Metabolic Capabilities

It is generally accepted that L. monocytogenes is an aerobically growing microaerophilic (carbon dioxydophilic) organism. However, L. monocytogenes is able to survive and to colonize the mammalian gut where it encounters anaerobic conditions or to multiply in decaying plants, an environment also devoid of oxygen. This is reflected in the genome sequence as many fermentative pathways are predicted (Figure 3.3.). Analysis of the genome sequence identified in L. monocytogenes and in L. innocua a single continuous locus, coding for the proteins necessary for vitamin B12 synthesis. Furthermore, the proteins necessary for degradation of the carbon sources ethanolamine and propanediol in a coenzyme B12-dependent manner are encoded within the same region. Vitamin B12 synthesis and degradation of ethanolamine and propanediol may be important for anaerobic growth of Listeria.

Not all organisms have the ability to synthesize vitamin B12. A search among genome sequences revealed that vitamin B12 biosynthesis genes are found

Vitamin B12 Metabolism Fermentation

Figure 3.3. Fermentative pathways as deduced from the Listeria monocytogenes genome analysis. Color code: Black, intermediary metabolites; Red, fermentation end products; Blue, enzymes involved in fermentation. Paralogous genes were detected for several key enzymes: lmo1917 (pflA) and lmo1406 (pflB) encode pyruvate formate lyases; lmo0210 (Idh) and lmo1057 encode lactate dehydrogenases; lmo1179 and lmo1634 encode acetaldehyde dehydrogenase/alcohol dehydrogenase; lmo1369 and lmo2103 (pta) encode phosphotransacetylases; lmo1581 (ackA) and lmo1168 (ackA2) encode acetate kinases.

Figure 3.3. Fermentative pathways as deduced from the Listeria monocytogenes genome analysis. Color code: Black, intermediary metabolites; Red, fermentation end products; Blue, enzymes involved in fermentation. Paralogous genes were detected for several key enzymes: lmo1917 (pflA) and lmo1406 (pflB) encode pyruvate formate lyases; lmo0210 (Idh) and lmo1057 encode lactate dehydrogenases; lmo1179 and lmo1634 encode acetaldehyde dehydrogenase/alcohol dehydrogenase; lmo1369 and lmo2103 (pta) encode phosphotransacetylases; lmo1581 (ackA) and lmo1168 (ackA2) encode acetate kinases.

in just over one-third of the bacteria sequenced at that time (for a review, see Raux et al. 2000). The proteins deduced from the genome sequence of L. monocytogenes and L. innocua share the highest homology with those of S. enterica serovar Typhimurium (38-65% protein identity). As described above, all three gene clusters seem to have been acquired by Listeria en bloc by horizontal gene transfer from Enterobacteriaceae, most probably from S. enterica (Buchrieser et al. 2003). Genes coding for CbiD, CbiG, and CbiK, specifically associated with the anaerobic pathway, are present in Listeria. This suggests that the two Listeria species also contain the oxygen-independent pathway like Salmonella. Downstream of the cobalamine biosynthesis genes, Listeria contains orthologs of genes necessary in Salmonella for the coenzyme B12-dependent degradation of ethanolamine and propanediol. Salmonella typhimurium synthesizes vitamin B12 anaerobically (Jeter et al. 1984) and can use ethanolamine and 1,2-propanediol as the sole carbon and energy source for growth (Price-Carter et al. 2001). Vitamin B12-dependent anaerobic degradation of ethanolamine and propanediol could enable L. monocytogenes to use ethanolamine and 1,2 propanediol as carbon and energy source for growth under anaerobic conditions encountered in the mammalian gut, where both substances are believed to be abundant. It is tempting to speculate that the vitamin B12 synthesis genes together with the pdu and eut operons play a role during listerial infection. This hypothesis is substantiated by a recent report that showed, by studying intracellular gene expression of L. monocytogenes, that this bacterium can use ethanolamine as alternative nitrogen sources during replication in epithelial cells (Joseph et al. 2006).

In-depth genome analysis facilitated the identification of a previously unknown and alternative route of gluthatione biosynthesis in Listeria, which probably has an important role in protection against oxidative stress (Gopal et al. 2005). GSH is the predominant low-molecular-weight peptide thiol present in living organisms. In bacteria it plays a pivotal role in many metabolic processes including thiol redox homeostasis, protection against reactive oxygen species, protein folding, and provision of electrons via NADPH to reductive enzymes, such as ribonucleotide reductase. A broad survey of the distribution of thiols in microorganisms revealed that several species of gram-positive bacteria, including Listeria, streptococci, and enterococci, produce significant amounts of GSH but the source of GSH in these bacteria has remained a puzzle, since their genomes do not contain a canonical gshB gene (Fahey et al. 1978; Newton et al. 1996). Copley and Dhillon (Copley and Dhillon 2002) identified in the L. monocytogenes genome, a gene containing an N-terminal domain that encodes a molecule significantly related to bacterial Y-glutamylcysteine ligases (GshA) and a C-terminal domain that encodes a molecule that bears little resemblance to typical bacterial glutathione synthetases (GshB) but is clearly related to the ATP-grasp super-family of proteins. Gopal and colleagues (Gopal et al. 2005) demonstrated that this gene encodes a multidomain protein (termed GshF) that carries out complete synthesis of GSH. Furthermore, gluthatione biosynthesis seems to be involved in virulence of L. monocytogenes as a gsf deletion mutant is not only defective in gluthatione synthesis but also impaired in growth and survival in mouse macrophages and in Caco-2 enterocyte like cells (Gopal et al. 2005).

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