Abstract
The bacterium Aeromonas salmonicida subsp. salmonicida is a common pathogen in fish farms worldwide. Since the antibiotic resistance of this bacterial species is on the increase, it is important to have a broader view on this issue. In the present study, we tested the presence of known plasmids conferring multi-drug resistance as well as antibiotic resistance genes by a PCR approach in 100 Canadian A. salmonicida subsp. salmonicida isolates. Our study highlighted the dominance of the conjugative pSN254b plasmid, which confers multi-drug resistance. We also identified a new multi-drug plasmid named pAsa8, which has been characterized by a combination of sequencing technologies (Illumina and Oxford nanopore). This new plasmid harbors a complex class 1 integron similar to the one of the Salmonella genomic island 1 (SGI1) found in Salmonella enterica and Proteus mirabilis. Consequently, in addition to providing an update on the A. salmonicida subsp. salmonicida isolates that are resistant to antibiotics, our data suggest that this bacterium is potentially an important reservoir of drug resistance genes and should consequently be monitored more extensively. In addition, we describe a screening method that has the potential to become a diagnostic tool that is complementary to other methods currently in use.
Highlights
Antibiotic resistance genes in A. salmonicida subsp. salmonicida are mostly located on plasmids[4]
In order to determine the fraction of isolates bearing antibiotic-resistance genes and to infer the geographic distribution of the R-plasmids, we investigated a collection of 100 A. salmonicida subsp. salmonicida Canadian isolates, using, among other methods, optimized multiplex PCR assays
We subsequently developed four multiplex PCR assays to detect the antibiotic resistance genes known to be present in A. salmonicida subsp. salmonicida isolates: floR, cat, sul[1], sul[2], tetA, tet(C), tet(E), and tet(H)
Summary
Antibiotic resistance genes in A. salmonicida subsp. salmonicida are mostly located on plasmids[4]. Antibiotic resistance genes in A. salmonicida subsp. PSN254b and pAB5S9b plasmids, which bear multi-drug resistance genes, as well as variants of pRAS3 have been recently described in isolates from Canada[5]. There are currently no government-approved antibiotics that are effective against bacterial isolates that host either one of these two plasmids For their part, pRAS3 variants contain a gene coding for tetracycline resistance[5]. Salmonicida Canadian isolates, using, among other methods, optimized multiplex PCR assays This approach was done to assess the presence of new R-plasmids. This screening showed that pSN254b is the most common R-plasmid in tested Canadian isolates of A. salmonicida subsp. A new R-plasmid, pAsa[8], was found, which had genetic features that provided additional evidence that A. salmonicida subsp. salmonicida is an important reservoir for mobile genetic elements as well as antibiotic resistance genes
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