Listeria welshimeri

Classification

Higher order taxa Domain; Phylum; Class; Order; Family; Genus Bacteria; Firmicutes; Bacilli; Bacillales; Listeriaceae; Listeria

Species Listeria welshimeri

Description and significance Listeria welshimeri, is one of six species that belongs to the genus Listeria. It is motile, gram-positive, facultative anaerobic rod-like bacteria that is ubiquitous in nature (2). L. welshimeri can be found in decaying plants, soil, sewage, dust and water. L. welshimeri was first isolated from decaying plants. These bacteria are small ( 0.5 to 2.0µm), non-spore-forming and are motile below 30degrees C by means of peritrichous flagella that uniformly cover its body (2).There are two pathogenic strains of Listeria, including L. monocytogenes and L. ivanovii and there are four non pathogenic strains which include L. L. innocua, L. seeligeri, L. grayi and L. welshimeri (3). L. welshimeri is an important organism because of it is similar to L. monocytogenes, which is a food-borne pathogen.L. welshimeri was analyzed to compare its genome sequence to that of L. monocytogenes in order to learn more about this pathogen.

Genome sequence Listeria welshimeri has a circular chromosome that is made of 2,814,130 base pairs, giving it the smallest genome size of the genus Listeria. It has a low G+C content (36.4%) giving its DNA a less rigid structure. The origin of replication and terminus are located about 1,400 kb apart. Its genome contains six complete copies of rRNA operons (16S-22S-5S) (4). L. welshimeri contains plasmids pBcloBAC11, pCC1FOS, pCR4Blunt-TOPO and pUC19 which are cloning vectors for E. coli. There are 233 gene deletions in L. welshimeri when compared to L. monocytogenes and the majority of the deleted portions are in gene clusters. These deletions originate at the same loci in the chromosome. Two surface-associated proteins, leucine-rich repeat (LRR) and LPXTG motifs have been lost in L. welshimeri. These proteins are required for adhering and invading nonphagocytic cells (4).

Cell Structure & Metabolism: L. welshimeri is gram-positive, giving it a thick cell wall for protection from its environment. L. welshimeri is rod-like and becomes motile using peritrichous flagella (4). L. welshimeri genome has the largest number of genes encoding 69 lipoproteins in comparison to other Listeria species (5). L. welshimeri contains protein p60, used to adhere and invade 3T6 mouse fibroblasts but do not adhere to human epithelial Caco-2.

L. welshimeri can catalyze reactions, however it cannot oxidize them because it does not have the oxidase enzyme. It acquires energy by fermentation of sugar and ∂-methyl-D-mannoside (4). BilE system is present and helps protect bacteria within the gall bladder, a specific niche for the parasite to avoid host defense responses. L. welshimeri uses proteins and peptides as a source of amino acids using trypsin like serine/cysteine proteases. Thus this organism carries out biosynthesis pathways that include peptidoglycan synthesis (4).

Ecology: Listeria species can endure low and high temperatures and can survive in a wide range of pH thus it is ubiquitous in nature. All Listeria species have been isolated from soil, decaying vegetable matter, sewage, water, animal feed, fresh and processed meats, raw milk, cheese, slaughterhouse waste and asymptomatic human and animal carriers. Because it is so widespread, it has many opportunities to enter food production and processing environments. They can grow in food, even at temperatures of refrigeration. The coexistence of Listeria species on the same food is not unlikely and occurs more often than just one species at a time. Since all Listeria species are potential food contaminants, the detection of one on foodstuff often indicates the presence of the others. Though L. welshimeri is present in the environment and has been isolated from animal and human carriers, studies monitoring its ecology are still rare (6).

Pathology: Listeri welshimeri is a non pathogenic organism and does not have the virulence gene. Virulence determinants in the Listeria pathogenesis are localized on the chromosomal locus between the prs and ldh, which are designated as the virulence gene cluster “vgc”. This gene is responsible for the intracellular life cycle of the bacterium. However, all of these genes are missing in the L. welshimeri, making this species non-pathogenic. This also suggests that the L. welshimeri probably evolved from the loss of the vgc region, leading to the generation of pathogenic species from a progenitor strain already containing the virulence gene (2). L. welshimeri s non-hemolytic and even in high infecting doses (>1 x 10 8 CFU/ml) which is at least 100,000 fold higher than the 50% lethal dose of L. monocytogenes, that causes listeriosis (1x10 3 CFU/ml), does not kill mice (2).