Low, medium and high heat tolerant strains of Listeria monocytogenes and increased heat stress resistance after exposure to sublethal heat

Authors: SHEN, QIAN - Mississippi State University; JANGAM, PRIYANKA - Mississippi State University; SONI, KAMLESH - Mississippi State University; NANNAPANENI, RAMAKRISHNA - Mississippi State University; SCHILLING, WES - Mississippi State University; SILVA, JUAN - Mississippi State University

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/30/2014
Publication Date: 8/1/2014
Citation: Shen, Q., Jangam, P.M., Soni, K., Nannapaneni, R., Schilling, W., Silva, J. 2014. Low, medium and high heat tolerant strains of Listeria monocytogenes and increased heat stress resistance after exposure to sublethal heat. Journal of Food Protection. 77:1298-1307.

Interpretive Summary: Thirteen different serotypes of L. monocytogenes are known, of which 1/2a, 1/2b and 4b are responsible for most listeriosis outbreaks in the United States. None evaluated the 13 serotypes of L. monocytogenes for diversity in their heat tolerance. Studies related to the effect of inherent variation in heat tolerance within L. monocytogenes strains against potential heat stress adaptation at varied sublethal heat stress conditions have not been attempted. Risk associated with microbial stress adaptation is a vital and timely area of research with direct practical implications. The stability (i.e. cellular imprint/memory) of heat stress adaptation is a very important aspect to study as heat stress adapted cells may not immediately encounter homologous (i.e. heat) or heterogeneous (i.e. acid, antimicrobial, HHP) lethal inactivation treatments during food processing. There is little information is available on whether the stability of heat stress adaptation in L. monocytogenes is strain dependent. To improve the existing knowledge on the heat stress adaptation of L. monocytogenes, our study focused on the following objectives: (1) evaluate the heat tolerance of 37 L. monocytogenes strains representing all known 13 serotypes of L. monocytogenes; (2) evaluate the heat stress adaptation in representative strains of low, medium and high heat tolerant L. monocytogenes after sublethal heat stress at 48°C; (3) evaluate the growth rate of representative strains of low, medium and high heat tolerant L. monocytogenes after sublethal heat stress at 48°C for different time periods; and (4) evaluate the stability of heat stress adaptation in representative strains of low, medium and high heat tolerant L. monocytogenes after sublethal heat stress at 48°C followed by cooling down to 22°C or 4°C before lethal heat stress at 60°C. Our results show that there is a high diversity in heat tolerance within strains of L. monocytogenes serotypes and their heat stress adaptation once acquired is still preserved after cooling step which should be taken into account while conducting risk analysis for this pathogen. Such a risk analysis may contribute to the critical control point practice of the pathogens in the food industry.

Technical Abstract: Listeria monocytogenes exhibits sophisticated adaptive mechanisms to counteract higher levels of lethal acid, heat, salt or oxidative stresses after pre-exposure to sublethal concentrations of homogenous stress. A group of 37 strains representing all 13 serotypes of Listeria monocytogenes with initial cell density of 107 CFU/ml were analyzed for their heat tolerance at 60°C for 10 min. These L. monocytogenes strains were categorized into three heat tolerance groups: low (strains with < 2 log survival), medium (2 to 4 log survival) and high (4 to 6 log survival) heat tolerance. Serotype 1/2a strains exhibited relatively lower heat tolerance since 7 out of 8 tested strains were classified as low heat tolerant. Of the two 1/2b serotypes tested, one was very heat sensitive (non-detectable) and the other very heat resistant (5.4 log CFU/ml survival). Among the 16 serotype 4b strains, survival varied from non-detectable to 4 log CFU/ml. When one L. monocytogenes strain from each representing group was subjected to sublethal heat stress at 48°C for 30 or 60 min, the survival of heat stressed cells at 60°C for 10 min increased by 5 log CFU/ml (or D60°C values nearly doubled) compared to the non-stressed control cells. Sublethal heat stress at 48°C for 60 or 90 min increased lag phase of L. monocytogenes in tryptic soy broth supplemented with 0.6% yeast extracts (TSB-YE) at room temperature by 3 to 5 hours compared to non-stressed control cells. The heat stress adaptation in L. monocytogenes was reversed within 2 h at room temperature but well maintained up to 24 h at 4°C. Our results show that there is a high diversity in heat tolerance within strains of L. monocytogenes serotypes and their heat stress adaptation once acquired is still preserved after cooling step which should be taken into account while conducting risk analysis for this pathogen.