Transport Systems and Sugar Utilization

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The Listeria genomes encode an abundance of transport proteins (e.g., 11.6% of all predicted genes of L. monocytogenes EGDe). These comprise, in particular, proteins dedicated to carbohydrate transport conferring Listeria probably in part its ability to colonize a broad range of ecosystems. The overall array of sugar transporters is similar in all Listeria genomes, in particular among the four sequenced L. monocytogenes strains, but also with L. innocua. Listeria are predicted to transport and metabolize many simple as well as complex sugars including fructose, rhamnose, rhamnulose, glucose, mannose, chitin, sucrose, cellulose, pullan, trehalose, and tagatose. These sugars are largely associated with the environments where Listeriae are found. As in most bacterial genomes the predominant class corresponds to ABC transporters. Interestingly, most of the carbohydrate transport proteins belong to phosphoenolpyruvate-dependent phosphotransferase system (PTS)-mediated carbohydrate transport. The PTS allows the use of different carbon sources and in many bacteria studied so far the PTS is a crucial link between metabolism and regulation of catabolic operons (Barabote and Saier 2005; Kotrba et al. 2001). The Listeria genomes contain an unusually large number of PTS loci (e.g., nearly twice as many as E. coli and nearly thrice as many as B. subtilis). Most of these PTS systems are conserved in the different sequenced genomes; however, subtle differences can be observed, probably allowing niche-specific adaptation. An example is the family of P-glucoside-specific PTSs, of which eight are present in L. monocytogenes serotype 1/2a, two of those are missing in the L. monocytogenes serotype 4b strains and five are missing from L. innocua. As one of these P-glucoside-specific PTS systems named BvrABC was shown to be implicated in virulence of L. monocytogenes (Brehm et al. 1999) these differences might play a role in virulence differences among strains.

The different PTS systems should allow Listeria to use many different carbon sources during extracellular growth. However, during intracellular growth L. monocytogenes also needs a supply of carbon sources. This is in part achieved by a protein similar to UhpT, a hexose phosphate permease found in enteric bacteria, encoded by hpt. In addition, hpt is absent from the nonpathogenic L. innocua. Functional analysis showed that Hpt allows L. monocytogenes to utilize phosphorylated sugars such as glucose-1-phosphate within the host cytosol and thus contributes to the bacterial virulence within the mammalian host cell. Deletion of the hpt gene resulted in impaired listerial intracytosolic proliferation and attenuated virulence in mice. Thus Hpt is involved in the replicative phase of the intracellular parasitism of L. monocytogenes and is an example that adaptation to intracellular parasitism involves exploitation of physiological mechanisms of the eukaryotic host cell (Chico-Calero et al. 2002). Recently it was shown that this transporter can mediate the in vivo uptake of the antibiotic fosfomycin, thus leading to fosfomycin sensitivity of intracellular L. monocytogenes, despite the fact that L. monocytogenes is resistant to fosfomycin when tested in vitro with conventional methods (Scortti et al. 2006).

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