This collaborative study was carried out by researchers of CReSA (IRTA-UAB), Universidad Complutense de Madrid and Universitat de Lleida.

Dr Migura, researcher of CReSA, briefly explains: “The study showed that both treatments generate an increase in the prevalence of CR E. coli during the treatment period, but resistance diminished after treatment and by the time of finishing, CR E. coli bacteria was not recovered from the animals”.

The study was designed as follows. One hundred 7-day-old piglets were divided into two groups, a control group and a group parenterally treated with ceftiofur.

During the fattening period, both groups were subdivided in two and a second treatment with amoxicillin was administered in feed to two of the four groups, as follows: group untreated, group treated with amoxicillin, group treated with ceftiofur, and group treated with ceftiofur and amoxicillin.

During treatment, fecal samples were collected and CR E. coli was isolated. The presence of cephalosporin resistance genes, and replicon typing of plasmids harbouring CR genes were analyzed in those isolates.

The results suggest that the use of ceftiofur and amoxicillin at different stages of the rearing cycle are independent risk factors for the selection of CR E. coli. Both beta-lactam antimicrobials select for resistant E. coli during the course of treatment. However, CR E. coli bacteria were not detected in the absence of the selective pressure or when the animals departed to the abattoir.

Additionally, results from this study have shown the coexistence of many different CR genes within one farm. The experiments revealed the presence of a great variety of plasmids of many different sizes harboring the same resistance genes. Co-selection by plasmids bearing resistance genes for different antimicrobial families probably plays an important role in the maintenance of resistance mechanisms.

During the last decade, resistance to extended-spectrum beta-lactams (ESBLs), especially third- and fourth-generation cephalosporins and penems, has raised the concern of the scientific community.

The World Health Organization has defined third- and fourth-generation cephalosporins as being “critically important” for use in humans.

A third-generation cephalosporin, ceftiofur, and a fourth-generation cephalosporin, cefquinome, have been developed strictly for veterinary use. The systemic use in food animals of cephalosporins that could potentially select for resistant organisms is worrisome due to the role that food-producing animals may play in the spread of extended-spectrum cephalosporinases into the community.

Further studies should be designed to identify other risk factors associated with the persistence of resistance determinants to minimize the recirculation of isolates and/or plasmids within farms.