A new photometric assay with bromocresol purple for testing in vitro antitrichomonal activity in aerobic environment

A new colorimetric assay relying on the acidic metabolism of Trichomonas va ginalis was developed for in vitro screening of various compounds against a xenically grown trichomonads. Parasites from continuous culture were expose d to series of drug dilutions in a microtiter plate. After an incubation pe riod of 48 h at 37 degrees C, the pH indicator of the medium had changed it s colour in non-inhibited cultures due to the production of lactate and ace tate. Inhibited cultures showed no colour changes. The use of bromocresol p urple, a pH indicator, was suitable for several reasons: it is not toxic at the concentration used in the assay; the absorbance of bromocresol purple at 405 nm showed a linear correlation with both the pH of the medium and th e viability of the trichomonads observed microscopically; plates could easi ly be read by eye or using an enzyme-linked immunosorbant assay (ELISA) rea der. By comparison of the decreases in absorbance in test cultures with tho se in control cultures, IC50 values could be determined. Thus, IC50 of metr onidazole was calculated at 25.5 +/- 2.3 mu mol/l (n = 5) with the bromocre sol purple assay and about 25 mu mol/l after microscope observation. Minima l lethal concentrations (MLCs) could be read by eye. This test is now routi nely used for anti-trichomonas evaluation of various chemical compounds and natural products.