This study demonstrated that 15 species of ruminal bacteria with no previous history of contact with antibiotics are susceptible to bacitracin, chloramphenicol, chlortetracycine, erythromycin, novobiocin, oleandomycin, oxytetracycline, penicillin, tetracycline, tylosin, and vancomycin. A number of the species were not inhibited by kanamycin, neomycin, polymyxin, and streptomycin. The data suggest that antibiotic-resistant cells occur within susceptible cultures of these species. Streptococcus bovis FD-10 and a nonruminal anaerobe, Bacteroides melaninogenicus BE-1, showed similar antibiotic susceptibilities. MATERIALS AND METHODS Bacterial strains used in this study were, except as noted, isolated by Marvin P. Bryant. Organisms used 1 Published with the approval of the Director of the North Dakota Agricultural Experiment Station as Journal Article No. 123. included: Bacteroides amylophilus, Bryant strain no. H-18 (isolated by P. N. Hobson); B. melaniniogenicus Bryant strain no. BE-i (isolated by R. J. Gibbons); B. rwninicola subspecies brevis, Bryant strain no. GA-33 (9); B. rwninicola subspecies ruminicola, Bryant strain no. 23 (9); B. succinogenes, Bryant strain no. S-85 (4, 5); Butyrivibrio fibrisolvens, Bryant strain no. 49 (7); Eubacterium limosum, Bryant strain no. L-34 (11); Eubacteriwn runinantiwn, Bryant strain no. B,C-23 (3); Lachnospira multiparus, Bryant strain no. D-32 (8); Peptostreptococcus elsdenii, Bryant strain no. B-159 (11); Ruminococcus albus, Bryant strain no. 7 (10); R. flavefaciens, Bryant strain no. C-94 (10); Selenomonas ruminantiwn, Bryant strain no. HD-1 (2; isolated by Lorraine Gall); Spirillum sp. Bryant no. B-385 (2); Streptococcus bovis, Bryant strain no. FD-10 (isolated by Bryant from a dried rumen product); Succinimonas amylolytica, Bryant strain no. B24 (9); and Succinivibrio dextriniosolvens, Bryant strain no. 24 (8). [Eubacterium limosum, Bryant strain no. L-34 was received from M. P. Bryant as Ramibacterium sp., Bryant strain no.

Fifteen species of ruminal bacteria were tested against 10 antibiotics in concentrations ranging from 0.1 to 200 /ig/ml in an anaerobic tube dilution system.

This study demonstrated that 15 species of ruminal bacteria with no previous history of contact with antibiotics are susceptible to bacitracin, chloramphenicol, chlortetracycline, erythromycin, novobiocin, oleandomycin, oxytetracycline, penicillin, tetracycline, tylosin, and vancomycin. A number of the species were not inhibited by kanamycin, neomycin, polymyxin, and streptomycin. The data suggest that antibiotic-resistant cells occur within susceptible cultures of these species. Streptococcus bovis FD-10 and a nonruminal anaerobe, Bacteroides melaninogenicus BE-1, showed similar antibiotic susceptibilities.