In a recent article in Environmental Mutagenesis [9:289-295, 19871, "Mutagenicity and Toxicity Studies of Several a$-Unsaturated Aldehydes in the Salmonella typhimurium Mutagenicity Assay," K.O. Cooper et al provided support for a phenomenon that had been described earlier by Ames et al [1975], Maron and Ames [1983], and others. The primary conclusion of this article is correct, ie, in the presence of high toxicity on the plate, his-cells can scavenge the available histidine and form pinpoint colonies which may mimic small his+ revertant colonies. The quality of photographs illustrating the changes in the background lawns of the plates at different levels of toxicity is excellent. Unfortunately, this article runs into difficulty on two major points. First, the data presented do not support the primary conclusion, and second, the statement that the three chemicals discussed are not mutagenic in strain TA100 is not supported by the presented data.
In the "Results" section, data from a 30-min preincubation test are presented in Table , yet the data used to support the report's primary conclusion are from a 90min preincubation test and are not presented in their entirety. In fact, the 90-min incubation results from only one dose level of each chemical are mentioned. It is obvious from Table I that crotonaldehyde was not toxic in 30 min at the doses tested, and trans,trans-muconaldehyde was only minimally toxic. Because the data presented in Table show that toxicity is underestimated when trypticase soy agar (TSA) plates are used, the survival values obtained on minimal agar plates should have been presented in Table , rather than the TSA survival values. This is supported by the authors themselves (p 293, lines 16-17), who state, "...survivals on TSA plates are inaccurate for this class [a,P-unsaturated aldehydes] of compounds. " The authors did not follow their own advice (p 294, lines 7-9).
The 30-min (TSA) survival with 0.10 mM trans,trans-muconaldehyde was 25% (Table ), and it is stated in "Results" that the 90-min minimal agar survival at the same concentration was 29.0%. This does not demonstrate a difference between survivals on the two media. It is reasonable to assume, therefore, that the survival values presented in Table for this chemical are the same as were obtained at all doses on minimal agar. Given this assumption, it is difficult to understand how >9,000 "apparent" revertants owing to microcolonies were obtained at 0.05 mM (87% survival). Based on the fluctuations in survival seen with crotonaldehyde, 87% is probably not significantly different from the control (nontoxic) value.
I am puzzled as to why trans,trans-muconaldehyde and trans-4-hydroxynonenol were selected for this exercise. There are no reports of the mutagenicity of trans,trans-