ObjectiveTo evaluate the susceptibility ofStreptococcus mutans (S. mutans)to NaF and the effects ofCandida albicans (C. albicans)metabolites on the metabolic activity and drug resistance ofS. mutans.
MethodsThe minimum inhibitory concentration (MIC) and MIC based on metabolic activity (MIC-MA) of NaF againstS. mutanswere measured through the broth dilution test and D2O-labeled single-cell Raman microspectroscopy, respectively, to explore the inhibitory effect of NaF on the growth and metabolism ofS. mutans. The effects of different concentrations ofC. albicanssupernatant on the metabolic activity ofS. mutanswere evaluated by using D2O-labeled single-cell Raman microspectroscopy. The correlation betweenC. albicansandS. mutanswas explored throughassessing the drug resistance ofS. mutansagainst NaF in a mixed system containingC. albicanssupernatant by using D2O-labeled single-cell Raman microspectroscopy.
ResultsThe MIC and MIC-MA of NaF againstS. mutanswere 0.4 g·L-1and 1.2 g·L-1, respectively. At the MIC level,S. mutansshowed completely inhibited growth but retained high metabolic activity. Therefore,S. mutansmay regain its virulence. At the MIC-MA level, the metabolic activity ofS. mutanswas inhibited only when the NaF concentration reached 3×MIC. At the concentration of OD600≥0.5, theC. albicanssupernatant significantly promoted the metabolic activity ofS. mutansat different time points (P<0 .05). in the mixed system, the susceptibility ofS. mutansto NaF decreased, and statistical differences at different time points were observed (P<0 .05). the metabolic activity ofS. mutanswas inhibited completely only at the concentration of 4×MIC (1.6 g·L-1).
ConclusionD2O-labeled single-cell Raman microspectroscopy is suitable for evaluating the effects of drugs on bacterial metabolic activity. In the mixed system, the metabolites in theC. albicanssupernatant at the concentration of OD600≥0.5 significantly promoted the metabolic activity ofS. mutansand reducedthe susceptibility ofS. mutansto NaF.
