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Australia relies on periodic antimicrobial resistance (AMR) surveys to determine trends and changes in AMR in animal production systems. This study is a follow-up to a survey of Escherichia colifrom healthy cattle at slaughter conducted in 2013, which provided baseline data on AMR prevalence across cattle groups and production practices. In this study, 591 beef cattle, 194 dairy cattle, and 216 veal calf fecal samples were collected from 25 beef and veal processing establishments in Australia, representing approximately 77% of total export volume. A total of 969 matrix-assisted laser desorption–ionization results confirmed commensal E. coliisolates from 574 beef cattle, 186 dairy cattle, and 209 veal calves were recovered, and antimicrobial susceptibility testing was carried out by microbroth dilution to 16 drugs from 10 classes interpreted against epidemiological cutoff breakpoints. Overall, a high proportion of E. coliisolates (83.8%) were wild type for all antimicrobials assessed. In addition, isolates that were non–wild type (NWT) for three or more classes of antimicrobial did not exceed 4% for any of the cattle groups. The prevalence of E. colithat were NWT for antimicrobials that are critical or of high importance to human health was very low, with 1.4% of all isolates tested determined to be NWT for fluoroquinolones, third-generation cephalosporins, or polymyxins. Genomic analysis of NWT isolates identified one beef cattle isolate (ST-10) harboring blaCMY-2and a dairy isolate (ST-58) and two veal calf isolates (ST-58 and ST-394) that had qnrS1,which confer resistance to extended-spectrum cephalosporins and fluoroquinolones, respectively. The low levels of AMR reported in this study confirm previous Australian studies, which indicated that there is minimal evidence that specific production practices lead to widespread disproportionate development of NWT isolates.

Escherichia coliO157 and six non-O157 Shiga toxin–producing E. coli(STEC) serotypes (O26, O45, O103, O111, O121, and O145, colloquially referred to as the “big 6”) have been classified as adulterants of raw nonintact beef products in the United States. While beef cattle are a known reservoir for the prototype STEC serotype, E. coliO157, less is known about the dissemination of non-O157 STEC serotypes in Australian cattle. In the present study, 1,500 fecal samples were collected at slaughter from adult (n=628) and young (n=286) beef cattle, adult (n=128) and young (n=143) dairy cattle, and veal calves (n= 315) across 31 Australian export-registered processing establishments. Fecal samples were enriched and tested for E. coliO157 and the big 6 STEC serotypes using BAX System PCR and immunomagnetic separation methods. Pathogenic STEC (pSTEC; isolates that possess stx, eae, and an O antigen marker for O157 or a big 6 serotype) were isolated from 115 samples (7.7%), of which 100 (6.7%) contained E. coliO157 and 19 (1.3%) contained a big 6 serotype. Four of the 115 samples contained multiple pSTEC serotypes. Among samples confirmed for big 6 pSTEC, 15 (1%) contained E. coliO26 and 4 (0.3%) contained E. coliO111. pSTEC of serotypes O45, O103, O121, and O145 were not isolated from any sample, even though genes indicative of E. colibelonging to these serotypes were detected by PCR. Analysis of animal classes revealed a higher pSTEC prevalence in younger animals, including veal (12.7%), young beef (9.8%), and young dairy (7.0%), than in adult animals, including adult beef (5.1%) and adult dairy (3.9%). This study is the largest of its kind undertaken in Australia. In contrast to E. coliO157 and consistent with previous findings, this study reports a relatively low prevalence of big 6 pSTEC serotypes in Australian cattle populations.

Antimicrobial agents are used in cattle production systems for the prevention and control of bacteria associated with diseases. Australia is the world’s third largest exporter of beef; however, this country does not have an ongoing surveillance system for antimicrobial resistance (AMR) in cattle or in foods derived from these animals. In this study, 910 beef cattle, 290 dairy cattle, and 300 veal calf fecal samples collected at slaughter were examined for the presence of Escherichia coliand Salmonella,and the phenotypic AMR of 800 E. coliand 217 Salmonellaisolates was determined. E. coliwas readily isolated from all types of samples (92.3% of total samples), whereas Salmonellawas recovered from only 14.4% of samples and was more likely to be isolated from dairy cattle samples than from beef cattle or veal calf samples. The results of AMR testing corroborate previous Australian animal and retail food surveys, which have indicated a low level of AMR. Multidrug resistance in Salmonellaisolates from beef cattle was detected infrequently; however, the resistance was to antimicrobials of low importance in human medicine. Although some differences in AMR between isolates from the different types of animals were observed, there is minimal evidence that specific production practices are responsible for disproportionate contributions to AMR development. In general, resistance to antimicrobials of critical and high importance in human medicine was low regardless of the isolate source. The low level of AMR in bacteria from Australian cattle is likely a result of strict regulation of antimicrobials in food animals in Australia and animal management systems that do not favor bacterial disease.