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

Rapid construction of Campylobacter jejuni deletion mutants

AIMS

To develop a novel method for rapid construction of Campylobacter jejuni deletion mutants.

METHODS AND RESULTS

We used overlapping extension PCR protocol to amplify a target sequence region of Camp. jejuni genomic DNA in which an internal fragment, Cj0618 coding sequence, was replaced by a chloramphenicol resistance cassette. After the resulting PCR product was introduced into electrocompetent Camp. jejuni 81-176, chloramphenicol-resistant mutants in which the wild type allele has been replaced by the deletion cassette were selected. DNA sequencing confirmed precise deletion in the Cj0618 gene. As expected from the previously reported role of Cj0618 in chick colonization, the resulting deletion mutant showed a caecal colonization defect in chick infection.

CONCLUSIONS

This method can be used for rapid construction of Camp. jejuni deletion mutants.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of this method should facilitate functional characterization of various Camp. jejuni genes.

Antibiotic resistance in bacteria associated with food animals: a United States perspective of livestock production

The use of antimicrobial compounds in food animal production provides demonstrated benefits, including improved animal health, higher production and, in some cases, reduction in foodborne pathogens. However, use of antibiotics for agricultural purposes, particularly for growth enhancement, has come under much scrutiny, as it has been shown to contribute to the increased prevalence of antibiotic-resistant bacteria of human significance. The transfer of antibiotic resistance genes and selection for resistant bacteria can occur through a variety of mechanisms, which may not always be linked to specific antibiotic use. Prevalence data may provide some perspective on occurrence and changes in resistance over time; however, the reasons are diverse and complex. Much consideration has been given this issue on both domestic and international fronts, and various countries have enacted or are considering tighter restrictions or bans on some types of antibiotic use in food animal production. In some cases, banning the use of growth-promoting antibiotics appears to have resulted in decreases in prevalence of some drug resistant bacteria; however, subsequent increases in animal morbidity and mortality, particularly in young animals, have sometimes resulted in higher use of therapeutic antibiotics, which often come from drug families of greater relevance to human medicine. While it is clear that use of antibiotics can over time result in significant pools of resistance genes among bacteria, including human pathogens, the risk posed to humans by resistant organisms from farms and livestock has not been clearly defined. As livestock producers, animal health experts, the medical community, and government agencies consider effective strategies for control, it is critical that science-based information provide the basis for such considerations, and that the risks, benefits, and feasibility of such strategies are fully considered, so that human and animal health can be maintained while at the same time limiting the risks from antibiotic-resistant bacteria.

Bacteriophage influence Campylobacter jejuni types populating broiler chickens

The characteristics that allow one Campylobacter jejuni genotype to succeed over another under the influence of bacteriophage predation have been examined in experimental broiler chickens following the observation that this succession appeared to occur in naturally colonized broiler chicken flocks. Examination of three C. jejuni strains from a single flock indicated that horizontal transfer of at least 112 kb of genomic DNA from strain F2C10 (bacteriophage sensitive) to strain F2E1 (bacteriophage insensitive) had created strain F2E3. Transfer of this DNA was associated with acquisition of sensitivity to 6 of 25 lytic bacteriophage isolated from the same flock. All strains tested were capable of colonizing broiler chickens but cocolonization revealed that the bacteriophage sensitive strains F2E3 and F2C10 had a competitive advantage over the bacteriophage insensitive strain F2E1. With the addition of lytic bacteriophage the situation was completely reversed, with F2E1 dominating. The inability to replicate bacteriophage is associated with a significant fitness cost that renders the insensitive strain competitive only in the presence of bacteriophage. We demonstrate that interstrain recombination in vivo can generate genome diversity in C. jejuni and that bacteriophage predation is a strong selective pressure that influences the relative success of emergent strains in broiler chickens.

A fitness cost associated with the antibiotic resistance enzyme SME-1 beta-lactamase

The bla(TEM-1) beta-lactamase gene has become widespread due to the selective pressure of beta-lactam use and its stable maintenance on transferable DNA elements. In contrast, bla(SME-1) is rarely isolated and is confined to the chromosome of carbapenem-resistant Serratia marcescens strains. Dissemination of bla(SME-1) via transfer to a mobile DNA element could hinder the use of carbapenems. In this study, bla(SME-1) was determined to impart a fitness cost upon Escherichia coli in multiple genetic contexts and assays. Genetic screens and designed SME-1 mutants were utilized to identify the source of this fitness cost. These experiments established that the SME-1 protein was required for the fitness cost but also that the enzyme activity of SME-1 was not associated with the fitness cost. The genetic screens suggested that the SME-1 signal sequence was involved in the fitness cost. Consistent with these findings, exchange of the SME-1 signal sequence for the TEM-1 signal sequence alleviated the fitness cost while replacing the TEM-1 signal sequence with the SME-1 signal sequence imparted a fitness cost to TEM-1 beta-lactamase. Taken together, these results suggest that fitness costs associated with some beta-lactamases may limit their dissemination.

