Effect of bacterial toxins in serum on the chromosomes of WI-38

Two batches of foetal calf serum, free from detectable bacterial, mycotic, mycoplasmal and bovine viral conta mination and possessing good growth-promoting properties induced an abnormally high number of chromatid breaks in WI-38 cells to the extent that the cells were unacceptable as a vaccine substrate. This phenomenon, immediately reversible on changing to a different batch of serum, was unaffected by pasteurisation, suggesting that it was caused by some unidentified toxic factor(s). It was found that the same effect could be brought about by the addition of subcytotoxic concentrations of some bacterial toxins to the culture media. These findings re-emphasise the importance of improved methods for the collection and the maintenance of sterility throughout the processing of such sera, especially when they are intended to be incorporated in media used to establish vaccine substrates.