The direct experimental evidence presented here shows that Escherichia coli cells can lose a part of their DNA during prolonged starvation. Under stringent conditions cells with a reduced DNA content achieve reproductive advantage over those that maintain their original genome size. Thus, the majority or nearly all of the cells of a long-starved bacterial population undergo genome size reduction. The loss of DNA seems to occur at random in different cells of a population and, thus, their DNA content may vary significantly from one another. The heterogeneity at the DNA level seems to be reflected in conspicuous morphological variability as well. We suggest that, in evolutionary terms, the general dynamics of bacterial genome organization involve two contrasting mechanisms: genome economization (size reduction by DNA loss) and genome loading (acquisition of exogenous DNA and its maintenance in the genome). The former, strengthening the so-called r strategy, might have resulted in the limited genome size of prokaryotes ranging up to 9.5 Mb. The latter explains the widespread horizontal, interspecific gene transfer (general genetic mixing) in bacteria. In the light of the above findings we propose a species concept in bacteria which is comparable to the biological species concept based on reproductive incompatibility.