High prevalence of tetracycline resistance in Enterococcus isolates from broilers carrying the erm(B) gene

A total of 73 isolates of Enterococcus spp. carrying the erm(B) gene were obtained from cloacal swabs of broiler chickens derived from 13 different farms in Belgium. The erm(B) gene encodes resistance to macrolides, lincosamides and streptogramin B antibiotics (MLS(B)). The isolates belonged to eight different species: Enterococcus avium (eight isolates), Enterococcus casseliflavus (11 isolates), Enterococcus cecorum (eight isolates), Enterococcus durans (seven isolates), Enterococcus faecalis (10 isolates), Enterococcus faecium (17 isolates), Enterococcus gallinarum (seven isolates) and Enterococcus hirae (five isolates). Acquired resistance to tetracycline was detected in 68 of the isolates, and in 62 of these it was associated with the presence of the resistance genes tet(L), tet(M), tet(O) or tet(S). In three E. faecium isolates that were phenotypically susceptible to tetracycline, tet(L) or tet(M) was present. The transposon integrase gene (int gene) of the Tn916/Tn1545 transposon family was detected in 18 of the 54 isolates that contained the tet(M) gene. It was concluded that acquired resistance to tetracycline antibiotics is often present in enterococci from poultry carrying the erm(B) gene. The use of tetracyclines in poultry may therefore co-select for resistance to MLS(B) antibiotics, which may be important as alternative therapy for enterococcal infections in humans.