Reliability of muramic acid as a bacterial biomarker is influenced by methodological artifacts from streptomycin

The muramic acid (MurA) assay is a powerful tool for the detection and quantification of bacteria with no need to enrich samples by culturing. However, the analysis of MurA in mixed biological and environmental matrices is potentially more complex than analysis in isolated bacterial cells. In this study, we employed one commonly used procedure for extraction of MurA from environmental samples and found that the presence of streptomycin interfered with the determination of MurA by creating chemical species that coeluted with the aldononitrile derivative of MurA prepared in this method. On a molar basis, streptomycin yields a signal that is approximately 0.67 times that of MurA. Mass spectrometry analysis confirmed that the interference from hydrolyzed streptomycin is not actually by MurA, but rather is likely to be N-methyl glucosamine. Because streptomycin is widely applied for selective growth of eukaryotes both in situ and in vitro, our findings may have implications for the significance of results from MurA assays. We conclude that MurA remains an effectual bacterial biomarker due to its unique bacterial origin, but care must be applied in interpreting results from the assay when performed in the presence of streptomycin.