Purpose: The objective of this study is to evaluate the effect of NaCl on stress resistance and biofilm production of S. aureus.
Methods: S. aureus ATCC13565 was exposed to 0, 2, 4 and 6% NaCl supplemented in tryptic soy broth (TSB) for 24 h at 35°C, followed by plating 0.1 ml of the culture on tryptic soy agar plus NaCl 0, 2, 4 and 6%. After incubation at 35°C for 24 h, the colonies on plates were collected and diluted to OD600 = 0.1. The diluents were spread on mueller hinton agar plus NaCl 0, 2, 4 and 6%. Twenty-two antibiotic disks (aminoglycoside, penicillin, macrolide, tetracycline, lincosamide, nitrofuran, aminocoumarin, chloramphenicol, fluoroquinoline, peptide, and rifampin group) were placed on the plates and then incubated at 35°C for 24 h. The inocula were inoculated in TSB adding NaCl 0, 2, 4 and 6% in 96-well microtiter plate and incubated at 35°C for 18 h. Biofilm formation was measured by crystal violet.
Results: In antibiotic disk diffusion assay, the antibiotic resistance of S. aureus to aminoglycoside antibiotics such as streptomycin, kanamycin, gentamicin and neomycin increased significantly (P < 0.05), but the pathogen did not have increased resistance to other antibiotics as NaCl concentration increased. Moreover, biofilm formation of S. aureus also increased (P < 0.05) as NaCl concentration increased.
Significance: Theses results indicate that NaCl in foods may increase antibiotic resistance and biofilm formation of S. aureus.