Fluoroquinolone resistance in Salmonella: a web discussion

Fluoroquinolone resistance in non-typhoidal Salmonella enterica isolated from slaughtered pigs in Thailand

Introduction. The emergence and spread of non-typhoidal Salmonella enterica (NTS) serovars resistant to fluoroquinolones and third- and higher-generation cephalosporins is a matter of great concern. Antimicrobial-resistant NTS is increasingly being discovered in humans, animals, food animals, food products, and agricultural environments. Pigs are considered a major reservoir of antimicrobial-resistant Salmonella spp.Hypothesis/Gap Statement. Fluoroquinolone-resistant Salmonella spp. warrant further surveillance and characterization for a better understanding of the bacteria isolated from animals.Aim. NTS isolated from pork from slaughterhouses across Thailand were characterized in terms of their serovars; resistance to fluoroquinolones, third-generation cephalosporins, and carbapenems; and antimicrobial resistance genes.Methodology. A total of 387 NTS isolates, collected from slaughtered pigs in ten provinces across Thailand between 2014 and 2015, were characterized based on their serovars, antimicrobial resistance genes, and susceptibility to fluoroquinolones, third-generation cephalosporins, and carbapenems.Results. Among all NTS isolates, S. enterica serovar Rissen was predominant. Antimicrobial resistance was exhibited in 93/387 isolates (24 %). Although 24 (6.2 %) isolates were susceptible to all the tested antimicrobials, they were found to possess β-lactamase genes, such as bla _TEM, bla _SHV, or bla _CTX-M. Mobilized colistin-resistant genes (mcr) and resistance to colistin were not observed in any tested isolate. Carbapenem resistance was detected in ten isolates (10.7 %); however, bla _KPC, bla _NDM, bla _OXA-48-like, and bla _IMP were not present. Among the 93 antimicrobial-resistant isolates, 87.1 % showed fluoroquinolone resistance with the quinolone resistance gene (qnrS) combined with topoisomerase genes parC (T57S) or gyrA (S83E/Y and D124E/G) substitutions, or topoisomerase gene substitutions alone.Conclusion. We found high fluoroquinolone resistance rates among the NTS isolates from pigs from slaughterhouses. The fluoroquinolone resistance mechanism in NTS was associated with the combination of qnrS and substitutions in gyrA, parC, or both. To prevent the transmission of antimicrobial-resistant NTS between animals and humans, continuous monitoring, surveillance, and regulation of Salmonella in the pork supply chain are pivotal.

Detection of Salmonella spp. in retail raw food samples from Vietnam and characterization of their antibiotic resistance

A study was conducted to examine the levels of Salmonella spp. contamination in raw food samples, including chicken, beef, pork, and shellfish, from Vietnam and to determine their antibiotic resistance characteristics. A total of 180 samples were collected and examined for the presence of Salmonella spp., yielding 91 Salmonella isolates. Sixty-one percent of meat and 18% of shellfish samples were contaminated with Salmonella spp. Susceptibility of all isolates to a variety of antimicrobial agents was tested, and resistance to tetracycline, ampicillin/amoxicillin, nalidixic acid, sulfafurazole, and streptomycin was found in 40.7%, 22.0%, 18.7%, 16.5%, and 14.3% of the isolates, respectively. Resistance to enrofloxacin, trimethoprim, chloramphenicol, kanamycin, and gentamicin was also detected (8.8 to 2.2%). About half (50.5%) of the isolates were resistant to at least one antibiotic, and multiresistant Salmonella isolates, resistant to at least three different classes of antibiotics, were isolated from all food types. One isolate from chicken (serovar Albany) contained a variant of the Salmonella genomic island 1 antibiotic resistance gene cluster. The results show that antibiotic resistance in Salmonella spp. in raw food samples from Vietnam is significant.

