Effect of in-feed administered coccidiostatics on ESBL-forming E.coli-bacteria, isolated from Dutch broilers

In broiler chickens, an extensive spread of ESBL-forming E.coli was observed in the first week on fattening farms. This rapid spread was not observed in grandparent animals. A difference between broilers and grandparent animals is that the broilers are given coccidiostatics as feed additives, while the grandparents are vaccinated against coccidiosis. This study was performed to investigate if coccidiostatics can influence the growth of ESBL-forming E.coli, isolated from Dutch broilers. The effects of three ionophore coccidiostatics (used as feed additives in broiler feed) on the growth of ESBL-forming E.coli were studied in vitro. Furthermore a growth experiment with whole broiler feed was performed to detect other substances in the feed that might affect the growth of ESBL-forming E.coli.\nMinimal Inhibitory Concentrations (MICs) were determined for salinomycin (concentration range 0,125 - 256 µg/ml), narasin (0,3125 - 64 µg/ml) and monensin (0,25 - 512 µg/ml). Six ESBL-positive strains and six ESBL-negative strains were tested. A growth experiment with a Bioscreen apparatus was performed with narasin and salinomycin for the same strains. Growth curves were made for each strain with concentrations of 1.25, 12.5 and 125 µg/ml salinomycin and 0.5, 5 and 50 µg/ml narasin (concentrations were chosen considering the concentrations of the substances in broiler feed). A growth experiment with 3 ESBL-positive and 3 ESBL-negative strains was performed with whole sterilised broiler feed suspended in the test medium.The results of determining the MICs of the tested strains for the ionophores showed that the growth of the tested ESBL-positive strains as well as the growth of ESBL-negative strains was unaffected by the presence of the coccidiostatics. For salinomycin and monensin, the MICs of all tested strains were >256 µg/ml and for narasin >64 µg/ml. The growth curves of the tested strains made with the Bioscreen showed no indication that salinomycin or narasin had a consistent inhibitory or stimulatory effect on the growth of the tested E.coli-strains. The inhibitory effects of salinomycin and narasin compared to the effects of the solvent ethanol were calculated, no trend in the effects on the growth of the tested strains was observed and there was no difference between the group ESBL-positive and ESBL-negative strains. The growth experiment with broiler feed showed no apparent effect of the broiler feed on the growth of the tested strains. \nIt can be concluded from this study that the coccidiostatics salinomycin, narasin and monensin have no consistent effect on the growth of ESBL-forming E.coli in vitro. The tests with broiler feed in the test medium did not indicate that other compounds of the broiler feed consistently affect the growth of ESBL-forming E.coli. However, the test with the broiler feed should be repeated to give more certainty about these observations.