Selection and characterization of fluoroquinolone-resistant mutants of Campylobacter jejuni using enrofloxacin

Fluoroquinolone-resistant Campylobacter isolates from conventional and antibiotic-free chicken products

The use of fluoroquinolones (FQs) in poultry production is an important issue in public health today. In February 2002, two prominent U.S. poultry companies pledged to stop using FQs for flock-wide treatment. One year later, we began a survey of Campylobacter isolates on chicken products from these two companies and from two producers claiming total abstention from antibiotic use. Using both standard isolation methods and new methods modified to enhance detection of FQ-resistant Campylobacter, we compared rates of FQ-resistant Campylobacter among these products. Four major findings were drawn from this study: a) antibiotic-free brands were not more likely to be contaminated with Campylobacter; b) a high percentage of products from the two conventional brands were contaminated with FQ-resistant Campylobacter (43 and 96%); c) these conventional brands had significantly higher odds of carrying resistant strains compared with antibiotic-free products; and d) supplementing media with FQs increased the sensitivity of detecting FQ-resistant strains among mixed populations of Campylobacter, thus reducing a bias toward underestimating the prevalence of FQ-resistant Campylobacter on samples. These results suggest that FQ resistance may persist in the commercial poultry environment in the absence of FQ-selective pressure and that these strains contaminate a larger proportion of foods than reported previously.

Incidence and mechanism of ciprofloxacin resistance in Campylobacter spp. isolated from commercial poultry flocks in the United Kingdom before, during, and after fluoroquinolone treatment

Five commercial broiler flocks were treated with a fluoroquinolone for a clinically relevant infection. Fresh feces from individual chickens and environmental samples were cultured for campylobacters before, during, and weekly posttreatment until slaughter. Both Campylobacter jejuni and C. coli were isolated during all treatment phases. An increased proportion of quinolone-resistant strains was seen during treatment, and these strains persisted posttreatment. One quinolone-resistant isolate of each species, each serotype, and each phage type from each sample at all treatment phases was examined for its phenotype and mechanism of resistance. Two resistant phenotypes were isolated: Nal(r) Cip(r) and Nal(r) Cip(s). The majority (269 of 290) of fluoroquinolone-resistant isolates, whether they were C. jejuni or C. coli, had a mutation in gyrA that resulted in the substitution Thr-86-->Ile. The other gyrA mutations detected were Thr-86-->Ala (n = 17) and Asp-90-->Asn (n = 10). The genotypic variation, based on the silent mutations in gyrA identified by the denaturing high-performance liquid chromatography pattern and DNA sequencing, was used to supplement typing data and provided evidence for both the spread of preexisting resistant strains and the selection of spontaneous resistant mutants in treated flocks. Multidrug resistance was significantly (P < 0.01) associated with resistance to ciprofloxacin. Twenty-five percent (73 of 290) of ciprofloxacin-resistant isolates but only 13% (24 of 179) of susceptible isolates were resistant to three or more unrelated antimicrobial agents. In conclusion, quinolone-resistant campylobacters were isolated from commercial chicken flocks in high numbers following therapy with a veterinary fluoroquinolone. Most ciprofloxacin-resistant isolates had the GyrA substitution Thr-86-->Ile. Resistant isolates were isolated from the feces of some flocks up to the point of slaughter, which may have consequences for public health.

Enhanced in vivo fitness of fluoroquinolone-resistant Campylobacter jejuni in the absence of antibiotic selection pressure

Campylobacter jejuni, a major foodborne human pathogen, has become increasingly resistant to fluoroquinolone (FQ) antimicrobials. By using clonally related isolates and genetically defined mutants, we determined the fitness of FQ-resistant Campylobacter in chicken (a natural host and a major reservoir for C. jejuni) in the absence of antibiotic selection pressure. When monoinoculated into the host, FQ-resistant and FQ-susceptible Campylobacter displayed similar levels of colonization and persistence in the absence of FQ antimicrobials. The prolonged colonization in chickens did not result in loss of the FQ resistance and the resistance-conferring point mutation (C257 --> T) in the gyrA gene. Strikingly, when coinoculated into chickens, the FQ-resistant Campylobacter isolates outcompeted the majority of the FQ-susceptible strains, indicating that the resistant Campylobacter was biologically fit in the chicken host. The fitness advantage was not due to compensatory mutations in the genes targeted by FQ and was linked directly to the single point mutation in gyrA, which confers on Campylobacter a high-level resistance to FQ antimicrobials. In certain genetic backgrounds, the same point mutation entailed a biological cost on Campylobacter, as evidenced by its inability to compete with the FQ-susceptible Campylobacter. These findings provide a previously undescribed demonstration of the profound effect of a resistance-conferring point mutation in gyrA on the fitness of a major foodborne pathogen in its natural host and suggest that the rapid emergence of FQ-resistant Campylobacter on a worldwide scale may be attributable partly to the enhanced fitness of the FQ-resistant isolates.

