| Description: |
Lactobacillus sakei is a non-pathogenic psychrotrophic lactic acid bacterium found naturally on fresh meat and fish. It is the predominant organism observed in vacuum-packed fresh meat and is very often found in traditionally made dry sausage. Consequently, it is used as a starter in the industrial production of fermented meat products. It is also found occasionally on other substrates such as fish-based products or sauerkraut and it was recently described in the human gastrointestinal tract. It plays an important role in the preservation and microbial quality of meat products by inhibiting the development of other bacterial species. L.sakei (strain 23K) was originally isolated from a French sausage. Its genome consists of a single chromosome. One prophage remnant and 12 complete insertion sequences have been identified. It contains 7 rRNA clusters. It is auxotrophic for all amino acids, except aspartic and glutamic acids. Few transporters are present for sugar uptake. It is able of synthesizing purines and pyrimidines de novo. Interestingly, it contains numerous catabolic genes involved in exogenous nucleoside salvage pathways. Growth experiments have shown that it catabolizes inosine and adenosine for energy production. Nucleoside metabolism might improve survival on meat as glucose, the preferred carbon source for L.sakei, is rapidly exhausted in meat. The effective survival of L.sakei on meat products probably involves catabolism of arginine, which is abundant in meat. It does not possess genes coding for tyrosine and histidine catabolism. Therefore, unlike many meat-borne microorganisms, it does not produce tyramine and histamine, which is an interesting feature from a biopreservation point of view, as these substances are biogenic amines with deleterious effects on human health. L.sakei is able to grow on meat during refrigeration and in the presence of curing salts (3-9% NaCl). Its ability to efficiently accumulate osmo- and cryoprotective solutes may account for its acclimation to cold and salt. It has more putative cold stress proteins than other lactobacilli but as many as L.monocytogenes, another bacterium known for its resistance to cold and salt. It is well equipped to cope with changing oxygen conditions. Heme acquisition is important for the oxidative stress response of L.sakei as indicated by the presence of a heme-dependent catalase. It cannot synthesize heme de novo and probably take it up from meat. At least four putative iron-transport systems have been identified. Three iron-dependent regulators belonging to the Fur family have been observed that might be involved in resistance to oxidative stress or regulation of iron-transport proteins. L.sakei lacks homologs of MUB proteins for adherence to intestinal mucus. It may colonize the meat surface through aggregation and biofilm formation with a set of components having no homolog in other lactobacilli. It contains two gene clusters potentially involved in the production of surface polysaccharides, which could mediate attachment to the meat surface or intestinal mucosa. Unlike other L.sakei, strain 23K is not a bacteriocin producer and it seems to have an innate resistance towards bacteriocins produced by its closely related competitors, as suggested by the presence of genes coding for putative bacteriocin-like peptides with cognate immunity proteins. |
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