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

Abstract

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.

Keywords:

• Bacillus cereus
• Antibiotic susceptibility
• Postantibiotic effect
• Time-killing effect, Sporulation

Acknowledgements

We would like to thank the Ministry of Science, Education and Sports, Agency for Mobility and EU Programmes of Croatia for financial support.