Inter-phylum circulation of a beta-lactamase - encoding gene: a rare but observable event

Beta-lactam degradation by beta-lactamases is the most common mechanism of beta-lactam resistance in Gram-negative bacteria. Beta-lactamase encoding genes can be transferred between closely-related bacteria, but spontaneous inter-phylum transfers (between distantly related bacteria) has never been reported. Here, we describe an extended-spectrum beta-lactamase (ESBL)-encoding gene ( bla MUN-1) shared between the Peudomonadota and Bacteroidota phyla. An Escherichia coli strain was isolated from a patient in Münster (Germany). Its genome was sequenced (Illumina and Nanopore). The ESBL encoding gene was cloned and the corresponding enzyme was characterised. Distribution of the gene among bacteria was studied with BLASTN using RefSeq Genomes databases. Frequency of its closest homolog in the Global Microbial Gene Catalog (GMGC) was also analysed. The bla MUN-1 gene found in the E. coli strain, encoded for an Ambler subclass A2 beta-lactamase with 82.2% amino acid identity to TLA-1 and it was found to confer an ESBL phenotype. bla MUN-1 was found in four copies, two chromosomal copies and two located on a phage-plasmid p0111. Each copy was found on a 7.6kb genomic island associated with mobility. bla MUN-1 was found distributed among the Bacteroidales order and in Sutterella wardsworthensis (Pseudomonadota). Its closest homolog in the GMGC was found predominantly and frequently in the Human gut sub-catalog (found in 26.8% of the samples). This is the first reported case of inter-phylum transfer of an ESBL-encoding gene, between the Bacteroidota and Pseudomonadota phyla. While the gene was frequently found in the human gut, inter-phylum transfer was rare, suggesting that inter-phylum barriers are strong but not impassable. ### Competing Interest Statement The authors have declared no competing interest.