Muller proposed that an asexual organism will inevitably accumulate deleterious 
mutations, resulting in an increase of the mutational load and an inexorable, 
ratchet-like, loss of the least mutated class [Muller, H.J. (1964) Mutat. Res. 
1, 2-9]. The operation of Muller's ratchet on real populations has been 
experimentally demonstrated only in RNA viruses. However, these cases are 
exceptional in that the mutation rates of the RNA viruses are extremely high. We 
have examined whether Muller's ratchet operates in Salmonella typhimurium, a 
DNA-based organism with a more typical genomic mutation rate. Cells were grown 
asexually under conditions expected to result in high genetic drift, and the 
increase in mutational load was determined. S. typhimurium accumulated mutations 
under these conditions such that after 1700 generations, 1% of the 444 lineages 
tested had suffered an obvious loss of fitness, as determined by decreased 
growth rate. These results suggest that in the absence of sex and with high 
genetic drift, genetic mechanisms, such as back or compensatory mutations, 
cannot compensate for the accumulation of deleterious mutations. In addition, we 
measured the appearance of auxotrophs, which allowed us to calculate an average 
spontaneous mutation rate of approximately 0.3-1.5 x 10(-9) mutations per base 
pair per generation. This rate is measured for the largest genetic target 
studied so far, a collection of about 200 genes.