Plasmids that carry antibiotic resistance genes occur frequently in Aeromonas salmonicida subsp. salmonicida, an aquatic pathogen with severe consequences in salmonid farming. Here, we describe a 67 kb plasmid found in the A. salmonicida subsp. salmonicida Strain SHY15-2939 from Quebec, Canada. This new plasmid, named pAsa-2939 and identified by high throughput sequencing, displays features never found before in this bacterial species. It contains a transposon related to the Tn21 family, but with an unusual organization. This transposon bears a catB3 gene (chloramphenicol resistance) that has not been detected yet in A. salmonicida subsp. salmonicida. The plasmid is transferable by conjugation into Aeromonas hydrophila, but not into Escherichia coli. Based on PCR analysis and genomic sequencing (Illumina and PacBio), we determined that the transposon is unstable in A. salmonicida subsp. salmonicida Strain SHY15-2939, but it is stable in A. hydrophila trans-conjugants, which explains the chloramphenicol resistance variability observed in SHY15-2939. These results suggest that this bacterium is likely not the most appropriate host for this plasmid. The presence of pAsa-2939 in A. salmonicida subsp. salmonicida also strengthens the reservoir role of this bacterium for antibiotic resistance genes, even those that resist antibiotics not used in aquaculture in Québec, such as chloramphenicol.

Aeromonas salmonicida subsp. salmonicida is a pathogenic bacterium responsible for furunculosis in salmonids. Following an outbreak of furunculosis, the infection can be treated with antibiotics, but it is common to observe ineffective treatment due to antibiotic resistance. This bacterium has a wide variety of plasmids responsible for this resistance. Among them, pRAS3 carries a tetracycline resistance gene. Several variants of this plasmid have been discovered over the years (pRAS3-3432 and pRAS3.1 to 3.4). During the present study, two new variants of the plasmid pRAS3 were identified (pRAS3.5 and pRAS3-3759) in strains of A. salmonicida subsp. salmonicida. Plasmid pRAS3-3759, which has been found in many strains from the same region over the past three years, has an additional genetic element identical to one found in pRAS3-3432. This genetic element was also found in Chlamydia suis, a swine pathogen. In this study, we analyzed the bacteria’s resistance to tetracycline, the number of copies of the plasmids, and the growth of the strains that carry five of the pRAS3 variants (pRAS3.3 to 3.5, pRAS3-3432, and pRAS3-3759). The results show no particular trend despite the differences between the plasmids, except for the resistance to tetracycline when analyzed in an isogenic background. Blast analysis also revealed the presence of pRAS3 plasmids in other bacterial species, which suggests that this plasmid family has widely spread. This study once again highlights the ability of A. salmonicida subsp. salmonicida to adapt to furunculosis antibiotic treatments, and the still-growing family of pRAS3 plasmids.

Avian colibacillosis is an infectious disease that causes significant economic losses worldwide in the poultry industry. The bacterium avian pathogenic Escherichia coli (APEC) is the etiological agent of the disease and is primarily treated with antibiotics. Unfortunately, due to antibiotic resistance in APEC, therapeutic options for treating this disease are becoming increasingly limited. Bacteriophages—viruses that infect bacteria—represent promising therapeutic candidates; however, understanding how these viruses adapt is essential before considering them for commercial applications. This study investigated the adaptation of an APEC-infecting phage to a laboratory strain E. coli K-12 host. Successive passages of the phage on the K-12 strain resulted in clear adaptation to this new host, with only a slight fitness cost on APEC. The adapted phages also exhibited smaller capsids. Genomic comparison between the adapted phages and the wild-type phage revealed three mutations that may explain the adaptation to the K-12 strain: two point mutations located in a gene encoding a hypothetical protein and a 42-bp in-frame deletion in a homologue of the gene encoding the tail spike protein TSP4. This study, in addition to identifying molecular determinants involved in the adaptation of an APEC-infecting phage, further highlights the remarkable capacity of viruses to evolve.

Background/Objectives: The bacterium Brochothrix campestris is closely related to Brochothrix thermosphacta, a known food spoilage agent, and Listeria monocytogenes, the causative agent of listeriosis. B. campestris garnered attention several years ago because it produces brochocin-C, a bacteriocin capable of inhibiting the growth of certain pathogens. It has been recently suggested that phages play a significant role in the evolution of B. thermosphacta, similar to the role they play for L. monocytogenes. However, understanding the role of phages in the evolution of B. campestris has been challenging because only a draft of genome sequences of a single B. campestris strain was previously available. Methods: In this study, DNA from the B. campestris type strain DSM 4712 was sequenced using Oxford Nanopore and Illumina technologies to obtain complete, high-quality genome sequences. Results: The assembly revealed the presence of a plasmid and a phage-plasmid. Additionally, chromosomal analysis identified several genomic islands, including one harboring the brochocin genes, suggesting that these genes may have undergone horizontal transfer. Conclusions: This study underscores the potential importance of phages in the evolution of B. campestris, and also highlights the need for further research into the interactions between Brochothrix species and their associated phages to better understand these complex biological relationships.