Listeria monocytogenes is one of the toughest challenges the food industry is facing today. While quaternary ammonium compounds (QAC) are typically applied at very high concentrations to kill L. monocytogenes, there are some factors that can cause its reduced efficacy in the food processing environment. Sublethal concentrations of stagnated QAC have been found both in processing plants and in waste waters released to the environment. Therefore, the purpose of this study was to investigate the ability of two L. monocytogenes strains, Bug600 (serotype 1/2a) and ScottA (serotype 4b) to persist in lethal and sublethal concentrations of first generation QAC (benzalkonium chloride, BAC) and second generation QAC (Ster-BAC), and if these strains would regrow when conditions had improved. Using BAC or Ster-BAC concentrations that mimic those found occurring in some environments, we determined L. monocytogenes survival, persistence, and recovery in high/low nutrient conditions, and also isolated two distinct morphotypes.

The prevalence and mode of persistence of Salmonella on commercial poultry processing equipment was examined in this study. The equipment surfaces of six commercial processing plants were sampled over three-time periods designated as A (after processing), B (after cleaning), and C (after sanitization) and each plant was visited three times. Salmonella prevalence was greater (p<0.0001) at time A than times B and C, while there were no differences (p=0.386) between times B and C. A total of twentyive Salmonella isolates from five distinct serovars were recovered from the processing plants. Further examination of the recovered isolates showed that tolerance to sanitizing agents (chlorine and quaternary ammonium compounds, QAC) and/or biofilm formation was the possible mode of persistence. In addition, all the isolates were resistant to multiple antibiotics in different classes of drugs and more than two genes that are responsible for ⅰ). biofilm formation, ⅱ). resistance to QAC, ⅲ). oxidative stress response, and ⅳ). resistance to antibiotics were detected. Three selected isolates were further characterized based on virulence factors and antimicrobial resistance using whole genome sequencing (WGS) to possibly predict phenotypic characteristics. The WGS data correlates with phenotypic characteristics that were previously observed in the isolates including the ability to produce biofilms and resistance to antibiotics including β-lactams, aminoglycosides, and cephalosporins. WGS predicted the isolates carried resistance genes for antibiotic drug classes that were not observed phenotypically. These include macrolides and fluoroquinolone, which is a concern due to its use in treating foodborne infection. Furthermore, the genome of the three selected isolates were predicted to have over sixty virulence genes that allow Salmonella to invade, attach, and colonize the host cells. The results from this study suggest that the antimicrobials used for sanitization may be insufficient to inactivate Salmonella because of the ability to produce biofilms on processing surfaces. Most importantly, the results suggest the ability of the isolates to cause infection in humans meaning if contamination were to occur, the right antibiotic treatment could be a challenge. Further research is imperative to determine the effective antimicrobial for Salmonella biofilms.

Quaternary ammonium compounds (QACs) are widely used in commercial and consumer applications as disinfectants, fabric softeners, hair conditioners, and emulsifying agents. The massive production and utilization of QACs has led to their extensive discharge into the environment, raising concerns globally. Several studies have reported on potential risks and detrimental effects of QACs on the natural environment and public wastewater treatment plants. Biological treatment has been found to be an effective way to remove QACs and especially aerobic treatment processes can provide rapid biodegradation via a consortium of bacteria. Although extensive research has been conducted on the fate and effect of QACs, relatively little is known about their effect on aerobic biological treatment processes, especially on nitrification. Research was conducted on the fate and effect of alkyl benzyl dimethyl ammonium chloride (AB), a QAC widely used as disinfectant, in mixed aerobic and nitrifying cultures. The results of this study demonstrated that up to 50 mg/L AB was efficiently degraded in a mixed aerobic culture fed with dextrin and peptone, although trace residual AB levels were observed. Nitrification of the produced ammonia was complete at an AB concentration of 20 mg/L after an acclimation period, but was almost completely inhibited at 50 mg/L. Mixed aerobic cultures maintained only with AB as external nitrogen and carbon source achieved a high degree of AB degradation at both 20 and 50 mg/L. Ammonia oxidation by a nitrifying culture, enriched with ammonium chloride and sodium bicarbonate, was inhibited with increasing AB concentration and completely ceased at 15 mg/L AB. Degradation or utilization of AB was not observed for all tested AB concentrations between 2 to 20 mg/L. Based on these experimental results, and assuming non-competitive inhibition, a relatively low value of the AB inhibition coefficient was obtained, which indicates a relatively high susceptibility of the ammonia oxidizers to AB. The results of this study have significant implications for both engineered and natural systems relative to the fate and effect of QACs.