In general, wild-type gram-negative enteric bacteria are not able to utilize D-amino acids as the precursors of respective L-amino acids. We found, however, that an L-histidine auxotroph mutant, TA100, derived from
Salmonella typhimurium strain LT2 and used in the Ames test, showed a biphasic growth curve in the presence of both L- and D-histidine at concentrations of 5μg/ml and 100μg/ml, respectively. L-histidine may be utilized preferentially and then, after a short lag, D-histidine may be utilized. The short lag time is therefore considered to be the time required for induction of such an enzyme that converts D-histidine to L-histidine and for uptake of D-histidine by the bacterial cells.
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