Susceptibility and resistance of ruminal bacteria to antimicrobial feed additives.

Susceptibility and resistance of ruminal bacterial species to avoparcin, narasin, salinomycin, thiopeptin, tylosin, virginiamycin, and two new ionophore antibiotics, RO22-6924/004 and RO21-6447/009, were determined. Generally, antimicrobial compounds were inhibitory to gram-positive bacteria and those bacteria that have gram-positive-like cell wall structure. MICs ranged from 0.09 to 24.0 micrograms/ml. Gram-negative bacteria were resistant at the highest concentration tested (48.0 micrograms/ml). On the basis of their fermentation products, ruminal bacteria that produce lactic acid, butyric acid, formic acid, or hydrogen were susceptible and bacteria that produce succinic acid or ferment lactic acid were resistant to the antimicrobial compounds. Selenomonas ruminantium was the only major lactic acid-producing bacteria resistant to all the antimicrobial compounds tested. Avoparcin and tylosin appeared to be less inhibitory (MIC greater than 6.0 micrograms/ml) than the other compounds to the two major lactic acid-producing bacteria, Streptococcus bovis and Lactobacillus sp. Ionophore compounds seemed to be more inhibitory (MIC, 0.09 to 1.50 micrograms/ml) than nonionophore compounds (MIC, 0.75 to 12.0 micrograms/ml) to the major butyric acid-producing bacteria. Treponema bryantii, an anaerobic rumen spirochete, was less sensitive to virginiamycin than to the other antimicrobial compounds. Ionophore compounds were generally bacteriostatic, and nonionophore compounds were bactericidal. The specific growth rate of Bacteroides ruminicola was reduced by all the antimicrobial compounds except avoparcin. The antibacterial spectra of the feed additives were remarkably similar, and it appears that MICs may not be good indicators of the potency of the compounds in altering ruminal fermentation characteristics.