Relative contribution of target gene mutation and efflux to fluoroquinolone and erythromycin resistance, in French poultry and pig isolates of Campylobacter coli

Thirty-eight avian and swine French isolates of Campylobacter coli were studied for their mechanisms of co-resistance to fluoroquinolones and erythromycin. A Thr86Ile modification of GyrA, responsible for fluoroquinolone resistance, was found in all the strains. Two different levels of resistance to erythromycin (MIC of 8-16 or >/=256 mg/l) were observed. A A2075G mutation in the 23S rRNA genes was found only in the highly-resistant strains. Phe-Arg-beta-naphthylamide, an efflux pump inhibitor, potentiated erythromycin in all the strains examined but restored susceptibility only in the strains with a low-level of resistance. This suggests the involvement of efflux in intrinsic and in acquired low-level of resistance to erythromycin in C. coli.

Prevalence and antimicrobial resistance of Campylobacter coli isolated from fattening pigs in France

Campylobacter are a leading cause of human diarrhea. The usual source of infection is contaminated food, particularly poultry but pork has also been described. The veterinary use of antimicrobial drugs has been suggested to be largely responsible for resistance in human isolates of this zoonotic pathogen. A study was carried out to investigate the occurrence and antimicrobial resistance of Campylobacter isolated from French fattening pigs. From March 1998 to June 1999, stomach samples were collected at slaughter from 240 fattening pigs originating from 24 different farms. Half of the pigs were found to be positive for Campylobacter but considerable variation was observed between farms. Isolates all belong to the Campylobacter coli species. Susceptibilities of the strains were determined for five antimicrobial drugs using agar dilution. Resistance to tetracycline and erythromycin was high (79 and 55%, respectively). For nalidixic acid, enrofloxacin and ampicillin, resistance was observed in 34, 15 and 20% of the isolates, respectively. More than one-third of the strains was resistant to at least three antimicrobial drugs. A Thr86Ile modification in GyrA was observed in the enrofloxacin-resistant strains studied. The multiresistant strains analyzed expressed the multidrug transporter CmeB at a high level. Results indicated a high prevalence of C. coli in the stomach of the French pigs examined. In addition, a high proportion of the strains was resistant to antimicrobial drugs, particularly to tetracycline and erythromycin, or were multiresistant.

Analysis of gyrA mutations in quinolone-resistant and -susceptible Campylobacter jejuni isolates from retail poultry and human clinical isolates by non-radioactive single-strand conformation polymorphism analysis and DNA sequencing

AIMS

The aims of this study were to characterize the molecular variations in the quinolone resistance-determining region (QRDR) of gyrA among quinolone-resistant and -susceptible Campylobacter jejuni isolates originating from foods of animal origin and human infections and to evaluate the suitability of the single-strand conformation polymorphism (SSCP) method as a screening method for molecular characterization of fluoroquinolone resistance.

METHODS AND RESULTS

Alterations in QRDR of gyrA from 182 C. jejuni isolates were determined by nonradioisotopic SSCP analysis and direct sequencing. A total of 13 types of nucleic acid sequence combinations within the QRDR of the gyrA gene resulted in 11 different SSCP patterns. All nalidixic acid resistant strains possessed nucleotide substitution at either codon Thr-86 or Asp-90. Silent mutations were detected additionally. Thr-86 to Ile mutation was detected in all 139 ciprofloxacin resistant strains, which showed cross-resistance to nalidixic acid.

CONCLUSIONS

The SSCP method is suitable for a molecular screening of quinolone resistant C. jejuni isolates and in combination with DNA sequencing suitable to detect genetic variations of the QRDR of gyrA.

SIGNIFICANCE AND IMPACT OF STUDY

This study provides data of the genetic variations of the QRDR of gyrA from C. jejuni isolates of foods and human beings.