| Abstract: | A breeder flock and a control group of progeny birds were fed antimicrobial-free rations; a second group of progeny received rations supplemented with 50 g chlortetracycline (Ctc)/ton. Effects of dietary Ctc on the distribution of species and biotypes of faecal Gram-positive cocci and their relative resistance to 12 antimicrobial agents were studied. Antimicrobial resistance (AMR) pattern diversity and modal AMR patterns were determined for bacterial species common to all three groups. Numerical taxonomic analysis placed 1321 (97%) of 1360 isolates into eight species or biotypes. The largest cluster (n = 659, 48%) was a biotype of Streptococcus faecalis. Three clusters were biotypes of Streptococcus faecium and contained 580 isolates (42%). The isolates were susceptible to ampicillin and almost uniformly resistant to methicillin, neomycin, streptomycin, sulfadiazine and tetracycline. There were 54 and 47 different AMR patterns, including 0 to 11 and 1 to 11 resistance determinants, in isolates from control and Ctc-fed birds, respectively. Modal AMR patterns for Strep. faecalis and one biotype of Strep. faecium were very similar for all three groups of birds. However, modal patterns in a second biotype of Strep. faecium varied considerably for all three groups. Interpretation of AMR pattern diversities were equivocal among biotypes from both progeny groups. The variable distribution of isolates, proportions of resistant strains, modal patterns and diversity indices among the progeny were probably due to their exposure to different environmental sources of bacteria. |