Evolution and ecology of antibiotic resistance genes

A new perspective on the topic of antibiotic resistance is beginning to emerge based on a broader evolutionary and ecological understanding rather than from the traditional boundaries of clinical research of antibiotic-resistant bacterial pathogens. Phylogenetic insights into the evolution and diversity of several antibiotic resistance genes suggest that at least some of these genes have a long evolutionary history of diversification that began well before the 'antibiotic era'. Besides, there is no indication that lateral gene transfer from antibiotic-producing bacteria has played any significant role in shaping the pool of antibiotic resistance genes in clinically relevant and commensal bacteria. Most likely, the primary antibiotic resistance gene pool originated and diversified within the environmental bacterial communities, from which the genes were mobilized and penetrated into taxonomically and ecologically distant bacterial populations, including pathogens. Dissemination and penetration of antibiotic resistance genes from antibiotic producers were less significant and essentially limited to other high G+C bacteria. Besides direct selection by antibiotics, there is a number of other factors that may contribute to dissemination and maintenance of antibiotic resistance genes in bacterial populations.

Effect of macrolide usage on emergence of erythromycin-resistant Campylobacter isolates in chickens

In this work we conducted both in vitro and in vivo experiments to examine the development and mechanisms of erythromycin (Ery) resistance in Campylobacter jejuni and Campylobacter coli. In vitro plating revealed that both Campylobacter species had similar but low spontaneous mutation frequencies (3 x 10(-9) to <5.41 x 10(-10)) for Ery resistance. Chickens infected with C. jejuni or C. coli were subjected to single or multiple treatments with medicated water containing tylosin (0.53 g/liter), which transiently reduced the level of Campylobacter colonization but did not select for Ery-resistant (Ery(r)) mutants in the treated birds. However, when tylosin was given to the chickens in feed at a growth-promoting dose (0.05 g/kg feed), Ery(r) mutants emerged in the birds after prolonged exposure to the antibiotic. The vast majority of the in vitro- and in vivo-selected Campylobacter mutants with Ery MICs of 8 to 256 microg/ml lacked the known resistance-associated mutations in the 23S rRNA gene, while the highly resistant mutants (Ery MIC > 512 microg/ml) had the A2074G mutation in the 23S rRNA gene. Inactivation of CmeABC, a multidrug efflux pump, dramatically reduced the Ery MIC in all of the examined mutants regardless of the presence of the A2074G mutation. Together, these results reveal distinct features associated with Ery resistance development in Campylobacter, demonstrate the significant role of CmeABC in Ery resistance, and suggest that long-term use of a macrolide as a growth promoter selects for the emergence of Ery(r) Campylobacter in animal reservoirs.

Comparison of antimicrobial resistance of Campylobacter jejuni isolated from humans, chickens, raw milk, and environmental water in Québec

This study compares the occurrence of antimicrobial resistance to erythromycin, ciprofloxacin, and tetracycline among 384 Campylobacter jejuni isolates from humans (245), fresh whole retail chickens (56), raw milk (33), and environmental water (41) collected between 2000 and 2003 in Québec, Canada. Resistance to ciprofloxacin was significantly more frequent in human isolates acquired abroad than in those acquired locally (50 versus 5.9%; P < 0.001); ciprofloxacin resistance was almost absent in water, chicken, and raw milk isolates. In contrast, resistance to erythromycin was significantly more common in chicken than in locally acquired human isolates (16 versus 3.0%, respectively; P < 0.001); no erythromycin resistance was found among water, raw milk, and human isolates acquired abroad. Resistance to tetracycline was significantly more common in chicken and human isolates acquired locally (58.9 and 45.8%, respectively) than in raw milk and water isolates (9.1 and 7.3%, respectively, P < 0.001). Tetracycline resistance was also observed in 44.4% of human isolates acquired abroad. No human isolate was resistant to both ciprofloxacin and erythromycin, but one chicken isolate was resistant to all three antimicrobial agents. Our results suggest that from 2000 to 2003 in Québec, antimicrobial resistance remained stable among locally acquired C. jejuni human clinical isolates and might even have decreased. However, the high erythromycin resistance rate observed among chicken isolates is concerning because of the risk of transmission of such isolates to humans. Additional studies are needed to monitor trends in antimicrobial resistance among food, environment, and human C. jejuni isolates as well as antibiotic use in animals.

