Characterizations of the B 12 biosynthetic pathways of other organisms has shown that the anaerobic pathway was found to include oxygen-sensitive enzymes, intermediates, or both. The aerobic system was found to require oxygen for function. K. aerogenes offers a unique opportunity to study B 12 synthesis under both conditions in the same organism. The introduction of novel functions by horizontal transfer, like the cob and pdu operons into Salmonella , allow rapid expansion into new niches. Saltation in phenotypic states may provide the environmental separation necessary for bacterial speciation.
Bacterial genomes have long been considered streamlined, that is reduced in size for rapid growth.
Yet natural variation in genome size within a species and the propensity for large duplications in growing strains both belie this conclusion. Rather, it is likely that bacterial genomes may be as large as is possible for a species to maintain, depending on its effective population size. Enteric species synthesize B 12 in at least three different ways. These analyses allow us to speculate that bacterial chromosomes have gained and lost tremendous amounts of DNA over evolutionary time.