Growth and Filamentation of Cold-Adapted, Log-Phase Listeria monocytogenes Exposed to Salt, Acid, or Alkali Stress at 3°C.
ABSTRACT 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 · 3.16 Impact Factor