Prevalence, genetic diversity, and antibiotic resistance patterns of Campylobacter jejuni from retail raw chickens in Korea

Campylobacter jejuni is a frequently detected food-borne pathogenic bacterium. Clinical cases are mostly sporadic but campylobacteriosis can have serious consequences, such as the development of Guillain-Barré syndrome as well as diarrheal diseases. We examined 265 retail raw chickens from Korean markets for the presence of C. jejuni using the US Food and Drug Administration standard cultural method and multiplex polymerase chain reaction (mPCR). The mPCR-confirmed C. jejuni isolates were subtyped by pulsed field gel electrophoresis (PFGE) and flaA-typing for investigating the genetic diversity of the microorganism in retail raw chickens. Restriction enzymes SmaI and DdeI were used for PFGE and flaA-typing, respectively. Campylobacter spp. were found in 181 samples (68.3%) and C. jejuni in 100 samples (37.74%). For C. jejuni, 73 pulsotypes and 30 flaA types were detected. Antibiotic resistance tests performed by disk diffusion assay indicated that most C. jejuni isolates were resistant to tetracycline, nalidixic acid, and ciprofloxacin, and 87 composite types were revealed by PFGE, flaA-typing, and the antibiotic resistance tests. Our results show that the genetic diversity of C. jejuni isolates is very high and the correlation between genotype and antibiotic resistance was low even though many bacteria showed multi-drug resistance.

High level of ciprofloxacin resistance and its molecular background among Campylobacter jejuni strains isolated in the United Arab Emirates

The antibiotic sensitivity and the serotype and molecular type (MT) distribution of 41 Campylobacter jejuni strains isolated from individual patients in Tawam Hospital, Al Ain, United Arab Emirates, were investigated. While all strains were sensitive to erythromycin (MIC 0.5-4 mg l(-1)), 35 isolates (85.4 %) exhibited resistance to ciprofloxacin (MIC 8-64 mg l(-1)). All resistant strains carried the Thr-86 to Ile mutation in the gyrase A (gyrA) gene, as shown by mismatch amplification mutation assay (MAMA) and confirmed by sequencing. Based on the partial sequences of gyrA, resistant isolates carried 10 distinct alleles, eight of them representing new variants. Strains were assigned to 30 MTs based on the combined results of PFGE and flaA PCR-RFLP typing. Eight of the 35 ciprofloxacin-resistant strains, isolated over a period of more than 1 year, represented the largest MT, also carrying the same allelic variant of the gyrA gene. These results show that the local incidence of fluoroquinolone resistance among C. jejuni is one of the highest reported worldwide. It was also demonstrated that stable MTs could persist for a relatively long time among the clonally unrelated antibiotic-resistant isolates of C. jejuni. The data also emphasize the need to replace fluoroquinolones as empirical therapy for diarrhoea of undiagnosed aetiology.

Antibiotic-resistant Campylobacter: could efflux pump inhibitors control infection?

Campylobacter is the most common cause of bacterial gastroenteritis in the world. Poultry is the main reservoir of human infections. The widespread use of antibiotics in agriculture and veterinary medicine has resulted in the emergence of an increasing number of antibiotic-resistant Campylobacter strains that can be transmitted to humans through the food chain. Of particular concern to public health is the prevalence of resistance to macrolides and fluoroquinolones that are used in the treatment of life-threatening campylobacteriosis. The CmeABC efflux system has been shown to contribute to the intrinsic and acquired resistance to these antibiotics. In addition, by mediating resistance to bile, it is essential for colonization of the chicken gut in vivo. Inhibition of CmeABC may provide an effective means of reversing antibiotic resistance and decreasing the transmission of Campylobacter via the food chain. This would positively impact on public health by decreasing the morbidity, mortality and increased healthcare costs associated with the treatment of antibiotic-resistant Campylobacter.

The biological cost of mutational antibiotic resistance: any practical conclusions?

