Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) for Twelve Antimicrobials (Biocides and Antibiotics) in Eight Strains of Listeria monocytogenes

Simple Summary Listeria monocytogenes is the bacterium responsible for the majority of cases of human listeriosis, a foodborne infection that, in certain groups in the population (children, elderly, pregnant women and immunocompromised individuals), exhibits a high fatality rate (of up to 30%), and the need for hospital admission in more than 90% of cases. An awareness of the minimal concentrations for disinfectants and antibiotics necessary to destroy L. monocytogenes, may assist with choosing the most effective antimicrobials for controlling this microorganism, whether in the food industry or in the health system. The lethal concentrations of three disinfectants (sodium hypochlorite, benzalkonium chloride, and peracetic acid) and eight antibiotics (ampicillin, cephalothin, cefoxitin, erythromycin, chloramphenicol, gentamicin, tetracycline, vancomycin, and fosfomycin) for eight different strains of L. monocytogenes were determined in this research work. It was demonstrated that the lethal concentrations for the disinfectants tested were much lower than the concentrations customarily used of these compounds. The characteristics of the cell surface play an important role in the tolerance of L. monocytogenes to these biocides. A considerable prevalence of resistance to most of the antibiotics tested was noted, making it clear that the necessary measures to control resistance in L. monocytogenes must be adopted. Abstract When selecting effective doses of antimicrobials, be they biocides or antibiotics, it is essential to know the minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of these substances. The present research determined the MICs and MBCs for three biocides, sodium hypochlorite (SH), benzalkonium chloride (BC), and peracetic acid (PAA), and nine antibiotics in eight strains of Listeria monocytogenes of varying serotypes. Marked intra-species differences were observed in the resistance of L. monocytogenes to the biocides and antibiotics. The MICs (ppm) for the biocides ranged between 1750 and 4500 for SH, 0.25 and 20.00 for BC, and 1050 and 1700 for PAA. Their MBCs (ppm) ranged from 2250 to 4500 for SH, 0.50 to 20.00 for BC, and 1150 to 1800 for PAA. The MICs (ppm) for antibiotics lay between 1 and 15 for ampicillin, 8 and 150 for cephalothin, 20 and 170 for cefoxitin, 0.05 and 0.20 for erythromycin, 4 and 50 for chloramphenicol, 3 and 100 for gentamicin, 2 and 15 for tetracycline, 2 and 80 for vancomycin, and 160 and 430 for fosfomycin. The corresponding MBCs (ppm) were from 5 to 20 for ampicillin, 9 to 160 for cephalothin, 70 to 200 for cefoxitin, 4 to 5 for erythromycin, 9 to 70 for chloramphenicol, 5 to 100 for gentamicin, 3 to 30 for tetracycline, 3 to 90 for vancomycin, and 160 to 450 for fosfomycin. Notably, erythromycin showed considerable efficacy, demonstrated by the low values for both MIC and MBC. Based on EUCAST and the CLSI criteria, all strains were susceptible to erythromycin. All strains were resistant to cephalothin, cefoxitin, gentamicin, and fosfomycin. Further values for resistance were 87.50% for ampicillin and vancomycin, 75.00% for tetracycline, and 62.50% for chloramphenicol. The high prevalence of antibiotic resistance is a matter for concern. A positive correlation was found between MIC and MBC values for most of the biocides and antibiotics. The higher the hydrophobicity of the cell surface, the higher the susceptibility to biocides, suggesting that surface characteristics of bacterial cells influence resistance to these compounds.