Efficacy and mode of action of selected non-ionophore antibiotics and direct-fed microbials in relation to Megasphaera elsdenii NCIMB 41125 during in vitro fermentation of an acidosis-causing substrate

Abstract The efficacy of prominent in-feed antibiotics and direct-fed microbials (DFM) to prevent or mitigate ruminal acidosis and lactate accumulation, in addition to whether their presence will enhance or inhibit Megasphaera elsdenii strain NCIMB 41125 (Me) were studied in vitro . The antibiotics studied were aureomycin+sulfamethazine as AS-700 (AS), terramycin as TM-200 (TM), zinc bacitracin (ZB), flavomycin (FM) and tylosin (TS). The DFM were Bovamine (BM) which contains a propionic bacterium and a lactobacillus, Levucell (LC) which contains a strain of the yeast Saccharomyces cerevisiae and Progut (PG) which contains a hydrolysate of S. cerevisiae . The antibiotics and DFM were introduced alone or in the presence of Me to an in vitro system with fermentation vessels containing a medium that promoted rapid gas production and lactate development. Dose sizes of the antibiotics were chosen to inhibit fermentation by 10–20% or 30–40% and for DFM dose sizes were according to the manufacturers. For Me the dose size was 100 µl/40 ml containing 2.5×10 5 colony forming units per ml. Me on average reduced lactate from 20.0 mM to 4.89 mM, increased VFA production and shifted VFA proportions to more butyrate and valerate (respectively from 5.80 to 16.0 mM/100 mM and from 0.51 to 4.71 mM/100 mM). The antibiotics moderately reduced lactate (26.7–16.8 mM), and AS, ZB and TS enhanced a VFA proportional shift towards propionate (from 22.6 to 28.7 mM/100 mM). In the presence of Me lactate was reduced to levels of Me alone and the ratio butyrate to propionate was reduced. None of the antibiotics inhibited the action of Me; on the contrary the interaction was additive. In contrast to the antibiotics and PG, the DFM BM and LC did not affect fermentation resulting in no response with respect to any of the variables measured. PG in the presence of Me apparently enhanced the action of Me, as noticed by an additional increase in butyrate and valerate proportions.