NEUTRAL RED ASSAY FOR MEASUREMENT OF QUANTITATIVE VERO CELL CYTOTOXICITY

A neutral red assay involving Vero cells was used to quantitate the cy totoxic activity of verotoxins (VT) produced by Escherichia coli and t o investigate changes in titer caused by altering the composition of t he cell culture medium. Three variations on medium 199 were investigat ed: one involved supplementing the medium with 5% fetal bovine serum ( FBS), a second was the use of serum-free (SF) medium that contained 5% bovine serum albumin and 5 mug of fibronectin per ml, and the third i nvolved the use of 4% Ultroser G, a commercial serum replacement. The level of cytotoxicity varied markedly with the type of VT and with the medium that was used. For VT1, there was no difference in cytotoxicit y between medium with 5% FBS and SF medium, but cytotoxicity was reduc ed more than 100-fold in medium containing Ultroser G compared with cy totoxicity in the other media. For VT2, VT2v, and VTe, there was a sli ght reduction in cytotoxicity in medium containing 4% Ultroser G and a more marked reduction in SF medium compared with cytotoxicity in medi um containing 5% FBS.