Potential of Antimicrobial Agents to Inhibit Fungi to Aid Isolation of Campylobacter

In preliminary studies conducted to isolate thermophilic Campylobacter spp. from cull dairy cow fecal samples, growth of fungal strains on Abeyta-Hunt-Bark agar and charcoal cefoperazone desoxycholate agar (CCDA) interfered with isolation of target bacteria. Concentrations of antifungal substances in the agar media were not effective in suppressing growth of antibiotic resistant fungi. The objective of the current study was to determine the efficacy of supplemental antimicrobial agents to inhibit growth of fungi of fecal origin on Campylobacter isolation media where microbial growth is overwhelmed by fungi. The inhibitory effect of rifampicin and seven antimycotic agents—cycloheximide, 5-fluorouracil, 5-fluorocytosine, sodium propionate, potassium sorbate, nystatin, and amphotericin B—was evaluated. Antimicrobial agents were added singly or combined at concentrations ranging from 12.3 to 227 mg/L. Incubation was conducted at 42°C for 24–48 h in gas jars (CO₂ + H₂). The most effective antibiotic combinations that inhibited fungal growth without affecting Campylobacter growth were CCDA selective supplement SR155E with amphotericin B (59.5 or 114 mg/L) or a mixture of rifampicin, 5-fluorocytosine, and nystatin (119 or 227 mg/L each). Results of the present study indicate that addition of supplemental antifungal agents in the presence of antibiotics such as rifampicin may enhance suppression of fungal overgrowth onCampylobacter isolation agar media.