Concentration-dependent inhibition of Streptococcus mutans metabolic activity by purified Croton lechleri latex (sangre de grado) in vitro

Abstract

Background. Chlorhexidine is the reference chemical antiplaque agent, but its adverse effects motivate the search for plant-derived alternatives. The proanthocyanidin-rich latex of Croton lechleri (sangre de grado) is a candidate, yet quantitative concentration-response data against Streptococcus mutans are scarce. Methods. In this exploratory in vitro study, ethanol-purified C. lechleri latex was tested at working concentrations of 3-100% v/v (final in-well concentrations 0.6-20% v/v) against S. mutans ATCC 25175 in a 96-well resazurin assay with dual-wavelength reading (570/600 nm) and correction for medium and latex color. Chlorhexidine 1% (0.2% in-well) was the positive control. Each condition was run in technical triplicate within a single experiment. Data were analyzed by one-way ANOVA, Tukey's test with simultaneous 95% confidence intervals (CI) and four-parameter logistic regression. Results. Metabolic activity decreased monotonically with concentration, from 101.3 +/- 1.7% (vehicle) to 54.0 +/- 4.7% (12%), 15.5 +/- 4.0% (50%) and 8.6 +/- 7.7% (100%), compared with 3.1 +/- 2.8% for chlorhexidine (F(7,16) = 160.7, p < 0.001). The lowest concentration that differed from the vehicle was 6% (difference 17.6 percentage points, 95% CI 2.1-33.1). The estimated half-maximal inhibitory concentration (IC50) was 12.3% v/v (95% CI 9.5-15.1). Differences from chlorhexidine at 50% and 100% were not statistically significant, but their confidence intervals were compatible with differences of up to 28 and 21 percentage points, respectively. Conclusions. Purified C. lechleri latex inhibited the metabolic activity of planktonic S. mutans in a concentration-dependent manner. These preliminary findings do not establish equivalence with chlorhexidine and warrant replication in independent experiments with chemically standardized extracts expressed in mg/mL, determination of MIC/MBC and biofilm models.