this inhibitor, and three Gram pos itive bacteria: one rod (Lactobacillus plantarum) and two cocci (Enter ococcus faecalis and Micrococcus varians). Ciprofloxacin is an efficie nt inhibitor of cell division in all six strains, although the effect on the morphology of the rods and cocci was clearly different. The per centage of viable bacteria was determined in the case of the rods by t he increase in either length or biovolume and, for the cocci, by the i ncrease in cell biovolume. Image analysis was required to detect varia tions in cell biovolume in the case of cocci and rods. For nalidixic a cid-sensitive strains, it was possible to use both, as similar percent ages of viable bacteria were detected with both inhibitors. The differ ences between the colony forming units (CFU) and direct viable (DVC) c ounts were much higher in the strains showing typical arrangements tha n in those strains without them. Finally, we found no significant diff erences in the number of viable bacteria in the natural population of the Butron river when we used either ciprofloxacin or nalidixic acid a s inhibitors of cell division.