The impact of food animals as a possible reservoir for extended–spectrum beta–lactamase (ESBL) producing Enterobacteriaceae, and the dissemination of such strains into the food production chain need to be assessed, according to Nadine Geser and co–authors at the Institute for Food Safety and Hygiene at the University of Zurich in Switzerland. In a study reported in BMC Veterinary Research, they report a study of 334 faecal samples from pigs, cattle, chicken and sheep taken at slaughter. Additionally, 100 raw milk samples, representing bulk tank milk of 100 different dairy farms, 104 minced meat (pork and beef) samples and 67 E. coli isolates from cattle E. coli mastitis were analysed.

As many as 15.3 per cent of the porcine, 13.7 per cent of the bovine, 8.6 per cent of the sheep and 63.4 per cent of the chicken faecal samples yielded ESBL producers after an enrichment step.

In contrast, none of the minced meat, none of the bulk tank milk samples and only one of the mastitis milk samples contained ESBL–producing strains.

Of the total of 91 isolates, 89 were E. coli, one was Citrobacter youngae and one was Enterobacter cloacae.

PCR analysis revealed that 78 isolates (85.7 per cent) produced CTX-M group 1 ESBLs while six isolates (6.6 per cent) produced CTX-M group 9 enzymes. Five detected ESBLs (5.5 per cent) belonged to the SHV group and two isolates (2.2 per cent) contained a TEM–type enzyme. A total of 27 CTX–M producers were additionally PCR–positive for TEM–beta–lactamase.

The ESBL–encoding genes of 53 isolates were sequenced of which 34 produced CTX–M–1, six produced CTX–M–14, five produced CTX–M–15 and also five produced SHV–12. Two isolates produced TEM–52 and one isolate expressed a novel CTX–M group 1 ESBL, CTX-M-117. One isolate – aside from a CTX–M ESBL – contained an additional novel TEM–type broad–spectrum beta–lactamase, TEM–186.

The relatively high rates of ESBL producers in food animals and the high genetic diversity among these isolates are worrisome and indicate an established reservoir in farm animals.

The researchers explained that the occurrence of ESBL–producing Enterobacteriaceae in the faecal microflora of farm animals represents an obvious risk for contamination of raw food products from animal origin. However, no ESBL producers were found in the examined food samples and the data concerning the ESBL type distribution in animals did not correlate closely with healthy human carriers. As a result, Geser and colleagues concluded that animal food products are unlikely to be the major vector for ESLB carriage in the human population in Switzerland.

They added, however, that the generally high ESBL occurrence in food animals in that country makes all the more important the prudent use of antibiotics in veterinary medicine and strict hygiene measures during slaughtering and milking.

Reference

Geser N., R. Stephan and H. Hachler. 2012. Occurrence and characteristics of extended-spectrum beta-lactamase (ESBL) producing Enterobacteriaceae in food producing animals, minced meat and raw milk. BMC Veterinary Research, 8:21. doi:10.1186/1746-6148-8-21

Further Reading

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You can view the full report (as a provisional PDF) by clicking here.