Purpose: This study evaluated the effects of sodium chloride, sodium phosphate, and sodium ascorbate on antibiotic susceptibility of Listeria monocytogenes, Staphylococcus aureus, and Escherichia coli.
Methods: L. monocytogenes strains (NCCP10805, NCCP10806, NCCP10807, NCCP10808, NCCP10809, NCCP10810, NCCP10811, NCCP10920, and NCCP10943), S. aureus strains (ATCC13565, ATCC14458, ATCC23235, ATCC27664, and NCCP10826), and E. coli strains (NCCP10439, NCCP14037, NCCP14038, NCCP14039, and NCCP15661) were exposed to sodium chloride (0, 1.2, and 3.6%), sodium phosphate (0, 0.3, and 1.5%), or sodium ascorbate (0, 0.3, and 1.5%) supplemented in tryptic soy broth (TSB) at 35°C for 24 h, and 0.1 ml portions of the cultures were subcultured in correspondent TSB at 35°C for 24 h. This subculture was repeated ten times. Subsequently, bacterial cells were diluted to OD600 = 0.2, and the diluents were spread-plated on Mueller Hinton agar. Ten antibiotic disks (ampicillin, bacitracin, penicillin G, tigecycline, rifampicin, streptomycin, vancomycin, gentamicin, erythromycin, lincomycin) for L. monocytogenes and S. aureus, and ten disks (tigecycline, rifampicin, streptomycin, florfenicol, gentamicin, neomycin, tetracycline, amoxicillin, ceftiofur, chloramphenicol) for E. coli were placed on the plates, followed by incubation at 30°C (L. monocytogenes) and 35°C (S. aureus and E. coli) for 24 h. Clear zone (mm) was measured to evaluate antibiotic susceptibility.
Results: Clear zone diameters of L. monocytogenes NCCP10805, NCCP10806, NCCP 10809, NCCP10920, and NCCP10943 exposed to sodium phosphate were decreased (P < 0.05) to penicillin G, rifampicin, vancomycin, and erythromycin. However, the diameters of S. aureus and E. coli strains were not decreased.
Significance: These results indicate that sodium phosphate may decrease antibiotic susceptibility of L. monocytogenes.