Direct viable count of Gram-positive and Gram-negative bacteria using ciprofloxacin as inhibitor of cellular division

Abstract

Following the method described by Kogure et al. (Can. J. Microbiol. 25 (1979) 415–420), we used ciprofloxacin, a quinolone which inhibits cell division of Gram-positive and Gram-negative bacteria, to quantify the number of viable bacteria in a series of cultures. As test bacteria we used three Escherichia coli strains, two of them sensitive to nalidixic acid and a third resistant to this inhibitor, and three Gram-positive bacteria: one rod ( Lactobacillus plantarum) and two cocci ( Enterococcus faecalis and Micrococcus varians). Ciprofloxacin is an efficient inhibitor of cell division in all six strains, although the effect on the morphology of the rods and cocci was clearly different. The percentage of viable bacteria was determined in the case of the rods by the increase in either length or biovolume and, for the cocci, by the increase in cell biovolume. Image analysis was required to detect variations in cell biovolume in the case of cocci and rods. For nalidixic acid-sensitive strains, it was possible to use both, as similar percentages of viable bacteria were detected with both inhibitors. The differences between the colony forming units (CFU) and direct viable (DVC) counts were much higher in the strains showing typical arrangements than in those strains without them. Finally, we found no significant differences in the number of viable bacteria in the natural population of the Butrón river when we used either ciprofloxacin or nalidixic acid as inhibitors of cell division.