Listeria monocytogenes PdeE, a phosphodiesterase that contributes to virulence and has hydrolytic activity against cyclic mononucleotides and cyclic dinucleotides

Manganese uptake mediated by the NRAMP-type transporter MntH is required for acid tolerance in Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen that is characterized by its ability to withstand mild stresses (i.e. cold, acid, salt) often encountered in food products or food processing environments. In the previous phenotypic and genotypic characterization of a collection of L. monocytogenes strains, we have identified one strain 1381, originally obtained from EURL-lm, as acid sensitive (reduced survival at pH 2.3) and extremely acid intolerant (no growth at pH 4.9, which supports the growth of most strains). In this study, we investigated the cause of acid intolerance in strain 1381 by isolating and sequencing reversion mutants that were capable of growth at low pH (pH 4.8) to a similar extent as another strain (1380) from the same MLST clonal complex (CC2). Whole genome sequencing showed that a truncation in mntH, which encodes a homologue of an NRAMP (Natural Resistance-Associated Macrophage Protein) type Mn²⁺ transporter, is responsible for the acid intolerance phenotype observed in strain 1381. However, the mntH truncation alone was not sufficient to explain the acid sensitivity of strain 1381 at lethal pH values as strain 1381R1 (a mntH⁺ revertant) exhibited similar acid survival to its parental strain at pH 2.3. Further growth experiments demonstrated that Mn²⁺ (but not Fe²⁺, Zn²⁺, Cu²⁺, Ca²⁺, or Mg²⁺) supplementation fully rescues the growth of strain 1381 under low pH conditions, suggesting that a Mn²⁺ limitation is the likely cause of growth arrest in the mntH^- background. Consistent with the important role of Mn²⁺ in the acid stress response was the finding that mntH and mntB (both encoding Mn²⁺ transporters) had higher transcription levels following exposure to mild acid stress (pH 5). Taken together, these results provide evidence that MntH-mediated Mn²⁺ uptake is essential for the growth of L. monocytogenes under low pH conditions. Moreover, since strain 1381 was recommended for conducting food challenge studies by the European Union Reference Laboratory, the use of this strain in evaluating the growth of L. monocytogenes in low pH environments where Mn²⁺ is scarce should be reconsidered. Furthermore, since it is unknown when strain 1381 acquired the mntH frameshift mutation, the ability of the strains used for challenge studies to grow under food-related stresses needs to be routinely validated.

Validation of Predicted Virulence Factors in Listeria monocytogenes Identified Using Comparative Genomics

Listeria monocytogenes is an intracellular facultative pathogen that causes listeriosis, a foodborne zoonotic infection. There are differences in the pathogenic potential of L. monocytogenes subtypes and strains. Comparison of the genome sequences among L. monocytogenes pathogenic strains EGD-e and F2365 with nonpathogenic L. innocua CLIP1182 and L. monocytogenes strain HCC23 revealed a set of proteins that were present in pathogenic strains and had no orthologs among the nonpathogenic strains. Among the candidate virulence factors are five proteins: putrescine carbamoyltransferase; InlH/InlC2 family class 1 internalin; phosphotransferase system (PTS) fructose transporter subunit EIIC; putative transketolase; and transcription antiterminator BglG family. To determine if these proteins have a role in adherence and invasion of intestinal epithelial Caco-2 cells and/or contribute to virulence, five mutant strains were constructed. F2365ΔinlC2, F2365Δeiic, and F2365Δtkt exhibited a significant (p < 0.05) reduction in adhesion to Caco-2 cells compared to parent F2365 strain. The invasion of F2365ΔaguB, F2365ΔinlC2, and F2365ΔbglG decreased significantly (p < 0.05) compared with the parent strain. Bacterial loads in mouse liver and spleen infected by F2365 was significantly (p < 0.05) higher than it was for F2365ΔaguB, F2365ΔinlC2, F2365Δeiic, F2365Δtkt, and F2365ΔbglG strains. This study demonstrates that aguB, inlC2, eiic, tkt, and bglG play a role in L. monocytogenes pathogenicity.