Growth and Filamentation of Cold-Adapted, Log-Phase Listeria monocytogenes Exposed to Salt, Acid, or Alkali Stress at 3°C
Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5.Journal of food protection (Impact Factor: 1.85). 12/2012; 75(12):2142-50. DOI: 10.4315/0362-028X.JFP-12-199
In Canada, there is a zero tolerance for Listeria in a 125-g sample of product in which growth of Listeria monocytogenes can occur, and a limit of ≤100 CFU/g in ready-to-eat (RTE) food products that support limited growth during the stated shelf life and/or RTE refrigerated foods with a shelf life of ≤5 days. L. monocytogenes can form filaments in response to pH and osmotic, atmospheric, and temperature stress, which can result in an underestimation of the risk of RTE foods as filaments form single colonies on plate count agars but can divide into individual cells once the stress is removed. The objective was to investigate the filamentation characteristics of three strains of L. monocytogenes exposed to saline, acidic, basic, and simultaneous acidic and saline environments at 3°C. After 4 days at 3°C, log-phase cells grown in tryptic soy broth (TSB) were longer than cells grown at 15°C, and 68% of cells were below the reference value of the 90th percentile of control cultures. When cultures growing at 3°C were exposed to additional stresses, increases in the proportion and length of filaments in the population were observed, while increases in log CFU per milliliter were reduced. After 4 days of incubation at 3°C, the log CFU per milliliter of L. monocytogenes increased by 1.1 U in TSB and 0.4 to 0.5 U in TSB with 4% NaCl, TSB with a pH of 6.0 with 4% NaCl, and TSB with a pH of 5.5. Moreover, the longest 10% of cells were 6.4 to 8.5 times longer than control cells, and only 20 to 30% of cells were below the reference value. Cultures grown in TSB at pH 6.0 with 4% NaCl experienced more sustained filamentation than cultures grown in TSB with 4% NaCl, but less than cultures grown in TSB at pH 6.0. The mechanism involved in filamentation could be different for cells exposed to NaCl than exposed to acid, and additional stress might not necessarily result in more extensive filament formation. These findings contribute to a better understanding of the widespread potential of filament formation and the potential implications for food safety.
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ABSTRACT: A number of studies have reported that pathogenic and nonpathogenic foodborne bacteria have the ability to form filaments in microbiological growth media and foods after prolonged exposure to sublethal stress or marginal growth conditions. In many cases, nucleoids are evenly spaced throughout the filamentous cells but septa are not visible, indicating that there is a blockage in the early steps of cell division but the mechanism behind filament formation is not clear. The formation of filamentous cells appears to be a reversible stress response. When filamentous cells are exposed to more favorable growth conditions, filaments divide rapidly into a number of individual cells, which may have major health and regulatory implications for the food industry because the potential numbers of viable bacteria will be underestimated and may exceed tolerated levels in foods when filamentous cells that are subjected to sublethal stress conditions are enumerated. Evidence suggests that filament formation under a number of sublethal stresses may be linked to a reduced energy state of bacterial cells. This review focuses on the conditions and extent of filament formation by foodborne bacteria under conditions that are used to control the growth of microorganisms in foods such as suboptimal pH, high pressure, low water activity, low temperature, elevated CO2 and exposure to antimicrobial substances as well as lack a of nutrients in the food environment and explores the impact of the sublethal stresses on the cell's inability to divide.International journal of food microbiology 05/2013; 165(2):97-110. DOI:10.1016/j.ijfoodmicro.2013.05.001 · 3.08 Impact Factor
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ABSTRACT: The genus Listeria contains ten species of Gram-positive bacteria, L. monocytogenes, L. fleischmannii, L. grayi, L. innocua, L. ivanovii, L. marthii, L. rocourtiae, L. seeligeri, L. weihenstephanensis, and L. welshimeri, and has been classified (along with members of the genus Brochothrix: B. thermosphacta and B. campestris) within the family Listeriaceae. Members of this family produce short rods that may form filaments. Cells stain Gram-positive, and the cell walls contain meso-diaminopimelic acid. The major lipid components include saturated straight-chain and methyl-branched fatty acids. Endospores are not produced; menaquinones are the sole respiratory quinones. Growth is aerobic and facultatively anaerobic; glucose is fermented to lactate and other products. L. monocytogenes (and to a lesser extent L. ivanovii) which are pathogenic to humans and a range of other animals, and the disease is primarily transmitted by consumption of contaminated food or feed. Human listeriosis is an opportunistic infection which most often affects those with severe underlying illness, the elderly, pregnant women, and both unborn and newly delivered infants. The reported incidence of human listeriosis varies between countries from 10 cases per million of the total population. Because of the severity of infection, listeriosis is one of the major causes of death from a preventable foodborne illness. Studies of the molecular biology of L. monocytogenes have identified a number of virulence factors that promote uptake into nonprofessional phagocytic cells and the process of movement from cell-to-cell by recruiting host cell proteins and remodeling the host cell cytoskeleton. This has made L. monocytogenes also of interest both as a tool to help understand eukaryotic cell biology and as a potential therapeutic agent for intracellular delivery of drugs and as a cancer vaccine. The presence of L. monocytogenes remains a major challenge for the food industry. Its psychrotrophic nature means that it can grow at or below refrigeration temperatures and it is also relatively tolerant of high solute concentrations, resists desiccation, and therefore can overcome mild food preservation techniques. L. monocytogenes is able to form biofilms and can colonize food processing equipment and environments, leading to cross-contamination of processed foods. Hence it is of particular concern in ready-to-eat foods.The Prokaryotes, 01/2014: pages 241-259; , ISBN: 978-3-642-30119-3
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ABSTRACT: The aim of this study was to examine the filament formation and differential gene expression of Listeria monocytogenes 08-5923 grown on refrigerated vacuum-packaged ham products with various NaCl concentrations. Filament formation of L. monocytogenes was observed on ham products with 1.35% and 2.35% NaCl, which was monitored using flow cytometry by measuring forward light scatter.Quantitative Real-Time PCR was used to study the differential expression of genes in filamented cells of L. monocytogenes grown on hams following 2 or 3 months of storage at 4°C. The genes involved in cell division (ftsX/lmo2506), cell wall synthesis (murZ/lmo2552) and NADPH production (gnd/lmo1376) were significantly down-regulated in filamented cells of L. monocytogenes grown on ham with 2.35% NaCl stored at 4°C. To our knowledge, this study reports the first evidence of filament formation of Listeria grown on meat products, which could impact the food safety risk and tolerance levels of L. monocytogenes set by regulatory agencies.This article is protected by copyright. All rights reserved.FEMS Microbiology Letters 09/2014; 360(2). DOI:10.1111/1574-6968.12599 · 2.12 Impact Factor
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