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.
Highlights
Bacteria of the genus Listeria are short, Gram-positive non-spore-producing rods that have the ability to grow in a wide range of temperatures (0.5 ◦ C to 45 ◦ C), pH values (4.7 to 9.2), and osmotic pressures
The higher the hydrophobicity, the higher the susceptibility to the biocides. These results reveal that cell surface plays an important role in the tolerance of L. monocytogenes to these antimicrobials, and especially to sodium hypochlorite (SH) and benzalkonium chloride (BC)
It was demonstrated that the minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) for the biocides tested, sodium hypochlorite, benzalkonium chloride, and peracetic acid, relative to L. monocytogenes, were much lower than the concentrations of these disinfectants customarily used
Summary
Bacteria of the genus Listeria are short, Gram-positive non-spore-producing rods that have the ability to grow in a wide range of temperatures (0.5 ◦ C to 45 ◦ C), pH values (4.7 to 9.2), and osmotic pressures. These characteristics, along with the fact that they are facultative anaerobes, allow these microorganisms to survive under adverse environmental conditions [1]. A total of 26 species have so far been identified within the genus Listeria (Table 1) Of all these species the most prominent is Listeria monocytogenes because it causes the most cases of listeriosis, be it in humans or in animals.
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