Transmission electron microscopy study of Listeria monocytogenes serotype 1/2a cells exposed to sublethal heat stress and carvacrol

Authors: SULAGNA, SAHA - Mississippi State University; NITIN, DHOWLAGHAR - Mississippi State University; AMANDA, LAWRENCE - Mississippi State University; RAMAKRISHNA, NANNAPANENI - Mississippi State University; CHANDER, SHARMA - Mississippi State University; BARAKAT, MAHMOUD - Mississippi State University

Submitted to: Journal of the Mississippi Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/2015
Publication Date: 7/1/2015
Citation: Sulagna, S., Nitin, D., Amanda, L., Ramakrishna, N., Chander, S., Barakat, M. 2015. Transmission electron microscopy study of Listeria monocytogenes serotype 1/2a cells exposed to sublethal heat stress and carvacrol. Journal of the Mississippi Academy of Sciences. 60:300-304.

Interpretive Summary: Prevalence of Listeria monocytogenes is very high in catfish. Several published studies have reported on the occurrence of 25% to 47% for L. monocytogenes in catfish samples. L. monocytogenes can get inactivated with proper cooking prior to consumption. However, contaminated catfish or salmon may cause problem to the preparation of down-stream value added smoked fish products. The natural persistence of L. monocytogenes is due to its wide-spread distribution and its ability to withstand adverse environmental conditions. Recent studies show that some strains of L. monocytogenes are able to persist in food processing environments for longer periods. Screening of heat resistance (at 60°C/10 min) of 37 L. monocytogenes strains collected from various sources showed significant variation with some strains yielding 7 log reductions while others with as little as 2 log CFU/ml reductions. Since there is a potential occurrence of heat stress-resistant strains of L. monocytogenes, understanding their environmental stability and persistence will be of immense significance. Our studies show that L. monocytogenes was found to develop high tolerance to heat by undergoing changes in its growth, ultrastructure, surface properties, and intracellular structures.

Technical Abstract: The objective of this study was to investigate the morphological changes that occurred in Listeria monocytogenes serotype 1/2a cells as visualized by transmission electron microscopy (TEM) after exposure to sublethal heat stress at 48°C for 60 min and in combination with lethal concentration of carvacrol for 30 min. The morphological changes in L. monocytogenes were classified into changes occurring in the cell surface, cell wall, cell membrane and cytoplasm. The TEM micrographs revealed the thickening of cell wall and cell membrane, clumping of cytoplasm, loss of cytoplasm, pore formation and membrane bleb formation in L. monocytogenes cells when subjected to sublethal heat stress followed by carvacrol treatment. These findings indicate that L. monocytogenes cells when subjected to different stresses may alter their cellular morphology which may aid in their survival. Further studies will investigate the effect of increased contact time with carvacrol and with other essential oils on the ultrastructural changes in L. monocytogenes cells adapted to sublethal heat stress.