Antibiotic resistance in Escherichia coli isolated from pre- and post-weaned piglets: a snapshot survey of Australia

The Australian pig industry experiences outbreaks of pre- and post-weaning diarrhoea caused by Escherichia coli, which is linked to reduced growth rates, high medication costs and high levels of mortality and morbidity (Fairbrother et al. 2005). Antibiotics are often used for treatment at weaning but E. coli can develop resistance over time. This is concerning for effective control of E. coli disease as well as abundance of antibiotic resistant strains in both humans and animals. The aim of this study was to isolate E. coli from healthy and sick piglets to determine resistance to antibiotics used in human medicine.

A snapshot survey was conducted from September 2013 to May 2014 in 22 commercial piggeries located in South Eastern Australia (New South Wales n = 9; Victoria n = 10; and South Australia n = 3). Faecal samples were collected from each herd (10 from pre-weaned and 40 from post-weaned piglets) and spread onto sheep blood agar (SBA) and CHROMagar orientation to isolate E. coli. A total of 325 E. coli isolates (15 from each herd) were tested for resistance to 27 human antibiotics using the BD Phoenix Automated Microbiology System (BD Diagnostics) according to human Clinical Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines.

Resistance to antibiotics was found in pre-weaning piglets despite greater exposure to antibiotics after weaning, suggesting possible colonisation by resistant bacteria from sows or the farrowing environment. Chloramphenicol (no longer used in the Australian pig industry) showed a significant increase (P < 0.001) in resistant E. coli from pre- to post-weaned, suggesting co-selection for the resistance phenotype due to exposure to other antibiotics (Fig. 1). Resistance to human third-generation cephalosporins (ceftriaxone and ceftazidime) was less common (Fig. 1), although continued monitoring for emerging resistance to these antimicrobials is essential, considering their importance in human therapeutics. Multi-drug resistance (resistant to ≥3 classes of antibiotics; Magiorakos et al. 2012) was observed in 34% of isolates in this study including drugs important for human health requiring further investigation. Surveillance of E. coli resistance in both healthy and diseased piglets is necessary to anticipate any potential threat to both animal and public health.

Fig. 1.  Antimicrobial resistance of E. coli isolated from pre-weaned () and post-weaned () piglets in South Eastern Australia. All other E. coli resistant isolates were < 2%. *SXT – trimethoprim-sulfamethoxazole. Chloramphenicol: significant increase (P < 0.001) in resistant E. coli from pre- to post-weaned.