Listeria monocytogenes Cold Shock Proteins: Small Proteins with A Huge Impact

Abstract

Listeria monocytogenes has evolved an extensive array of mechanisms for coping with stress and adapting to changing environmental conditions, ensuring its virulence phenotype expression. For this reason, L. monocytogenes has been identified as a significant food safety and public health concern. Among these adaptation systems are cold shock proteins (Csps), which facilitate rapid response to stress exposure. L. monocytogenes has three highly conserved csp genes, namely, cspA, cspB, and cspD. Using a series of csp deletion mutants, it has been shown that L. monocytogenes Csps are important for biofilm formation, motility, cold, osmotic, desiccation, and oxidative stress tolerance. Moreover, they are involved in overall virulence by impacting the expression of virulence-associated phenotypes, such as hemolysis and cell invasion. It is postulated that during stress exposure, Csps function to counteract harmful effects of stress, thereby preserving cell functions, such as DNA replication, transcription and translation, ensuring survival and growth of the cell. Interestingly, it seems that Csps might suppress tolerance to some stresses as their removal resulted in increased tolerance to stresses, such as desiccation for some strains. Differences in csp roles among strains from different genetic backgrounds are apparent for desiccation tolerance and biofilm production. Additionally, hierarchical trends for the different Csps and functional redundancies were observed on their influences on stress tolerance and virulence. Overall current data suggest that Csps have a wider role in bacteria physiology than previously assumed.

Keywords: Listeria monocytogenes; biofilm; cold; cold shock protein; osmotic; stress tolerance; virulence.

Figure 1

Csp protein sequences based on L. monocytogenes EGDe genome, highlighted under the red rectangles, are the two RNA-binding motifs (RNBP). Figure created using QIAGEN CLC Genomics Workbench 20.0 (available online: https://digitalinsights.qiagen.com/ (accessed on 9 April 2021)).

Figure 2

Overall effects of cold shock proteins (Csps) in L. monocytogenes: Csps are involved in (a) biofilm formation, (b) motility through impacting flagella expression, (c) they are dispensable during normal growth at optimal temperature. However, current data and inference from other bacterial species suggest that Csps are involved in nutrient acquisition and utilization. Nevertheless, this needs to be confirmed. (d) Csps are involved in stress tolerance responses to cold, osmotic, oxidative, high hydrostatic pressure (HHP), and desiccation stress. It is suggested that they might play roles in response to pH and bacteriocin (e.g., nisin) stress. However, this needs to be validated. (e) Csps are important for the full expression of virulence as observed in zebrafish embryos. They promote hemolysis, cell invasion, as well as growth and survival within macrophages. (f) Csps involvement in response to cleaning and disinfection chemical stress is yet to be confirmed, but inference from other bacterial species supports their involvement. Figure 2 was created with BioRender.com.