Growth inhibition of Acidiphilium species by organic acids contained in yeast extract

The growth of Acidiphilium species was inhibited by 0.4% yeast extract (Difco Laboratories). The growth inhibitory substances were purified from the yeast extract and identified as dl-lactic acid and succinic acid. The minimum inhibitory concentrations (MICs) of dl-lactic and succinic acids against Acidiphilium sp. 63 were 1 and 3 mM, respectively. The contents of dl-lactic and succinic acids in the yeast extracts were estimated at 43.9 and 1.2 mg/g using high-performance liquid chromatography. Acidiphilium species that we have been studying could be classified into two groups on the bases of the difference in susceptibility to these two organic acids.