Molecular characteristics of antimicrobial resistance and virulence factors of Salmonella enterica isolated from pig production and humans in NortheasternThailand and Thailand-Laos border area

Abstract

In this study, we aimed to characterize antimicrobial resistance and virulence factors in the Salmonella isolates from pig production and humans in Northeastern provinces of Thailand and Thailand-Laos border provinces. This study comprises three projects. Project 1 demonstrated high prevalence and molecular characteristics of multidrug resistant Salmonella in pigs, pork and humans in Thailand-Laos provinces. A total of 1,187 samples were collected from pigs, pig carcasses and workers in slaughterhouses; pork and butchers in retail markets and patients in hospitals in the provinces with cross border points including NongKhai, Ubon Ratchathani, Mukdahan of Thailand and Vientiane and Savannakhet in Laos during 2013-2014. Among these samples, 469 samples (39.5%) were positive to Salmonella enterica. The predominant serovars in Thailand provinces were S. Typhimurium (32.9%) and S. Rissen (20.3%). In Laos isolates, the predominant serovars were S. Anatum (24.1%) and S. Typhimurium (20.5%). Most isolates (98.2%) were multidrug resistance (MDR). Class 1 integrons carrying dfrA12-aadA2 gene cassette array were most commonly observed (19.2%). The presence of qnrB, qnrS and aac(6’)-Ib-cr was identified in 0.9%, 6.4%, 0.2% of the isolates, respectively. ESBL-producing Salmonella strains carrying blaCTX-M14 (2.4%) were identified in pig samples from Mukdahan province, Thailand. Project 2 described characterization of antimicrobial resistance in Salmonella enterica isolated from pork, chicken meat and humans in Northeastern Thailand. A total of 221 Salmonella isolates obtained from humans, raw pork and raw chicken in five Northeastern provinces of Thailand (Kalasin, Khon Kean, Loei, Nong Khai, Roi Et) during 2010-2011 were used in this project. The predominant serovars were S. Rissen (15.8%) and S. Anatum (14.9%). Most isolates (95.9%) were MDR. The dfrA12-aadA2 gene cassette array was commonly identified in the isolates from Northeastern region (40%). SGI-1 like gene cluster was observed in a serovar Anatum from pork. Single amino acid substitutions at position Ser 83 and Asp 87 in GyrA were most commonly observed in ciprofloxacin-resistant strains (1.8%). Of the plasmid-mediated quinolone resistance genes, only qnrB and qnrS genes were detected in 1.8% and 4% of all Salmonella isolates, respectively. The presence of spvC, pefA and rck was identified in 8.1%, 1.8% and 1.4% in the isolates, respectively. Project 3 described mutations in quinolone resistance-determining region in DNA gyrase and topoisomerase IV genes in nalidixic–acid resistant Salmonella enterica isolated from chicken meat, pork and humans. Twenty-eight nalidixic acid-resistant Salmonella isolates originated from chicken meat, pork and humans were examined for mutation in the quinolone determining region (QRDR) of gyrA, gyrB, parC and parE genes. Four single-point mutations in gyrA (i.e. C248A, C248T, G259T, G259A) leading to amino acid substitutions Ser83Tyr, Ser83Phe, Asp87Tyr and Asp87Asn in GyrA, respectively, were identified in 11 nalidixic acid-resistant strains. Amino acid change at position Ser83 was most frequently identified (28.5%). No mutations were observed in gyrB, parC and parE. Overall, our findings demonstrated the high prevalence of MDR Salmonella in humans, food-producing animals and their products in Thailand and Laos provinces. The circulation of their resistance determinants was highlighted in this study. The results indicate the need continuing antimicrobial resistance monitoring at nation, regional and global levels.