Abstract: Campylobacter species are the most common cause of foodborne disease in Australia and many countries throughout the World. Although campylobacteriosis is usually self-limiting, severe cases and those in the young, elderly and immunocompromised require antibiotic therapy. Antibiotic resistant Campylobacter isolates however may prolong illness and increase the risk of invasive disease. Antibiotic resistance in Campylobacter is thought to have arisen through the selective pressure of exposure to antimicrobial agents in veterinary medicine or animal husbandry, leading to the acquisition and dissemination of antibiotic resistance determinants, and genetic elements that harbour such genes, amongst isolates.

Little was known about tetracycline and trimethoprim resistance in Australian campylobacters, including the presence of resistance genes and associated genetic elements. Aims of this study were therefore to identify in Australian Campylobacter jejuni and Campylobacter coli isolates i). Tetracycline resistant determinants and associated genetic elements, ii). Trimethoprim resistant determinants and associated genetic elements, and iii). Integron like structures and associated genetic elements.

High-level tetracycline resistance was observed in 46 C. jejuni and C. coli isolates, with MICs ranging from 32 to >256mg/ml. All isolates examined harboured the tetO gene, confirming that tetracycline resistance in Australian campylobacters is also due to the previously reported TetO determinant. While several studies have described a significant role for plasmids in tetracycline resistance, this study demonstrated that in the majority of isolates (78%), including two thirds of strains that harboured plasmids, resistance was due to chromosomally encoded tetO. Six C. jejuni isolates were able to transfer a tetO harbouring plasmid to another C. jejuni strain. Plasmids were detected in approximately 74% of resistant strains, and ranged in size from small to larger plasmids (21 - 50kb). ClaI profiling of plasmids revealed genetic diversity and indicated that the tetO gene may be carried by a variety of plasmids.

High level trimethoprim resistance (MICs of 1000mg/ml and >1000mg/ml) was observed in all isolates (>100) examined from a second collection of C. jejuni, C. coli and non-C. jejuni/coli spp. Just over half of isolates harboured plasmids indicating that plasmids may not be involved in trimethoprim resistance in campylobacters. Isolates were also examined for the presence of the previously identified Campylobacter associated trimethoprim resistance genes dfr1 and dfr9. Although these genes play a significant role in this resistance, only approximately 16% of strains examined putatively harboured dfr1, and dfr9 was not detected.

Integrons, antibiotic resistance gene acquisition and expression systems, play an important role in trimethoprim resistance due to carriage of dfr genes as inserted gene cassettes. Trimethoprim resistant Campylobacter isolates were examined for the presence of the intI1 and intI2 genes, encoding the class 1 and class 2 integrons. Only 5.56% of strains examined for intI1 putatively carried this gene, and only 1.67% of isolates examined for intI2 putatively carried intI2. Both putative intI1 positive and negative isolates produced a variety of amplicons, ranging in size from »210bp to >1.5kb, when analysed for gene cassette sequences inserted into class 1 integrons.

This study has contributed to the knowledge of tetracycline and trimethoprim resistance, including the presence of resistance genes and associated genetic elements, in Australian isolates of C. jejuni and C. coli.