In-depth Analysis of Chlorine Dioxide Exposure on Listeria monocytogenes

Introduction: As application of novel sanitizers increases, elucidation of stress management systems employed by foodborne pathogens is pivotal.  Preliminary work in Listeria monocytogenes has shown increased activity of oxidative stress management genes upon exposure to chlorine dioxide, ClO₂.  Regulatory genes, sigB and ctsR are active during oxidative stress response.

Purpose: The purpose of this study was to further investigate the phenotypic and genotypic response of L. monocytogenes exposed to sublethal ClO₂ concentrations.  This work allowed for a more thorough understanding of L. monocytogenes’ stress response mechanism and ClO₂survival strategies.

Methods: L. monocytogenes 10403S wildtype, ΔsigB and ΔctsR null mutant strains were exposed to 300 mg/l aqueous ClO₂ at 37ºC, 230 rpm for up to 20 min, with cell populations enumerated every 2.5 min.  RNA was extracted from L. monocytogenes 10403S wildtype following exposure to 300 mg/l ClO₂ for 15 min and untreated cells; gene expression levels from untreated and ClO₂treated cells were compared using qRT-PCR with a total of 6 primer and probe gene sets.  All experiments were performed in triplicate. 

Results: Following 20 min exposure to 300 mg/l ClO₂, log death of L. monocytogenes wildtype, ΔsigB, and ΔctsR was 0.491, 0.678 and 0.415 CFU/ml, respectively.  Transcript levels of sigB, lmo0669, dnaK, clpC, and lmo1433 significantly increased upon exposure to ClO₂ compared to unexposed cells (P < 0.05). These genes are under direct regulation of sigB or ctsR, were shown to be over expressed via microarray analysis, and fall under the role categories of cell processes, energy metabolism and protein fate.

Significance: This study provides a more defined picture of the response of L. monocytogenes to oxidative stress resulting from ClO₂ exposure.  The heightened transcriptional activity of genes related to reductase enzymes and chaperone proteins provides insight into how foodborne pathogens respond and potentially survive sanitizer exposure.