A key parameter influencing the rate and trajectory of the evolution of antibiotic resistance is the fitness cost of resistance. Recent studies have demonstrated that antibiotic resistance, whether caused by target alteration or by other mechanisms, generally confers a reduction in fitness expressed as reduced growth, virulence or transmission. These findings imply that resistance might be reversible, provided antibiotic use is reduced. However, several processes act to stabilize resistance, including compensatory evolution where the fitness cost is ameliorated by additional mutation without loss of resistance, the rare occurrence of cost-free resistance mechanisms and genetic linkage or co-selection between the resistance markers and other selected markers. Conceivably we can use this knowledge to rationally choose and design targets and drugs where the costs of resistance are the highest, and where the likelihood of compensation is the lowest.

Effect of conventional and organic production practices on the prevalence and antimicrobial resistance of Campylobacter spp. in poultry

Intestinal tracts of broilers and turkeys from 10 conventional broiler farms and 10 conventional turkey farms, where antimicrobials were routinely used, and from 5 organic broiler farms and 5 organic turkey farms, where antimicrobials had never been used, were collected and cultured for Campylobacter species. A total of 694 Campylobacter isolates from the conventional and organic poultry operations were tested for antimicrobial resistance to nine antimicrobial agents by the agar dilution method. Although Campylobacter species were highly prevalent in both the conventional and organic poultry operations, the antimicrobial resistance rates were significantly different between the organic operations and the conventional operations. Less than 2% of Campylobacter strains isolated from organically raised poultry were resistant to fluoroquinolones, while 46% and 67% of Campylobacter isolates from conventionally raised broilers and conventionally raised turkeys, respectively, were resistant to these antimicrobials. In addition, a high frequency of resistance to erythromycin (80%), clindamycin (64%), kanamycin (76%), and ampicillin (31%) was observed among Campylobacter isolates from conventionally raised turkeys. None of the Campylobacter isolates obtained in this study was resistant to gentamicin, while a large number of the isolates from both conventional and organic poultry operations were resistant to tetracycline. Multidrug resistance was observed mainly among Campylobacter strains isolated from the conventional turkey operation (81%). Findings from this study clearly indicate the influence of conventional and organic poultry production practices on antimicrobial resistance of Campylobacter on poultry farms.

Emerging microbiological food safety issues related to meat

Avian influenza viruses and antibiotic-resistant pathogens have become topics of current public health interest. This paper will focus on the significance of these pathogens to the meat industry as well as other emerging microbiological food safety topics likely to impact the meat industry. These include surveillance of foodborne pathogens, microbial source tracking, risk assessment, and human populations at increased risk of infection by foodborne microbes. These emerging issues will likely lead to even greater challenges to producing microbiologically safe meat products than the industry has ever experienced. However, accompanying such challenges will be innovative solutions that provide even greater public health protection to meat-containing foods.

Fitness of antimicrobial-resistant Campylobacter and Salmonella

Campylobacter and Salmonella are the most commonly reported bacterial causes of human foodborne infections, and increasing proportions of these pathogens become resistant to medically important antimicrobial agents, imposing a burden on public health. Acquisition of resistance to antibiotics affects the adaptation and evolution of Salmonella and Campylobacter in various environments. Many resistance-conferring mutations entail a biological fitness cost, while others (e.g. fluoroquinolone resistance in Campylobacter) have no cost or even enhanced fitness. In Salmonella, the fitness disadvantage due to antimicrobial resistance can be restored by acquired compensatory mutations, which occur both in vitro and in vivo. The compensated or even enhanced fitness associated with antibiotic resistance may facilitate the spread and persistence of antimicrobial-resistant Salmonella and Campylobacter in the absence of selection pressure, creating a significant barrier for controlling antibiotic-resistant foodborne pathogens.

Antimicrobial susceptibilities of Campylobacter jejuni isolates from poultry from Alberta, Canada

One hundred four isolates of Campylobacter jejuni from poultry in Alberta, Canada, collected during 2001 were tested for resistance to 10 antimicrobial agents using agar dilution. This study provides a baseline of resistance profiles and the mechanisms of resistance observed in C. jejuni in poultry from Alberta, Canada.

Food commensal microbes as a potentially important avenue in transmitting antibiotic resistance genes

The rapid emergence of antibiotic-resistant (ART) pathogens is a major threat to public health. While the surfacing of ART food-borne pathogens is alarming, the magnitude of the antibiotic resistance (AR) gene pool in food-borne commensal microbes is yet to be revealed. Incidence of ART commensals in selected retail food products was examined in this study. The presence of 10(2)-10(7) CFU of ART bacteria per gram of foods in many samples, particularly in ready-to-eat, 'healthy' food items, indicates that the ART bacteria are abundant in the food chain. AR-encoding genes were detected in ART isolates, and Streptococcus thermophilus was found to be a major host for AR genes in cheese microbiota. Lactococcus lactis and Leuconostoc sp. isolates were also found carrying AR genes. The data indicate that food could be an important avenue for ART bacterial evolution and dissemination. AR-encoding plasmids from several food-borne commensals were transmitted to Streptococcus mutans via natural gene transformation under laboratory conditions, suggesting the possible transfer of AR genes from food commensals to human residential bacteria via horizontal gene transfer.

