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Elucidation of the mechanism of antimicrobial activity of pentacyclic triterpenoids on Staphylococcus aureus
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posted on 06.02.2017by Chung, Pooi Yin
Staphylococcus aureus strains with multiple resistances to antibiotics are increasingly widespread in hospitals and community settings, thus the development of new agents are urgently necessary. The availability of complete whole-genome nucleotide sequence data of various strains of S. aureus, presents an opportunity to explore novel compounds and their targets to address the challenges presented by antimicrobial drug resistance in this organism. Study compounds α-amyrin [3β-hydroxy-urs-12-en-3-ol (AM)], betulinic acid [3β-hydroxy-20(29)-lupaene-28-oic acid (BA)] and betulinaldehyde [3β-hydroxy-20(29)-lupen-28-al (BE)] belong to pentacyclic triterpenoids that are based on a 30-carbon skeleton comprising five six-membered rings or four six-membered rings and one five-membered rings. These compounds were reported to exhibit antimicrobial activities against bacteria and fungi, including S. aureus. The MIC values of these compounds against reference strains of methicillin-resistant S. aureus (MRSA ATCC 43300) and methicillin-sensitive S. aureus (MSSA ATCC 29213) ranged from 64 to 512 μg/ml while most combinations of the compounds and combinations with standard antibiotics methicillin and vancomycin showed synergistic effects. However, the response mechanisms of S. aureus to these compounds are still poorly understood. The genome-wide transcription of reference strains MRSA and MSSA treated with sub-inhibitory concentrations of the three compounds was determined using Affymetrix GeneChips. Treatment of the MRSA strain with AM resulted in the down-regulation of genes involved in the peptidoglycan biosynthesis, aminoacyl tRNA synthase and fatty acid biosynthesis pathways. When treated with BA, there was up-regulation of genes involved in β-lactam resistance, ABC transporters and DNA replication. Down-regulation of genes involved in three main pathways; i.e. fatty acid biosynthesis, peptidoglycan biosynthesis and ribosome were observed when the reference strain of MRSA was treated with BE. In addition, two genes in the ABC transporter pathway were at least 3-fold up-regulated. Similar to the effect of BE against MRSA, AM inhibits MSSA primarily via the ribosomes. The gene expression pattern of MSSA after treatment with BA exhibited minimal changes, while the effect of BE on MSSA is also primarily on the ribosome. In general, there were positive correlation between microarray data and real-time PCR data for all the genes validated. The findings showed that AM, BA and BE affected novel and previously targeted pathways which could be further explored in the development of therapeutic compounds for the treatment of S. aureus infections.