Sensitivity of sheep intestinal lactic acid bacteria to secondary compounds extracted from Acacia saligna leaves

The sensitivity of the lactic acid bacteria (LAB) Lactobacillus plantarum and Enterococcus faecium), isolated from sheep faeces, to secondary compounds extracted from Acacia saligna leaves was investigated. Secondary compounds (SC) extracted from A. saligna foliage were: total phenolics (TP), saponins (SP), alkaloids (AK), a methanol extract (ME) and an aqueous extract (AF). All SC were examined with and without PEG for inhibitory effects on L. plantarum, E. faecium and a LAB mixture (L. plantarum + E. faecium 1:1). The highest sensitivity (i.e., inhibition zone) occurred in L. plantarum to all SC, whereas E. faecium was more resistant. All LAB were more sensitive to TP than to other SC, as inhibition zones ranged from 7 to 26 mm2 for AK and TP in L. plantarum, from 5 to 15 mm2 for the AF and ME in E. faecium, and from 5 to 14 mm2 for AK and TP in the LAB mixture. Adding SC to the bacterial medium reduced (P<0.05) growth and lactic acid production. Within each bacterial species, growth, biomass and lactic acid production were lowest (P<0.05) when TP extract was added to cultures of L. plantarum and the LAB mixture, and when ME was added to cultures of E. faecium. Combining both LAB strains increased biomass production with all SC. Resistant colonies increased (P<0.001) with TP addition in L. plantarum, and with TP, SP and AK in E. faecium. PEG decreased inhibitory effects of SC and increased bacterial growth and production of lactic acid and biomass, probably due to its ability to form insoluble complexes with SC. Response to PEG was stronger against TP than other SC, especially L. plantarum and the LAB mixture. Overall, L. plantarum was more susceptible than E. faecium or the LAB mixture to SC, and had a higher response to PEG addition, which reduced toxicity in both bacterial strains. Lactic acid bacteria would be affected to a different extent by SC reaching the small intestine, whereas PEG may neutralize inhibitory effects of SC on these bacterial species.