Antibiotic resistance in bacteria that cause disease in man is an issue of major concern. Although misuse of antibiotics in human medicine is the principal cause of the problem, antibiotic-resistant bacteria originating in animals are contributory factors, with some types of resistance in some species of bacteria. Antibiotics are added to animal feeds to treat and prevent infections and to improve growth and production. Until recently, the major concerns about incorporation of antibiotics in animal feeds related to antibiotic residues in products from treated animals. Although, in 1969, the Swann (1969) report drew attention to the potential for antibiotic-resistant bacteria to spread from treated animals via the food chain, there was little response until the detection of vancomycin-resistant enterococci in animals fed a related glycopeptide, avoparcin. Subsequently, attention started to focus on the issue and other examples of transfer of resistant bacteria through the food chain, such as enterococci resistant to quinupristin–dalfopristin or to everninomicin, fluoroquinolone-resistant campylobacters and multiresistantEscherichia coli, and salmonella such asSalmonella typhimuriumDT104. Reviews and committees in many countries have highlighted the need for better control of licensing of antibiotics, and codes for prudent use of antibiotics by veterinary practitioners and farmers. The continued use of antibiotic growth promoters has been questioned and there is a need to ensure that antibiotics important in human medicine are not used therapeutically or prophylactically in animals.

Aims: To carry out a preliminary assessment of the occurrence of resistance to antimicrobials in bacteria that has been isolated from a variety of aquaculture species and environments in Australia. Method and Results: A total of 100 Gram-negative (Vibrio spp. and Aeromonas spp. predominantly) and four Gram-positive bacteria isolated from farmed fish, crustaceans and water from crab larval rearing tanks were obtained from diagnostic laboratories from different parts of Australia. All the isolates were tested for sensitivity to 19 antibiotics and Minimal Inhibitory Concentrations were determined by the agar dilution method. Plasmid DNA was isolated by the alkali lysis method. Resistance to ampicillin, amoxycillin, cephalexin and erythromycin was widespread; resistance to oxytetracycline, tetracycline, nalidixic acid and sulfonamides was common but resistance to chloramphenicol, florfenicol, ceftiofur, cephalothin, cefoperazone, oxolinic acid, gentamicin, kanamycin and trimethoprim was less common. All strains were susceptible to ciprofloxacin. Multiple resistance was also observed and 74·4% of resistant isolates had between one and ten plasmids with sizes ranging 2–51 kbp. Conclusions: No antibiotics are registered for use in aquaculture in Australia but these results suggest that there has been significant off-label use. Significance and impact of study: Transfer of antibiotic resistant bacteria to humans via the food chain is a significant health concern. In comparison with studies on terrestrial food producing animals, there are fewer studies on antibiotic resistance in bacteria from aquaculture enterprises and this study provides further support to the view that there is the risk of transfer of resistant bacteria to humans from consumption of aquaculture products. From the Australian perspective, although there are no products registered for use in aquaculture, antimicrobial resistance is present in isolates from aquaculture and aquaculture environments.

Fluoroquinolone antimicrobial drugs are highly bioavailable, broad-spectrum agents with activity against gram-negative pathogens, especially those resistant to other classes of antimicrobial drugs. Australia has restricted the use of quinolones in humans through its national pharmaceutical subsidy scheme; and, through regulation, has not permitted the use of quinolones in food-producing animals. As a consequence, resistance to fluoroquinolones in the community has been slow to emerge and has remained at low levels in key pathogens, such as Escherichia coli. In contrast to policies in most other countries, this policy has successfully preserved the utility of this class of antimicrobial drugs for treatment of most infections.