Twenty-nine hippuricase-positive Campylobacter strains were isolated from wild birds and river water. Previous characterization using atpA typing indicated that these strains were related to Campylobacter jejuni and Campylobacter coli but were most similar to three recently described hippuricase-positive Campylobacter species recovered from zebra finches, i.e. C. aviculae, C. estrildidarum and C. taeniopygiae. Phylogenetic analyses using 330 core genes placed the 29 strains into a clade well separated from the other Campylobacter taxa, indicating that these strains represent a novel Campylobacter species. Pairwise digital DNA–DNA hybridization and average nucleotide identity values were below 70 and 95 %, respectively, thus providing further supporting evidence of a novel taxon. Standard phenotypic testing was performed. All strains are microaerobic or anaerobic, motile, Gram-negative, spiral cells that are oxidase, catalase and nitrate reductase positive, but urease negative. Genomic analyses indicate that the 29 strains can potentially synthesize very few amino acids de novo and are auxotrophic for many amino acids and cofactors, similar to the species composing the Campylobacter lari group. In addition, these strains encode complete Entner–Doudoroff and Leloir pathways, suggesting that they may possess the ability to utilize both glucose and galactose; these pathways were also identified in the genomes of the zebra finch-associated taxa. The data presented here show that these strains represent a novel species within Campylobacter, for which the name Campylobacter molothri sp. nov. (type strain RM10537^T=LMG 32306^T=CCUG 75331^T) is proposed.

Campylobacter jejuni and Campylobacter coli are well known for their natural competence, i.e., their capacity for the uptake of naked DNA with subsequent transformation. This study identifies non-transformable C. jejuni and C. coli strains from domestic animals and employs genomic analysis to investigate the strain genotypes and their associated genetic mechanisms. The results reveal genetic associations leading to a non-transformable state, including functional DNase genes from bacteriophages and mutations within the cts-encoded DNA-uptake system, which impact the initial steps of the DNA uptake during natural transformation. Interestingly, all 38 tested C. jejuni ST-50 strains from the United States exhibit a high prevalence of non-transformability, and the strains harbor a variety of these genetic markers. This research emphasizes the role of these genetic markers in hindering the transfer of antimicrobial resistance (AMR) determinants, providing valuable insights into the genetic diversity of Campylobacter. As ST-50 is a major clone of C. jejuni globally, we additionally determined the prevalence of the genetic markers for non-transformability among C. jejuni ST-50 from different regions of the world, revealing distinct patterns of evolution and a strong selective pressure on the loss of competence in ST-50 strains, particularly in the agricultural environment in the United States. Our findings contribute to a comprehensive understanding of genetic exchange mechanisms within Campylobacter strains, and their implications for antimicrobial resistance dissemination and evolutionary pathways within specific lineages.

Nine Campylobacter strains were isolated from cattle and feral swine faeces: three were recovered during a 2007 Campylobacter-associated outbreak linked to a dairy, and the other six were isolated during a 2009–2010 survey of farms and ranches in Central California. The species identification of these strains could not be determined by 16S rRNA gene sequencing but were most similar to Campylobacter concisus and Campylobacter mucosalis. Additional atpA typing indicated that the nine strains composed a discrete novel clade related to C. concisus and C. mucosalis. A polyphasic study was undertaken here to clarify their taxonomic position. Phylogenetic analyses were performed based on 16S rRNA gene sequences and the concatenated sequences of 330 core genes. The core gene analysis placed the nine strains into a clade well separated from the other Campylobacter taxa, indicating that these strains represent a novel Campylobacter species. Pairwise digital DNA–DNA hybridization and average nucleotide identity values between these strains and other campylobacters are lower than 16 and 73%, respectively, further supporting their placement into a novel taxon. Standard phenotypic testing was performed. All strains are microaerobic or anaerobic, motile, Gram-negative, slightly-curved rods that are oxidase positive but catalase negative. Strains can be distinguished from the other catalase-negative Campylobacter species using phenotypic markers such as motility, oxidase activity, cephalothin resistance, hippuricase activity, growth at 30 °C, and α-haemolysis. The data presented here show that these strains represent a novel species within Campylobacter, for which the name Campylobacter californiensis sp. nov. (type strain RM6914^T=LMG 32304^T=CCUG 75329^T) is proposed.

Six urease-negative Campylobacter strains were isolated from cattle faeces over a 19-month period from 2009 to 2010. These strains were initially identified as Campylobacter sputorum by 16S rRNA gene and atpA typing. Initial studies characterizing these strains by multilocus sequence typing and genome sequencing further supported their classification as C. sputorum but indicated that these strains form a divergent clade within the species. A polyphasic study was undertaken here to clarify their taxonomic position. Phylogenetic analyses were performed based on 16S rRNA gene sequences and the concatenated sequences of 330 core genes, with the latter analysis also placing the six strains into a clade distinct from the three C. sputorum biovars. Pairwise digital DNA–DNA hybridization values identified these strains as C. sputorum, and the pairwise average nucleotide identity values were consistent with those observed between current Campylobacter subspecies pairs. Standard phenotypic testing was also performed. All strains are microaerobic, anaerobic, motile, Gram-negative and oxidase- and catalase-positive; cells are curved rods or spirals. Strains can be distinguished from the C. sputorum biovars by the presence of alkaline phosphatase activity and triphenyltetrazolium chloride reduction and absence of nitrate reduction. The data presented here show that these strains represent a novel subspecies within C. sputorum, for which the name C. sputorum subsp. bovis subsp. nov. (type strain RM8705^T=LMG 32300^T=CCUG 75470^T) is proposed.