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Dispatches Integronlike Structures in Campylobacter spp. of Human and Animal OriginBrigid Lucey,* D. Crowley,* P. Moloney, B. Cryan, M. Daly,*
F. O'Halloran,* E.J. Threlfall,§ and S. Fanning*
Campylobacter spp. are isolated from animals and birds and from the environment, particularly surface water. Poultry have been implicated as a major source of sporadic infection (1). Thermophilic Campylobacter spp., particularly Campylobacter jejuni and C. coli, are recognized as one of the etiologic agents of acute diarrheal disease in humans worldwide (2,3). Antimicrobial chemotherapy is usually reserved for patients with advanced infection or patients prone to relapse. Erythromycin, fluoroquinolones, and tetracycline are the antimicrobial drugs of choice. Bacterial resistance to antimicrobial agents, which is increasing worldwide, is frequently caused by the acquisition of new genes rather than by mutation (4,5). An efficient means of acquiring new genes is by mobile genetic elements such as resistance (R)-plasmids and transposons. Recently, a novel class of naturally occurring mobile genetic elements, integrons, have been described as vehicles for the acquisition of antimicrobial resistance genes (5). Horizontal and vertical transfer can occur readily, as shown by the widespread acquisition of these gene cassettes among the Enterobacteriaceae and Pseudomonas spp. Integrons comprise two conserved structural regions (5'CS and 3'CS) flanking an internal variable region containing one or more site-specific recombined gene cassettes. While most known cassette-associated genes located distal to the 5'CS region encode resistance to antimicrobial drugs, some cassettes may include one or more open reading frames whose product(s) and corresponding function(s) remain to be defined (5). In the 3'CS downstream of the gene cassette are two genes, one of which encodes resistance to quaternary ammonia compounds (qacED1), while the other is the sulphonamide resistance determinant (sul1). Antimicrobial resistance among Campylobacter spp. to drugs used in the treatment of human infection is increasing (6-8). This article reports the results of an investigation of a collection of Irish thermophilic Campylobacter spp. cultured from clinical cases of gastroenteritis and from porcine and poultry sources. We studied a representative sample of 55 isolates (47 C. jejuni and eight C. coli isolated between 1996 and 1998), cultured from intestinal tissue of animals at slaughter and from human fecal samples. Antimicrobial agent susceptibility tests were performed by the agar
diffusion method on IsoSensitest agar (Difco, Dublin, Ireland) with 5% horse
blood (9). Cultures were prepared by inoculating colonies from a fresh,
pure, 24-hour culture into sterile distilled water to give an inoculum
turbidity equivalent to a 0.5 McFarland turbidity standard. The McFarland
standard was prepared by adding 0.5 ml 0.048 M BaCl2 to 99.5 ml
0.18 M H2SO4 with constant stirring. Samples were
swabbed evenly onto agar plates and allowed to dry. Twelve antimicrobial
agents were tested on disks. Antimicrobial drugs tested, together with their
abbreviations and corresponding concentrations in parentheses, included
ampicillin (Ap, 10 µg/disc), chloramphenicol (C, 10 µg/disc),
ciprofloxacin (Cp, 5 µg/disc), colistin (Ct, 25 µg/disc), erythromycin (E,
5 µg/disc), gentamicin (G, 10 µg/disc), nalidixic acid (Na, 30 µg/disc),
spectinomycin (Sp, 10 µg/disc), streptomycin (S, 25 µg/disc),
sulphafurazole (Su, 100 µg/disc), tetracycline (T, 10 µg/disc), and
trimethoprim (Tm, 1.25 µg/disc). The plates containing the antibiotic disks
were incubated at 37°C under microaerophilic conditions for 24 hours.
Inhibition zone sizes were recorded according to the guidelines of the
National Committee for Clinical Laboratory Standards (10). Resistance
profiles were further confirmed by E-Test (AB Biodisc, Solna, Sweden).
