Resistant bacteria found on US swine farm

In the research, published recently in Antimicrobial Agents and Chemotherapy, investigators from The Ohio State University (OSU) report the finding of carbapenem-resistant Enterobacteriaceae (CRE). The findings raise concerns about possible foodborne transmission of CRE, which affect human health.

Highly mobile piece of DNA

From the environment of the swine operation, the researchers recovered in total 18 isolates of multiple Enterobacteriaceae species, harbouring the beta-lactamase gene IMP-27. This gene confers resistance to carbapenem antibiotics.

The finding is relevant as CRE were previously unreported from US livestock. The scientists do mention that ‘plasmid-mediated CREs have been reported from livestock in Europe and Asia’. In the United States, carbapenems are not approved for use in livestock.

Research

In their article in the scientific journal, the researchers describe how they collected environmental and faecal samples of the 1,500 sow operation during 4 visits over a 5-month period in 2015. They screened samples using selective media for the presence of CRE, with resulting carbapenemase-producing isolates further characterised.

They reported that, of 30 environmental samples collected from a nursery room on the initial visit, 2 samples yielded 3 isolates carrying the metallo-β-lactamase gene blaIMP-27 on IncQ1 plasmids: 2 isolates of ST 218 Escherichia coli and 1 isolate of Proteusmirabilis.

Enterobacteriaceae

They also reported that on their 3rd visit, they recovered 15 IMP-27-bearing isolates of multiple Enterobacteriaceae species from 11 of 24 environmental samples from 2 farrowing rooms. They noted that these isolates each also carried ^blaIMP-27 on IncQ1 plasmids. Interestingly, no CRE isolates were recovered from faecal swabs or samples in this study.

As is common in US swine production, piglets on this farm receive ceftiofur at birth, with males receiving a second dose at castration (around day 6). This selection pressure may favour the dissemination of ^blaIMP-27-bearing Enterobacteriaceae in this farrowing barn. The absence of this selection pressure in the nursery and finisher barns likely resulted in the loss of the ecological niche needed for maintenance of this carbapenem resistance gene.

Troubling new path for spread of pathogens

A reviewing article by the Center for Infectious Disease Research and Policy (Cidrap), part of the University of Minnesota, stated that the finding suggests a troubling new path for the spread of the drug-resistant pathogens. It pointed to the fact that “all of the isolates carried the gene on what the researchers say is a highly mobile piece of DNA that can spread to a broad range of bacteria.”

Although it is not known how IMP-27 was introduced to the farm, lead author Dr Thomas Wittum, OSU College of Veterinary Medicine, told Cidrap News he believes the pigs and sows were colonised with the CRE, then shed it in their faeces, which explains how it was spread into the barn environment.

No selection for carbapenem antibiotics

It remains to be seen how a carbapenem-resistance gene would spread on a farm where there is no selection pressure from carbapenem antibiotics. Wittum and his team pointed for the answer in the direction of ceftiofur usage. This is a common cephalosporin that is being given on this farm all piglets at birth and to all male pigs again at castration around day 6.

The research team wrote in their report for Antimicrobial Agents and Chemotherapy: “This selection pressure may favour the dissemination of ^blaIMP-27-bearing Enterobacteriaceae in this farrowing barn. The absence of this selection pressure in the nursery and finisher barns likely resulted in the loss of the ecological niche needed for maintenance of this carbapenem resistance gene.”

While the exact relationship between cephalosporin use and carbapenem resistance hasn’t been established, Wittum told Cidrap News that there is a “clear relationship between CRE and ceftiofur use in the farrowing barn at this farm.”

Reaction by National Pork Board

The National Pork Board replied to the findings pointing to the fact that “the resistant gene identified was not found in a market hog and there was no threat to food safety.”

The research was carried out by Dixie F. Mollenkopf, Jason W. Stull, Dimitria A. Mathys, Andrew S. Bowman, Sydnee M. Feicht, Susan V. Grooters, and Joshua B. Daniels and Thomas E. Wittum, all attached to The Ohio State University, Columbus, OH, United States.