Effect of sub-MIC values of metronidazole, ciprofloxacin, and imipenem on growth and toxin production in Clostridioides difficile

Aim: This study intends to investigate the effect of sub-minimum inhibitory concentration (sub-MIC) of metronidazole, ciprofloxacin, and imipenem on the growth and toxin production in Clostridioides difficile. Background: C. difficile is the most common causative agent of hospital-acquired diarrhea. Toxin production in C. difficile appears to be a critical process for induction of the disease. Several factors such as antibiotics can facilitate growth and toxin production in C. difficile. Methods: Five C. difficile strains were grown with and without sub-MIC concentrations of metronidazole, ciprofloxacin, and imipenem (0.5x MIC). The bacterial growth was measured by density at OD620 nm in 0, 4, 8, 12 and 24 h post inoculation. Toxin production was detected using ELISA in culture supernatants as well as in cell pellet. Results: The five strains showed minor growth variations in the presence and absence of antibiotic sub-MIC values, except for metronidazole, in which the sub-MIC concentration reduced the growth rate of the resistant isolate in comparison with the control without antibiotic. There were no significant variations in the levels of toxin production with the sub-MIC values of antibiotics examined in comparison with antibiotic-free controls. However, the amount of toxin production in the culture supernatant was greater than in the cell pellet. Conclusion: The findings of this study suggested that sub-MIC concentrations of antibiotics may have minor effects on bacterial growth and toxin production of C. difficile. Taken together, these findings suggest that presence of antimicrobial agents increased expression levels of certain genes, particularly virulence genes, which may help C. difficile to survive.