Effect of Sub-Inhibitory Concentrations of Quaternary Ammonium Compounds and Heavy Metals on Antibiotic Resistance and Expression of Virulence Factors Among Staphylococcus spp. from Dairy Products

Antimicrobial resistance is spreading rapidly throughout the world. The food chain can be one of the routes of transmission for microorganisms containing drug-resistance genes and thus serve as a channel for their transmission. Environmental stress and methods of preventing the spread of microorganisms trigger adaptive responses in bacterial cells. The aim of the present study was to determine the effect of the stress induced by sub-inhibitory concentrations (SICs) of cadmium chloride and benzalkonium chloride on antibiotic resistance and the expression of selected virulence factors in <i>Staphylococcus</i> isolates from food. The study was conducted on strains of the species <i>S. epidermidis, S. heamolyticus, S. saprophyticus</i>, and <i>S. aureus</i>. The values of the minimum inhibitory concentration against erythromycin, tetracycline, and oxacillin were determined before and after the incubation of the tested strains under stress conditions. The ability to form biofilm and slime production was also investigated. The expression levels of the genes responsible for antibiotic resistance (<i>bla</i>Z, <i>tet</i>K, <i>tet</i>M, <i>erm</i>B, and <i>mec</i>A) and virulence (<i>eno</i>) were conducted using Real-Time PCR. The MIC values of the antibiotics tested against the strains analyzed were found to be elevated in the presence of SICs of benzalkonium chloride and cadmium chloride. Furthermore, the intensity of biofilm production was also increased. SICs of benzalkonium chloride induced the expression of the <i>tet</i>M, <i>tet</i>K, <i>mec</i>A, and <i>bla</i>Z genes in 75%, 66.6%, 33.3%, and 40% of the isolates tested, respectively. Similar treatment with cadmium chloride induced the expression of the same genes in 75%, 100%, 66.6%, and 40% of the strains. In both cases, the expression of the <i>erm</i>B gene was reduced in 100% of the isolates. The <i>eno</i> gene was found to be overexpressed in 66.6% of the strains following benzalkonium chloride stress, and in 100% of the strains following cadmium chloride stress. These findings suggest that in <i>Staphylococcus</i> spp. strains, changes in the expression of the genes encoding antibiotic resistance and virulence factors may occur in response to the applied stress factors. The results indicate the possibility of selecting more resistant and virulent strains due to the use of too low concentrations of disinfectants, which emphasizes the need to use appropriate inhibitory doses of disinfectants in the food industry.