Purpose: This study elucidated the effect of MSG and DSI on antibiotic resistance of Listeria monocytogenes, Escherichia coli, and Staphylococcus aureus.
Methods: L. monocytogenes (NCCP10805, NCCP10806, NCCP10807, NCCP10808, NCCP10809, NCCP10810, NCCP10811, NCCP10920, and NCCP10943), E. coli (NCCP10439, NCCP14037, NCCP14038, NCCP14039, and NCCP15661), and S. aureus (ATCC13565, ATCC14458, ATCC23235, ATCC27664, and NCCP10826) were subjected to tryptic soy broth formulated with 0, 0.3, and 1.0% MSG and DSI at 35°C for 24 h, and 0.1 ml aliquots of each culture were subcultured in same media at 35°C for 24 h. Subculture was repeated ten times. Bacterial cells (OD600 = 0.2) were spread-plated on Mueller Hinton agar. Ten antibiotic disks (ampicillin, bacitracin, penicillin G, tigecycline, rifampicin, streptomycin, vancomycin, gentamicin, erythromycin, and lincomycin) for L. monocytogenes and S. aureus, and ten disks (tigecycline, rifampicin, streptomycin, gentamicin, florfenicol, neomycin, tetracycline, amoxicillin, ceftiofur, chloramphenicol) for E. coli were placed on the media, followed by incubation at 30°C (L. monocytogenes) and 35°C (S. aureus and E. coli) for 24 h. Clear zones were measured to evaluate antibiotic susceptibility.
Results: Clear zones of S. aureus and E. coli strains by MSG and DSI were not decreased. However, clear zone sizes of L. monocytogenes decreased (P < 0.05) from 34 to 14 mm (NCCP10811), from 28 to 18 mm (NCCP10920), and from 36 to 14 mm (NCCP10943) against penicillin G (10 μg) after exposure to 1.0% MSG. Exposure of L. monocytogenes to 1.0% MSG decreased (P < 0.05) clear zone sizes on vancomycin (30 μg) from 25 to 21 mm (NCCP10920) and from 25 to 20 mm (NCCP10943). DSI did not affect antibiotic susceptibility of L. monocytogenes.
Significance: These results indicate that MSG may increase the antibiotic susceptibility of L. monocytogenes, depending on strain and antibiotic.