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To test the latter hypothesis, genomic DNA was purified from all isolates
(11). Using the oligonucleotide primers Int 1 F 5'-GGC ATC CAA GCA CGA
AG-3'and Int 1 B 5'-AAG CAG ACT TGA CCT GA-3' designed to anneal to the 5'CS
and 3'CS flanking regions (12) of integrons, we tested the Campylobacter spp.
genome by polymerase chain reaction (PCR) for putative gene cassettes. Escherichia
coli containing the characterized plasmids R100.1 and R751 (13) together
with CIT-F 100, a Salmonella enterica serotype Typhimurium DT104
strain cultured from a contaminated food source (14), were included as
controls. Gene cassettes of 1.0-kb and 800 bp, respectively, from E. coli
(data not shown) and 1.0- and 1.1-kb (Figure 1a, lane 2), from Salmonella
Typhimurium were detected after amplification. These amplicon profiles
were designated as integron pattern (IP)-groups A, B, and C, respectively
(Table). After amplification and conventional agarose gel analysis of all Campylobacter
spp. isolates in the study population, DNA amplicons of 230 bp to 1.47
kb were detected. A total of 22 gene cassette structures were identified (Figure
2). The most commonly occurring amplified gene cassette pattern was
designated IP-group I, consisting of four DNA fragments of 350 bp to 700 bp
(Figure 2, lane 1 and Figure 1a, lane 1). This gene cassette pattern was
present in both poultry and clinically derived C. jejuni, accounting
for 38% of strains. IP-group II (Figure 2, lane 2) accounted for 7% of all C.
jejuni isolates only. The IP-groups III (Figure 2, lane 3), XIV
(Figure 2, lane 14), XVI (Figure
2, lane 16), and XXI (Figure 2, lane 21) each
accounted for 6% of the collection, with IP-groups XIV and XVI being unique
to C. coli. All other IP-groups (Table; Figure 2) were represented by
single isolates. A 350-bp amplified DNA fragment was common to all isolates,
with the exception of the poultry-derived C. coli CIT-P2 and a
clinical isolate C. jejuni CIT-H3. Amplicons of 230 and 250 bp were
conserved among C. coli isolates only. |
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Drug selection may promote recombinational events between Campylobacter spp., Enterobacteriaceae and other gram-negative organisms (15). A common habitat for these organisms is the human and animal gastrointestinal tract. Modern animal husbandry promotes the use of large animal housing facilities, thereby ensuring genetic interconnection between large populations of bacteria. Campylobacter spp. have a natural ability for transformation (18), and in shared animal reservoirs, interspecies transfer of DNA, including antimicrobial resistance encoding genes and other unrelated genes, may occur by strategies analogous to site-specific recombination (19,20). Our findings may indicate a novel mechanism by which unrelated DNA becomes incorporated into cells (21). Detailed characterization of these integronlike structures is an essential step in understanding the role(s) of these novel genetic elements. The existence of these structures may have interesting implications regarding the diversity of the Campylobacter spp. genome and the evolution of this species. Together with the corresponding DNA fingerprint profile (Lucey B., Fanning S., manuscript in preparation) the variation in genetic content and structure of these determinants may be used as a potential tool in elucidating the epidemiology of these pathogens (22,23). AcknowledgmentsThe authors thank Helen O'Shea, Alessandra Carattoli, Fred Angulo, and colleagues at the Department of Medical Microbiology, Cork University Hospital, for valuable comments on the manuscript; Michael Betts and Dorothe Sandvang for providing veterinary samples and Escherichia coli controls, respectively. Ms. Lucey, senior biomedical scientist, Molecular Diagnostics Unit, Cork's Institute of Technology, and Department of Medical Microbiology, Cork University Hospital, is completing a Masters thesis under the direction of Séamus Fanning. Her research interests include the molecular epidemiology of Campylobacter spp. and the genetic mechanisms underlying antimicrobial resistance in these organisms. She is a recipient of the Abbott Research Prize 1996 awarded to Medical Laboratory Scientists. Address for correspondence: Séamus Fanning, Molecular Diagnostics Unit, Cork Institute of Technology, Bishopstown, Cork, Ireland; fax: 353-21-545-343; e-mail: sfanning@cit.ie References
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