Abstract
We aimed to analyze the antimicrobial resistance phenotypes and to elucidate the molecular mechanisms underlying resistance to cephalosporins, ciprofloxacin, and azithromycin in Salmonella enterica serovar Typhimurium isolates identified from patients with diarrhea in Shanghai. The isolates showed high rates of resistance to traditional antimicrobials, and 20.6, 12.7, and 5.5% of them exhibited decreased susceptibility to cephalosporins, ciprofloxacin, and azithromycin, respectively. Notably, 473 (84.6%) isolates exhibited multidrug resistance (MDR), including 161 (28.8%) isolates that showed an ACSSuT profile. Twenty-two MDR isolates concurrently exhibited decreased susceptibility to cephalosporins and ciprofloxacin, and six of them were co-resistant to azithromycin. Of all the 71 isolates with decreased susceptibility to ciprofloxacin, 65 showed at least one mutation (D87Y, D87N, or D87G) in gyrA, among which seven isolates simultaneously had mutations of parC (S80R) (n = 6) or parC (T57S/S80R) (n = 1), while 49 isolates with either zero or one mutation in gyrA contained plasmid-mediated quinolone resistance (PMQR) genes including qnrB, qnrS, and aac(6′)-Ib-cr. Among the 115 cephalosporin-resistant isolates, the most common ESBL gene was blaCTX-M, followed by blaTEM-1, blaOXA-1, and blaSHV -12. Eight subtypes of blaCTX-M were identified and blaCTX-M-14 (n = 22) and blaCTX-M-55 (n = 31) were found to be dominant. To the best of our knowledge, this is the first report of the presence of blaCTX-M-123 and blaCTX-M-125 in S. Typhimurium. Besides, mphA gene was identified in 15 of the 31 azithromycin-resistant isolates. Among the 22 isolates with reduced susceptibility to cephalosporins and ciprofloxacin, 15 contained ESBL and PMQR genes. Coexistence of these genes lead to the emergence of MDR and the transmission of them will pose great difficulties in S. Typhimurium treatments. Therefore, surveillance for these MDR isolates should be enhanced.
Highlights
Salmonella infection is a major global public health problem, which has caused food-borne illnesses in many countries
Typhimurium isolates in Shanghai, China, exhibited high rates of resistance to traditional antimicrobials, such as tetracycline, ampicillin, nalidixic acid, and sulfisoxazole
We showed that the existence of plasmidmediated quinolone resistance (PMQR) genes remarkably enhances the minimum inhibitory concentrations (MICs) (≥2 μg/mL) of ciprofloxacin in the presence of a single or no point mutation in the gyrA gene in S
Summary
Salmonella infection is a major global public health problem, which has caused food-borne illnesses in many countries. Data from the Foodborne Diseases Active Surveillance Network (Foodnet) report showed that Salmonella has become a leading cause of death as a food-borne bacterial pathogens in the United States (Barton Behravesh et al, 2011). Typhimurium is one of the predominant serotypes in many developed and developing countries, and the global outbreak of food-borne diseases due to infection by S. S. Typhimurium infection was very common in the United States; for example, S. Typhimurium enteritis had been diagnosed in 600 persons in 44 states due to peanut butter contamination in 2008 (Maki, 2009). S. Typhimurium infection has been often reported in China and was found to be the second most prevalent serotype (Ran et al, 2011)
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