Inhibition of ruminal bacteria involved in lactic acid metabolism by extracts from Australian plants

Ethanolic extracts, essential oils and plant secondary compounds from selected Australian plants were tested in vitro for their potential to selectively inhibit bacteria associated with lactic acid production in ruminants. A combination of agar dilution and microbroth dilution assays were used to determine the minimum inhibitory concentration (MIC) of the plant extracts against a panel of ruminal bacteria. Ethanolic extract from Eremophila glabra inhibited lactate producers and all other rumen bacteria at 1.260 mg/ml, except for the ruminal lactate fermenter Megasphaera elsdenii (MIC 10 mg/ml). Extracts from Acacia decurrens, A. saligna, Kennedia eximia and K. prorepens inhibited ruminal lactate producer Lactobacillus spp. only (MIC from 5 to 10 mg/ml). The MIC of essential oils ranged from 0.003 to 0.020 mg/ml, but the inhibitory effect was not specific to lactate producers. E. glabra was identified as the plant with the most favourable effect and purified compounds from this plant were investigated for further analysis. Seven serrulatane diterpenes were isolated by chromatography and tested against the major ruminal lactate producer Streptococcus bovis and a lactate fermenter M. elsdenii in a microbroth dilution assay. All but one of these inhibited S. bovis, with the MIC ranging from 0.320 to 1.080 mg/ml, with only one compound also inhibiting M. elsdenii (MIC 1.080 mg/ml). Selective inhibition of lactate producing bacteria in the rumen by some Australian plant extracts and their secondary compounds was identified and may lead to further research into the application of bioactive plants in the management of lactic acidosis in ruminants.