Identification of the Ames Mutagen Tester Strains as Salmonella typhimurium
Busch et a1 [1986] present convincing evidence that the Ames mutagen tester strains speciate in standard microbiological tests as Salmonella typhimurium although several atypical biochemical reactions were observed. Most notable among atypical reactions was a failure to ferment arabinose on the part of strains TA1537, TA1538, TA98, and TA104. Actually, an arabinose-negative phenotype is expected in these strains since mutations hisC3076, hisD3052, and hisG428 were isolated in a strain carrying a stable mutation [Margolin, 19631, ara-9 [Oeschger and Hartman, 1970; Hartman et al, 19861. Unfortunately, the presence of the ara-9 mutation was omitted from the original strain description of TA104 [Levin et al, 1982al. It should be noted that mutation hidl3018 also was isolated in the ara-9 genetic background, although the ultimate tester strain, TA97, is arabinose-positive. This is because the hisD region was moved to an ara+ his01242 genetic background during construction of strain TA97 [Levin et al, 1982bl. The arabinose-negative phenotypes are in striking confirmation of the known history of these strains. Except for Proteus and Serratia species, the vast majority of wildtype strains of Enterobacteriaceae are arabinose-positive [Edwards and Ewing, 19721. At one point in their description, Busch et a1 [1986] give one possible identification of TA102 as a Salmonella pullorum strain. Although not widely acknowledged, S pullorum strains are interestingly cysteine-requiring [reviewed in Sanderson and Hartman, 19781; in contrast, none of the Ames tester strains requires cysteine for growth.
Finally, the source of histidine in complex bacteriological media should be noted. Kirsch et al [1978] showed that the usable histidine in Difco nutrient broth consisted of about 0.85 mM L-carnosine in the beef extract component and an equivalent of 0.15 mM L-histidine (free histidine and histidine-containing peptides) in the peptone component. This observation may be important because we have found carnosinase-negative (as well as non-flagellate) mutants among some stocks after single-colony isolation (eg, strain SB3095 = hisG46pepD102 $u-2055 in Kirsh et al, 1978). Loss of production of hydrogen sulfide [Voll et al, 19741 and export of glutathione [Owens and Hartman, 19861 are two other attributes that appear to be selected for in long-term Salmonella stock cultures.