Changes in Barotolerance, Thermotolerance, and Cellular Morphology throughout the Life Cycle of Listeria monocytogenes

Abstract

Changes in barotolerance, thermotolerance, and cellular morphology throughout the life cycle of Listeria monocytogenes were investigated. For part 1 of this analysis, L. monocytogenes ATCC 19115 was grown to log, stationary, death, and long-term-survival phases at 35°C in tryptic soy broth with yeast extract (TSBYE). Cells were diluted in whole milk that had been subjected to ultrahigh temperatures (UHT whole milk) and then high-pressure processed (HPP) at 400 MPa for 180 s or thermally processed at 62.8°C for 30 s. As cells transitioned from the log to the long-term-survival phase, the D 400 MPa and D 62.8°C values increased 10-and 19-fold, respectively. Cells decreased in size as they transitioned from the log to the long-term-survival phase. Rod-shaped cells transitioned to cocci as they entered the late-death and long-term-survival phases. L. monocytogenes strains F5069 and Scott A showed similar results. For part 2 of the analysis, cells in long-termsurvival phase were centrifuged, suspended in fresh TSBYE, and incubated at 35°C. As cells transitioned from the long-term-survival phase to log and the stationary phase, they increased in size and log reductions increased following HPP or heat treatment. In part 3 of this analysis, cells in long-term-survival phase were centrifuged, suspended in UHT whole milk, and incubated at 4°C. After HPP or heat treatment, similar results were observed as for part 2. We hypothesize that cells of L. monocytogenes enter a dormant, long-term-survival phase and become more barotolerant and thermotolerant due to cytoplasmic condensation when they transition from rods to cocci. Further research is needed to test this hypothesis and to determine the practical significance of these findings.

Effect of Lactose Monolaurate on Pathogenic and Nonpathogenic Bacteria

“…2B) are observed as cocci in chains, yet no change in cell surface was observed. The change in the morphology of Listeria from a rod to a coccus has been shown to be based on the medium type and the presence of stress (temperature or pressure) (14). Figure 3 shows SEM of Mycobacterium sp.…”

Transcriptomic Response of Listeria monocytogenes during the Transition to the Long-Term-Survival Phase

“…3) in death phase may increase the general resistance of L. monocytogenes during and after death phase, as DnaK stabilizes proteins under various types of stresses (18). This may partly explain why LTS-phase cells of L. monocytogenes had significantly higher D values at 400 MPa than that at stationary phase (41). When listerial cells entered death phase, the majority of cells died, whereas about 10% of the population survived (Fig.…”

“…The LTS phase was also observed in L. monocytogenes by Wen et al, during which the cell density was found to remain at ϳ10 8 CFU/ml in tryptic soy broth with yeast extract (TSBYE) for over 30 days (41). These LTS cells were found to be predominantly cocci and highly resistant to both heat and high pressure stresses (41). The mechanisms that trigger listerial cells to transit from bacilli to cocci during the LTS phase remain unclear yet intriguing.…”

“…Various forms of LTS cells were reported in saprotrophic bacterial species, such as Micrococcus luteus (34), and in some enteric bacterial species, such as E. coli (9). The LTS phase was also observed in L. monocytogenes by Wen et al, during which the cell density was found to remain at ϳ10 8 CFU/ml in tryptic soy broth with yeast extract (TSBYE) for over 30 days (41). These LTS cells were found to be predominantly cocci and highly resistant to both heat and high pressure stresses (41).…”

“…A total of 1/10 ml of cell cultures at different phases were Gram stained and examined at a magnification of ϫ1,000 using a BX51 light microscope equipped with a DP20 camera (Olympus Optical, Tokyo, Japan) as previously described (41). At each phase coccoid-and rod-shaped cells were enumerated in three fields.…”