Abstract
Diverse mobile genetic elements (MGEs) including plasmids, insertion sequences, and integrons play an important role in the occurrence and spread of multidrug resistance (MDR) in bacteria. It was found in previous studies that IS26 and class 1 integrons integrated on plasmids to speed the dissemination of antibiotic-resistance genes in Salmonella. It is aimed to figure out the patterns of specific genetic arrangements between IS26 and class 1 integrons located in plasmids in MDR Salmonella in this study. A total of 74 plasmid-harboring Salmonella isolates were screened for the presence of IS26 by PCR amplification, and 39 were IS26-positive. Among them, 37 isolates were resistant to at least one antibiotic. The thirty-seven antibiotic-resistant isolates were further involved in PCR detection of class 1 integrons and variable regions, and all were positive for class 1 integrons. Six IS26-class 1 integron arrangements with IS26 inserted into the upstream or downstream of class 1 integrons were characterized. Eight combinations of these IS26-class 1 integron arrangements were identified among 31 antibiotic-resistant isolates. Multidrug-resistance plasmids of the IncHI2 incompatibility group were dominant, which all belonged to ST3 by plasmid double locus sequence typing. These 21 IncHI2-positive isolates harbored six complex IS26-class 1 integron arrangement patterns. Conjugation assays and Southern blot hybridizations confirmed that conjugative multidrug-resistance IncHI2 plasmids harbored the different complex IS26-class 1 integron arrangements. The conjugation frequency of IncHI2 plasmids transferring alone was 10−5-10−6, reflecting that different complex IS26-class 1 integron arrangement patterns didn't significantly affect conjugation frequency (P > 0.05). These data suggested that class 1 integrons represent the hot spot for IS26 insertion, forming diverse MDR loci. And ST3-IncHI2 was the major plasmid lineage contributing to the horizontal transfer of composite IS26-class 1 integron MDR elements in Salmonella.
Highlights
Salmonella is recognized worldwide as a predominant pathogen causing foodborne diseases in humans (Yang et al, 2016)
Multidrug resistance (MDR) among Salmonella toward numerous first-line agents, especially fluoroquinolones and extended-spectrum cephalosporins (ESCs) that are recommended as primary treatment choices for severe infections, may jeopardize therapy options and reduce the effectiveness of invasive Salmonellosis treatment (Folster et al, 2015; Tadesse et al, 2016)
The recruitment, dissemination and rapid evolution of diverse antibiotic resistance in bacteria has been largely manipulated by mobile genetic elements (MGEs) such as plasmids, insertion sequences (ISs), transposons (Tns) and integrons via horizontal gene transfer (HGT) (Brown-Jaque et al, 2015)
Summary
Salmonella is recognized worldwide as a predominant pathogen causing foodborne diseases in humans (Yang et al, 2016). The recruitment, dissemination and rapid evolution of diverse antibiotic resistance in bacteria has been largely manipulated by mobile genetic elements (MGEs) such as plasmids, insertion sequences (ISs), transposons (Tns) and integrons via horizontal gene transfer (HGT) (Brown-Jaque et al, 2015). Integrons are DNA elements capable of capturing and mobilizing exogenously functional gene cassettes, potentially permitting rapid adaptation to selective pressure and endowing increased fitness to the host (Deng et al, 2015). The class 1 integron is the most prevalent type associated with MDR Salmonella, playing a critical role in the dissemination of antibiotic resistance among various bacterial species (Li R. et al, 2013; Abraham et al, 2014). There is very little research directly targeting on the correlation between IS26 and the class 1 integron in Salmonella, providing little highlights to trace IS26-class 1 integron-mediated MDR transmission and the evolution of MDR Salmonella under antibiotic selective pressure
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