AUTHOR=Muchaamba Francis , Wambui Joseph , Stephan Roger , Tasara Taurai 

TITLE=Cold Shock Proteins Promote Nisin Tolerance in Listeria monocytogenes Through Modulation of Cell Envelope Modification Responses

JOURNAL=Frontiers in Microbiology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.811939

DOI=10.3389/fmicb.2021.811939

ISSN=1664-302X

ABSTRACT=<p><italic>Listeria monocytogenes</italic> continues to be a food safety challenge owing to its stress tolerance and virulence traits. Several listeriosis outbreaks have been linked to the consumption of contaminated ready-to-eat food products. Numerous interventions, including nisin application, are presently employed to mitigate against <italic>L. monocytogenes</italic> risk in food products. In response, <italic>L. monocytogenes</italic> deploys several defense mechanisms, reducing nisin efficacy, that are not yet fully understood. Cold shock proteins (Csps) are small, highly conserved nucleic acid-binding proteins involved in several gene regulatory processes to mediate various stress responses in bacteria. <italic>L. monocytogenes</italic> possesses three <italic>csp</italic> gene paralogs; <italic>cspA</italic>, <italic>cspB</italic>, and <italic>cspD</italic>. Using a panel of single, double, and triple <italic>csp</italic> gene deletion mutants, the role of Csps in <italic>L. monocytogenes</italic> nisin tolerance was examined, demonstrating their importance in nisin stress responses of this bacterium. Without <italic>csp</italic> genes, a <italic>L. monocytogenes</italic> Δ<italic>cspABD</italic> mutant displayed severely compromised growth under nisin stress. Characterizing single (Δ<italic>cspA</italic>, Δ<italic>cspB</italic>, and Δ<italic>cspD</italic>) and double (Δ<italic>cspBD</italic>, Δ<italic>cspAD</italic>, and Δ<italic>cspAB</italic>) <italic>csp</italic> gene deletion mutants revealed a hierarchy (<italic>cspD</italic> &gt; <italic>cspB</italic> &gt; <italic>cspA</italic>) of importance in <italic>csp</italic> gene contributions toward the <italic>L. monocytogenes</italic> nisin tolerance phenotype. Individual eliminations of either <italic>cspA</italic> or <italic>cspB</italic> improved the nisin stress tolerance phenotype, suggesting that their expression has a curbing effect on the expression of nisin resistance functions through CspD. Gene expression analysis revealed that Csp deficiency altered the expression of DltA, MprF, and penicillin-binding protein-encoding genes. Furthermore, the Δ<italic>cspABD</italic> mutation induced an overall more electronegative cell surface, enhancing sensitivity to nisin and other cationic antimicrobials as well as the quaternary ammonium compound disinfectant benzalkonium chloride. These observations demonstrate that the molecular functions of Csps regulate systems important for enabling the constitution and maintenance of an optimal composed cell envelope that protects against cell-envelope-targeting stressors, including nisin. Overall, our data show an important contribution of Csps for <italic>L. monocytogenes</italic> stress protection in food environments where antimicrobial peptides are used. Such knowledge can be harnessed in the development of better <italic>L. monocytogenes</italic> control strategies. Furthermore, the potential that Csps have in inducing cross-protection must be considered when combining hurdle techniques or using them in a series.</p>