title: 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.
authors.affiliation: Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5.
authors.forename: K M
authors.initials: KM
authors.lastname: Vail
authors.forename: L M
authors.initials: LM
authors.lastname: McMullen
authors.forename: T H
authors.initials: TH
authors.lastname: Jones
mesh_terms: D000222:Adaptation, Physiological; D015169:Colony Count, Microbial; D005506:Food Contamination / Q000032:analysis* / Q000517:prevention & control; D005511:Food Handling / Q000379:methods*; D005516:Food Microbiology; D059022:Food Safety; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008089:Listeria monocytogenes / Q000254:growth & development*; D008954:Models, Biological; D009994:Osmolar Concentration; D013696:Temperature; D013997:Time Factors