Antimicrobial susceptibility and the in vitro postantibiotic effects of vancomycin and ciprofloxacin against Bacillus cereus isolates.

Background: Vancomycin and ciprofloxacin were often used in the therapy of infections associated with Bacillus cereus.

Methods: Four B. cereus food and clinical isolates were chosen for determination of time-kill curves and postantibiotic effects (PAE) of ciprofloxacin and vancomycin.

Results: According to the minimum inhibition concentration (MIC), breakpoints defined by CLSI for Staphylococcus spp. were all four strains intermediate for vancomycin (MIC = 4 μg/ml) and sensitive to ciprofloxacin (MIC = 0.2 μg/ml) except the strain Bc63 resistant to the last antimicrobial (MIC = 1.6 μg/ml). The lowest CFU values of tested strains were reached after 3-5 hours of exposure to 4 ×  MIC of vancomycin, and after 6-7 hours exposure to 10 ×  MIC of ciprofloxacin. The maximum reduction of the CFU in the presence of vancomycin and ciprofloxacin was about 2.46 log10 and 2.48 log10, respectively. The average duration of the PAE of vancomycin and ciprofloxacin was 0.94 and 1.60 hours, respectively. The statistically significant differences between PAEs induced with 3 ×  MIC, 4 ×  MIC and 8 ×  MIC of vancomycin were observed (P < 0.05). Both antibiotics did not affect the sporulation of tested bacterial strains.

Conclusions: The differences in PAE duration were strain and antimicrobial dependent.