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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. 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. [ABSTRACT FROM AUTHOR]

The susceptibility of Cronobacter sakazakii ATCC 29544 (CS) and Yersinia enterocolitica ATCC 9610 (YE) to sodium hypochlorite (10% of active chlorine; SHY), peracetic acid (39% solution of peracetic acid in acetic acid; PAA) and benzalkonium chloride (BZK) was tested. Minimum inhibitory concentration (MIC) values (planktonic cells; microdilution broth method) of 3,800 ppm (SHY), 1,200 ppm (PAA) and 15 ppm (BZK) for CS, and 2,500 ppm (SHY), 1,275 ppm (PAA) and 20 ppm (BZK) for YE, were found. In some instances, an increase in growth rate was observed in presence of sub-MICs (0.25MIC, 0.50MIC or 0.75MIC) of biocides relative to the samples without biocides. The cultures exhibited an acquired tolerance to biocides and an increase in antibiotic resistance after exposure to sub-MICs of such disinfectants. Strains were able to form strong biofilms on polystyrene after 48 hours (confocal laser scanning microscopy), with average biovolumes in the observation field (14,161 µm2) of 242,201.0 ± 86,570.9 µm3 (CS) and 190,184.5 ± 40,860.3 µm3 (YE). Treatment of biofilms for 10 minutes with disinfectants at 1MIC or 2MIC reduced the biovolume of live cells. PAA (YE) and BZK (CS and YE) at 1MIC did not alter the percentage of dead cells relative to non-exposed biofilms, and their effect of countering biofilm was due principally to the detachment of cells. These results suggest that doses of PAA and BZK close to MICs might lead to the dissemination of live bacteria from biofilms with consequent hazards for public health. [ABSTRACT FROM AUTHOR]

The susceptibility of four L. monocytogenes isolates from pork to sodium hypochlorite (SHY) and benzalkonium chloride (BZK) was tested. Minimum inhibitory concentration (MIC) values of 3500 ppm (SHY), or between 3 ppm and 13 ppm (BZK), were found. Minimum bactericidal concentration (MBC) values ranged from 3500 ppm to 4500 ppm (SHY), and from 3 ppm to 14 ppm (BZK). The effect of SHY and BZK on the architecture and cellular viability of 24-h-old biofilms formed by such strains on polystyrene was determined through confocal laser scanning microscopy (CLSM) in conjunction with fluorescent dyes for live cells (SYTO 9) and dead cells (propidium iodide). Strains were able to form biofilm (biovolume values in the observation field of 14,161 μm2 ranged between 103,928.3 ± 6730.2 μm3 and 276,030.9 ± 42,291.9 μm3). Treatment of biofilms for 10 min with SHY (1MIC or 1.5MIC) or BZK (0.5MIC, 1MIC or 1.5MIC) decreased the biovolume of live (potentially dangerous) cells. SHY reduced the cellular viability of biofilms by more than 90%. On the other hand, BZK was able to remove most biofilm mass (live and dead cells), but decreased cellular viability only to a lesser extent, this suggesting strong biofilm detachment and dissemination of live cells. • The susceptibility of four L. monocytogenes strains to two disinfectants was tested. • L. monocytogenes biofilms were exposed to disinfectants at 0.5MIC, 1MIC or 1.5MIC. • Most bacteria in biofilms were inactivated after contact with sodium hypochlorite. • Exposure of biofilms to benzalkonium chloride caused strong biofilm detachment. • Low doses of biocides could lead to the spread of live cells, a worrying finding. [ABSTRACT FROM AUTHOR]