Antimicrobial Mechanism of Oleanolic and Ursolic Acids on Streptococcus mutans UA159

Triterpenoid saponin derivatives oleanolic acid (OA) and ursolic acid (UA), but not betulinic acid (BA), were previously found to have strong antimicrobial activity against Streptococcus mutans. OA and UA inhibited the transcription of genes related to peptidoglycan biosynthesis, thereby preventing bacterial growth. However, it is not clear whether this is the only pathway involved in the antimicrobial activity of these compounds against S. mutans. Therefore, we used quantitative real-time PCR (qPCR) and microarray analyses to examine the expression of genes related to essential metabolic pathways in S. mutans UA159 following incubation with OA, UA, or BA. An oligonucleotide array consisting of 5363 probes was designed to survey 1928 of the 1963 genes in the genome of S. mutans UA159. Genes that showed >2-fold changes in expression in response to the treatment conditions were annotated, and selected target genes involved in central metabolism were analyzed by qPCR. Microarray analysis confirmed that the gene expression patterns of the OA- and UA-treated cells differed from that of the BA-treated culture, indicating differences in the antimicrobial mechanism. In particular, the expression of pfk and pykF, coding for glycolysis regulatory proteins phosphofructokinase and pyruvate kinase, respectively, were significantly decreased in the OA and UA groups (P < 0.05), as were genes involved in fatty acid and amino acid synthesis. In addition, the microarray analysis confirmed previous qPCR results showing that peptidoglycan synthesis is down-regulated in the OA- and UA-treated groups. OA and UA also appear to decrease the generation of organic acids by S. mutans UA159, which would have an anticaries effect. Overall, these findings suggest that OA and UA affect multiple genes involved in the central metabolism of S. mutans, with inhibition of glycolysis, fatty acid synthesis, amino acid synthesis, and peptidoglycan synthesis, all contributing to their antimicrobial activity.