AcrAB-TolC directs efflux-mediated multidrug resistance in Salmonella enterica serovar typhimurium DT104

Multidrug-resistant Salmonella enterica serovar Typhimurium definitive phage type 104 (DT104) strains harbor a genomic island, called Salmonella genomic island 1 (SGI1), which contains an antibiotic resistance gene cluster conferring resistance to ampicillin, chloramphenicol, florfenicol, streptomycin, sulfonamides, and tetracyclines. They may be additionally resistant to quinolones. Among the antibiotic resistance genes there are two, i.e., floR and tet(G), which code for efflux pumps of the major facilitator superfamily with 12 transmembrane segments that confer resistance to chloramphenicol-florfenicol and the tetracyclines, respectively. In the present study we determined, by constructing acrB and tolC mutants, the role of the AcrAB-TolC multidrug efflux system in the multidrug resistance of several DT104 strains displaying additional quinolone resistance or not displaying quinolone resistance. This study shows that the quinolone resistance and the decreased fluoroquinolone susceptibilities of the strains are highly dependent on the AcrAB-TolC efflux system and that single mutations in the quinolone resistance-determining region of gyrA are of little relevance in mediating this resistance. Overproduction of the AcrAB efflux pump, as determined by Western blotting with an anti-AcrA polyclonal antibody, appeared to be the major mechanism of resistance to quinolones. Moreover, chloramphenicol-florfenicol and tetracycline resistance also appeared to be highly dependent on the presence of AcrAB-TolC, since the introduction of mutations in the respective acrB and tolC genes resulted in a susceptible or intermediate resistance phenotype, according to clinical MIC breakpoints, despite the presence of the FloR and Tet(G) efflux pumps. Resistance to other antibiotics, ampicillin, streptomycin, and sulfonamides, was not affected in the acrB and tolC mutants of DT104 strains harboring SGI1. Therefore, AcrAB-TolC appears to direct efflux-mediated resistance to quinolones, chloramphenicol-florfenicol, and tetracyclines in multidrug-resistant S. enterica serovar Typhimurium DT104 strains.

Ciprofloxacin-resistant Salmonella enterica Typhimurium and Choleraesuis from pigs to humans, Taiwan

We evaluated the disk susceptibility data of 671 nontyphoid Salmonella isolates collected from different parts of Taiwan from March 2001 to August 2001 and 1,261 nontyphoid Salmonella isolates from the National Taiwan University Hospital from 1996 to 2001. Overall, ciprofloxacin resistance was found in 2.7% (18/671) of all nontyphoid Salmonella isolates, in 1.4% (5/347) of Salmonella enterica serotype Typhimurium and in 7.5% (8/107) in S. enterica serotype Choleraesuis nationwide. MICs of six newer fluoroquinolones were determined for the following isolates: 37 isolates of ciprofloxacin-resistant (human) S. Typhimurium (N = 26) and Choleraesuis (N = 11), 10 isolates of ciprofloxacin-susceptible (MIC <1 mg/mL) (human) isolates of these two serotypes, and 15 swine isolates from S. Choleraesuis (N = 13) and Typhmurium (N = 2) with reduced susceptibility to ciprofloxacin (MIC >0.12 microg/mL). Sequence analysis of the gryA, gyrB, parC, parE, and acrR genes, ciprofloxacin accumulation, and genotypes generated by pulsed-field gel electrophoresis with three restriction enzymes (SpeI, XbaI, and BlnI) were performed. All 26 S. Typhimurium isolates from humans and pigs belonged to genotype I. For S. Choleraesuis isolates, 91% (10/11) of human isolates and 54% (7/13) of swine isolates belonged to genotype B. These two genotypes isolates from humans all exhibited a high-level of resistance to ciprofloxacin (MIC 16-64 mg/mL). They had two-base substitutions in the gyrA gene at codons 83 (Ser83Phe) and 87 (Asp87Gly or Asp87Asn) and in the parC gene at codon 80 (Ser80Arg, Ser80Ile, or Ser84Lys). Our investigation documented that not only did these two S. enterica isolates have a high prevalence of ciprofloxacin resistance nationwide but also that some closely related ciprofloxacin-resistant strains are disseminated from pigs to humans.

Fluoroquinolone resistance in Salmonella serovars isolated from humans and food animals

Quinolone-resistant Salmonella enterica usually contain a mutation in gyrA within the region encoding the quinolone resistance determining region of the A subunit of DNA gyrase. These mutations confer substitutions analogous to Escherichia coli Ser83-->Phe and Asp87-->Gly or Tyr, or a novel mutation resulting in Ala119-->Glu or Val. Mutations in gyrB are rare, and no mutations in parC or parE have been described. Quinolone-resistant Salmonella can also be cross-resistant to other agents including chloramphenicol and tetracycline. Increased efflux has been demonstrated and for some strains this has been associated with increased expression of acrB. Mutation in soxR has also been shown for one isolate. Detection of low level resistance (minimum inhibitory concentrations <0.5 microg ml(-1)) to fluoroquinolones is proving an increasing problem in the treatment of invasive Salmonella infections.