Changes in the bactericidal activity of the antibioticsa

ExposureAverage decline in log (CFU/ml) after 2 h of exposure ± SD
CIP (0.0625)AMP (48)RIF (48)STR (32)TET (48)
First exposure3.6 ± 0.33.3 ± 0.32.1 ± 0.23.7 ± 0.32.0 ± 0.4
Second exposure2.9 ± 0.73.2 ± 0.52.0 ± 0.83.3 ± 0.92.9 ± 0.3
P values0.1250.50.3750.3751
  • a Decline in the log10 (CFU per milliliter) of viable cells after 2 h of exposure to fresh antibiotic-containing LB (first exposure) and to antibiotic-containing filtrates of cultures previously exposed to antibiotics (second exposure). Average declines in cell densities, standard deviations, and P values (Wilcoxon signed-rank test, paired) of three independent experiments are listed. The bacteria were exposed to ciprofloxacin (CIP), ampicillin (AMP), rifampin (RIF), streptomycin (STR), and tetracycline (TET). The antibiotic concentrations (in micrograms per milliliter) are shown in parentheses after the drug.