Susceptibility and resistance of ruminal bacteria to antimicrobial feed additives

Abstract

Susceptibility and resistance of ruminal bacterial species to avoparcin, narasin, salinomycin, thiopeptin, tylosin, virginiamycin, and two new ionophore antibiotics, R022-6924/004 and R021-6447/009, 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 jig/lmn. Gram-negative bacteria were resistant at the highest concentration tested (48.0 ,ug/ml). On the basis of their fermentation produicts, 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 > 6.0 ,ug/ml) than the other compounds to the two major lactic acid-producing bacteria, Streptococcus bovis and Lactobacillus sp. lonophore compounds seemed to be more inhibitory (MIC, 0.09 to 1.50 ,ug/ml) than nonionophore compounds (MIC, 0.75 to 12.0 ,ug/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. lonophore 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.

Potentiation by metal ions of the efficacy of the ionophores, monensin and tetronasin, towards four species of ruminal bacteria

“…The selective toxicity of ionophores towards certain ruminal bacteria is a function of their ability to permeate the cell envelopes of some bacteria but not others (Chen & Wolin, 1979;Henderson et al, 1981;Bergen & Bates, 1984;Nagaraja & Taylor, 1987;Newbold et al, 1988;Russell & Strobel, 1989). Ionophores by definition translocate ions through biological membranes (Pressman, 1968), and this has been assumed to be their mode of action at the cellular level: ionophores that permeate the cell envelope will then disrupt transmembrane ionic gradients in accordance with their ion-translocating properties and cause toxicity.…”

“…Their nutritional effects are due largely to changes in the fermentation stoichiometry and the metabolism of dietary nitrogen by ruminal microorganisms (Bergen & Bates, 1984;Russell & Strobel, 1989;Duffield et al, 2012). These changes arise partly from the elimination of many Gram-positive bacteria (Chen & Wolin, 1979;Henderson et al, 1981;Nagaraja & Taylor, 1987;Newbold et al, 1988) and partly from adaptations which resistant Gram-negative bacteria undergo when grown in the presence of ionophores (Morehead & Dawson, 1992;Newbold et al, 1992;Callaway & Russell, 1999).…”

Binding of radiolabeled monensin and lasalocid to ruminal microorganisms and feed

“…The potency of antibiotics has often been designated by the minimum inhibitory concentration, and this term has been used in the assessment of the effect of ruminal ionophores (Dennis et al, 1981;Nagaraja and Taylor, 1987). The extrapolation of in vitro experiments to in vivo conditions can, however, present problems in interpretation.…”

Effects of antibacterial agents on in vitro ovine ruminal biotransformation of the hepatotoxic pyrrolizidine alkaloid jacobine

“…The ruminal broad-spectrum antibiotics were the most inhibitory to PA biotransformation, except chloramphenicol, which is not inhibitory in the mixed rumen ecosystem, and streptomycin, which does not inhibit most ruminal bacteria. The ionophores inhibited PA biotransformation at approximate levels (1 to 10 ,ug/ml) which inhibit gram-positive but not gram-negative ruminal bacteria (26,28,29,37,39,40). Crystal violet, which inhibited PA biotransformation, generally inhibits gram-positive bacteria (18).…”