The antimicrobial activity of the novel
ionophore tetronasin (formerly
ICI 139603) was compared with that of
monensin for the growth of ruminal bacteria, protozoa, and an anaerobic fungus. The potency of
tetronasin toward most bacteria and the fungus was an order of magnitude or more greater than that of
monensin. Lactobacillus casei was 55 times more sensitive to
tetronasin than to
monensin, indicating a potential role for
tetronasin in reversing
lactic acidosis. Bacteria with a gram-positive ultrastructure were generally sensitive to the
ionophores and unable to adapt to grow in their presence. The exception was the cellulolytic Ruminococcus flavefaciens, which adapted during successive cultivation on media with increasing
ionophore concentrations to grow at 100-fold higher concentrations of
tetronasin than were initially lethal to the organism. Gram-negative bacteria were more resistant and generally able to adapt to grow in the presence of both
ionophores. An in vivo trial with cattle and in vitro growth experiments indicated that the effect of
tetronasin on ciliate protozoa was minor. In vitro experiments measuring
hydrogen production by Neocallimastix frontalis suggested that this fungus would be unable to survive in ruminants receiving
tetronasin.