Antimicrobial drug resistance genes do not convey a secondary fitness advantage to calf-adapted Escherichia coli

Maintenance of antimicrobial drug resistance in bacteria can be influenced by factors unrelated to direct selection pressure such as close linkage to other selectively advantageous genes and secondary advantage conveyed by antimicrobial resistance genes in the absence of drug selection. Our previous trials at a dairy showed that the maintenance of the antimicrobial resistance genes is not influenced by specific antimicrobial selection and that the most prevalent antimicrobial resistance phenotype of Escherichia coli is specifically selected for in young calves. In this paper we examine the role of secondary advantages conveyed by antimicrobial resistance genes. We tested antimicrobial-susceptible null mutant strains for their ability to compete with their progenitor strains in vitro and in vivo. The null mutant strains were generated by selection for spontaneous loss of resistance genes in broth supplemented with fusaric acid or nickel chloride. On average, the null mutant strains were as competitive as the progenitor strains in vitro and in newborn calves (in vivo). Inoculation of newborn calves at the dairy with antimicrobial-susceptible strains of E. coli did not impact the prevalence of antimicrobial-resistant E. coli. Our results demonstrate that the antimicrobial resistance genes are not responsible for the greater fitness advantage of antimicrobial-resistant E. coli in calves, but the farm environment and the diet clearly exert critical selective pressures responsible for the maintenance of antimicrobial resistance genes. Our current hypothesis is that the antimicrobial resistance genes are linked to other genes responsible for differential fitness in dairy calves.

Genetic diversity in fluoroquinolone and macrolide-resistant Campylobacter coli from pigs

The genetic diversity of 115 Campylobacter coli strains, isolated from pigs of 59 geographical distant farms in Switzerland, were characterized on the basis of their DNA fingerprints and resistance to macrolides and fluoroquinolones. Sequence analysis showed that the macrolide-resistant isolates had a point mutation in the 23S ribosomal RNA (rRNA) genes (A2075G) and that the fluoroquinolone-resistant isolates had a point mutation in the gyrase gene gyrA (C257T). One fluoroquinolone-resistant strain had an additional transition mutation in the gyrB gene (A1471C). The flaA restriction fragment length polymorphism (RFLP) genotyping revealed that 57% of the isolates were genetically different. Point mutations in the 23S rRNA and gyrA genes could be found in both genetically distant and genetically related isolates. Additionally, isolates with and without point mutations were found within individual farms and on different farms. This study showed that the ciprofloxacin and erythromycin-resistant C. coli population present on the pig farms is not issued from a common ancestral clone, but individual Campylobacter strains have most likely mutated independently to acquire resistances under the selective pressure of an antibiotic.

Campylobacter jejuni

This review describes characteristics of the family Campylobacteraceae and traits of Campylobacter jejuni. The review then focuses on the worldwide problem of C. jejuni antimicrobial resistance and mechanisms of pathogenesis and virulence. Unravelling these areas will help with the development of new therapeutic agents and ultimately decrease illness caused by this important human pathogen.

Quinolones in 2005: an update

Quinolones are one of the largest classes of antimicrobial agents used worldwide. This review considers the quinolones that are available currently and used widely in Europe (norfoxacin, ciprofloxacin, ofloxacin, levofloxacin and moxifloxacin) within their historical perspective, while trying to position them in the context of recent and possible future advances based on an understanding of: (1) their chemical structures and how these impact on activity and toxicity; (2) resistance mechanisms (mutations in target genes, efflux pumps); (3) their pharmacodynamic properties (AUC/MIC and Cmax/MIC ratios; mutant prevention concentration and mutant selection window); and (4) epidemiological considerations (risk of emergence of resistance, clonal spread). Their main indications are examined in relation to their advantages and drawbacks. Overall, it is concluded that these important agents should be used in an educated fashion, based on a careful balance between their ease of use and efficacy vs. the risk of emerging resistance and toxicity. However, there is now substantial evidence to support use of the most potent drug at the appropriate dose whenever this is required.