Abstract
Leuconostoc carnosum is a known colonizer of meat-related food matrices. It reaches remarkably high loads during the shelf life in packaged meat products and plays a role in spoilage, although preservative effects have been proposed for some strains. In this study, the draft genomes of 17 strains of L. carnosum (i.e., all the strains that have been sequenced so far) were compared to decipher their metabolic and functional potential and to determine their role in food transformations. Genome comparison and pathway reconstruction indicated that L. carnosum is a compact group of closely related heterofermentative bacteria sharing most of the metabolic features. Adaptation to a nitrogen-rich environment, such as meat, is evidenced by 23 peptidase genes identified in the core genome and by the autotrophy for nitrogen compounds including several amino acids, vitamins, and cofactors. Genes encoding the decarboxylases yielding biogenic amines were not present. All the strains harbored 1–4 of 32 different plasmids, bearing functions associated to proteins hydrolysis, transport of amino acids and oligopeptides, exopolysaccharides, and various resistances (e.g., to environmental stresses, bacteriophages, and heavy metals). Functions associated to bacteriocin synthesis, secretion, and immunity were also found in plasmids. While genes for lactococcin were found in most plasmids, only three harbored the genes for leucocin B, a class IIa antilisterial bacteriocin. Determinants of antibiotic resistances were absent in both plasmids and chromosomes.
Highlights
Leuconostoc is a genus of heterofermentative Lactobacillaceae (Zheng et al, 2020)
The charge of contaminant bacteria, often dominated by L. carnosum, tends to increase and peaks up to 108 cells/g in some foods (Raimondi et al, 2018, 2019). These remarkably high levels of L. carnosum contamination were observed in modified atmosphere packaged (MAP) cooked ham, where it prevailed over the microbiota well before the expiry date (Björkroth et al, 1998; Vasilopoulos et al, 2010; Raimondi et al, 2019); in other meat products, such as sausages, vacuum-packaged smoked bacon, and sliced cooked poultry (Geeraerts et al, 2018; Li et al, 2019); and in processed vegetables (Hong et al, 2014; Jung et al, 2014)
Genome comparison indicated that the 17 strains of L. carnosum are a compact group of bacteria, with values of sequence similarity much higher than the threshold required for species demarcation, albeit their pangenome remained open
Summary
Leuconostoc is a genus of heterofermentative Lactobacillaceae (Zheng et al, 2020). The general traits of Leuconostoc species include facultative anaerobiosis, intrinsic vancomycin resistance, catalase negativity, ovococcoid morphology, and dextran production. The charge of contaminant bacteria, often dominated by L. carnosum, tends to increase and peaks up to 108 cells/g in some foods (Raimondi et al, 2018, 2019) These remarkably high levels of L. carnosum contamination were observed in modified atmosphere packaged (MAP) cooked ham, where it prevailed over the microbiota well before the expiry date (Björkroth et al, 1998; Vasilopoulos et al, 2010; Raimondi et al, 2019); in other meat products, such as sausages, vacuum-packaged smoked bacon, and sliced cooked poultry (Geeraerts et al, 2018; Li et al, 2019); and in processed vegetables (Hong et al, 2014; Jung et al, 2014). Any potential impact of L. carnosum on human health has never been assessed
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