Abstract
The bacterium Morganella morganii can produce the biogenic amines (BA) cadaverine, putrescine, and histamine in vitro and is responsible for high histamine concentrations in fish products. These BA can have toxic effects upon ingestion and are undesired in food. The purpose of this study was to characterize the phenotype and genotype of 11 M. morganii isolated from cheese in regard to the BA formation. In addition, we investigated the phylogeny, trehalose fermentation ability, and antibiotic resistance of the cheese isolates. To do so, we sequenced their genomes using both long and short read technologies. Due to the presence of the trehalose operon and the ability to ferment trehalose, the cheese isolates can be assigned to the subsp. sibonii. Comparative genomics with public available M. morganii genomes shows that the genomes of the cheese isolates cluster together with other subsp. sibonii genomes. All genomes between subsp. morganii and subsp. sibonii are separated by an average nucleotide identity (ANI) of less than 95.0%. Therefore, the subspecies could represent two distinct species. Nine of the strains decarboxylated lysine yielding cadaverine in vitro. This metabolic activity is linked to a previously unknown gene cluster comprising genes encoding a lysine-tRNA ligase (lysS), an HTH-transcriptional regulator (argP), a cadaverine-lysine antiporter (cadB), and a lysine decarboxylase (cadA). The formation of putrescine is linked to the speF gene encoding an ornithine decarboxylase. The gene is disrupted in five strains by an insertion sequence, and these strains only exhibit a weak putrescine production. Antimicrobial susceptibility profiling revealed that all cheese strains are resistant to tetracycline, chloramphenicol, tigecycline, colistin, and ampicillin. These phenotypes, except for colistin which is intrinsic, could be linked to antimicrobial resistance genes located on the chromosome.
Highlights
Morganella morganii is a facultative anaerobic Gram-negative bacterium (Janda and Abbott, 2015)
During routine analyses of various cheeses for the presence of E. coli and other coliforms using chromogenic media, individual colonies were identified as M. morganii using the MALDI Biotyper
We sequenced the genomes of 11 cheese isolates and the type strain M. morganii subsp. morganii DSM 30164 using both short read and long read sequencing technologies
Summary
Morganella morganii (formerly Proteus morganii) is a facultative anaerobic Gram-negative bacterium (Janda and Abbott, 2015). Trehalose fermenting strains are designated as M. morganii subsp. M. morganii was found in fish products, where it produces the biogenic amine (BA) histamine (Kim et al, 2003; Kanki et al, 2007). Various strains can decarboxylate ornithine (ODC+) and/or lysine (LDC+) to form the BA putrescine and cadaverine, respectively (Janda et al, 1996). Because cadaverine and putrescine enhance the toxicity of histamine, they are undesired in food (Linares et al, 2011). De las Rivas et al (2007, 2008) cloned two genes encoding ornithine decarboxylases and showed that the gene products decarboxylate ornithine. The genetic elements responsible for cadaverine formation in M. morganii have not been described
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