Escherichia coli WP2 bacteria with an ochre amino acid auxotrophy show no 
evidence of growth during the first few days after plating at densities above 
10(8) on plates lacking the required amino acid. They lose viability for some 
days, and then a subpopulation recovers and there is cell turnover. At very low 
plating densities (around 10(2) per plate), almost every cell will eventually 
form a small but visible colony. At intermediate plating densities (10(6) to 
10(7) per plate), there is an immediate increase in the number of viable 
bacteria. The results are consistent with a model that assumes that growth is 
dependent on trace amounts of tryptophan or a tryptophan-complementing substance 
and that death is due to extracellular toxic species in the medium, including 
active oxygen species. Mutations in mutT bacteria under these conditions result 
from incorporation of 7,8-dihydro-8-oxo-dGTP into DNA and thus largely reflect 
DNA synthesis associated with the increase in the number of viable cells at the 
initial density used (10(7) per plate). We show that the increase in cell number 
and much of this DNA synthesis can be eliminated by the presence of 10(8) 
scavenger bacteria and by removal of early-arising mutant colonies that release 
the required amino acid. The synthesis that remains is equivalent to less than a 
quarter of a genome per day and is marginally reduced, if at all, in a polA 
derivative. We cannot exclude the possibility that this residual DNA synthesis 
is peculiar to mutT bacteria due to transcriptional leakiness, although there is 
no evidence that this is a major problem in this strain. If such DNA synthesis 
also occurs in wild-type bacteria, it may well be important for adaptive 
mutation since use of a more refined agar in selective plates both eliminated 
the initial increase in cell number seen at low density (10(7) per plate) and 
reduced the rate of appearance of mutants at plating densities above 10(8) per 
plate.