An increased concern for the possible transfer of resistant bacteria or mobile resistance elements from food animals to humans has resulted in rigorous legislation preventing i.e. practical use of fluoroquinolones in the Danish broiler industry (Olesen et al., 2004; Petersen et al., 2006). In Denmark, antimicrobial resistance is annually monitored in both clinical and indicator E. coli isolated from poultry (DANMAP, 2006). However, very little is known on the prevalence of resistance at the flock level. The aim of this study was to determine the prevalence of flocks positive for E. coli resistant to quinolones or ß-lactams.

Sock samples were collected from 10 broiler parent flocks and 10 broiler offspring flocks. Five pairs of socks were collected from each house. Samples were enriched in McConkey broth and streaked on McConkey agar added with nalidixic acid (32 µg/ml), ciprofloxacin (2 µg/ml), ampicillin (32 µg/ml), cefotaxime (2 µg/ml) or ceftiofur (8 µg/ml). Repeated sampling was performed: five plates for each type of antibiotic were used for each sample in order to increase precision of the method. The ß-glucuronidase test was used for verification of presumptive E. coli.

Ampicillin and nalidixic acid resistances were detected in all flocks. The numbers of E. coli resistant to these drugs were higher in plates from parent flocks than in those from offspring flocks. A broiler parent flock without any history of quinolone usage tested positive for ciprofloxacin-resistant E. coli, although only few resistant colonies were detected. Cephalosporin resistance was not detected in any of the flocks.

In 2005, the prevalences of ampicillin and nalidixic acid resistance in indicator E. coli from Danish poultry were 15% and 10%, respectively (DANMAP, 2006). However, according to our study, these types of resistance are widespread in Danish broiler flocks. The observed high prevalence of nalidixic acid-resistant E. coli is surprising since the use of quinolones in poultry is restricted in Denmark. The low prevalence (5%) of flocks positive for ciprofloxacin resistance and the absence of resistance towards ceftiofur or cefotaxime might reflect the restricted use of antimicrobials in Danish poultry production.

Further investigations are in progress to assess the epidemiological relationship of the isolates resistant to ampicillin and nalidixic acid as well as to characterize the genes and mutations responsible for these types of resistance.

References

DANMAP 2005. 2006. Use of antimicrobial agents and occurrence of antimicrobial resistance in bacteria from food animals, foods and humans in Denmark. Danish Veterinary Laboratory, Copenhagen, Denmark, ISSN 1600-2032.

Olesen, I., H. Hasman, and F. M. Aarestrup. 2004. Prevalence of ß-lactamases among ampicillin-resistant Escherichia coli and Salmonella isolated from food animals in Denmark. Micr. Drug Res. 10:334-340.

Petersen, A., J. P. Christensen, P. Kuhnert, M. Bisgaard, J. E. Olsen. 2006. Vertical transmission of a fluoroquinolone-resistant Escherichia coli within an integrated broiler operation. Vet. Microbiol. 116:120-128.