Why it pays for bacteria to delete disused DNA and to maintain megaplasmids.

Genetic information for disused metabolic systems is easily lost. This can be understood on the basis of a mathematic model for the description of the growth of individual microbial cells. The essential features of the model are that the uptake of nutrients is proportional to the surface area of the cell and maintenance costs are proportional to the volume. This explains why disused genes are easily lost. Growth of the individual cell continues until DNA replication of the genome is completed. The consequence is that cells with a large genome size are bigger at the moment of cell division. This results in a less favourable surface to volume ratio, which has a negative influence on the population growth rate. This means that cells with a smaller genome size will have a selective advantage. In cells in which the total DNA is divided over several units which replicate simultaneously (e.g. a cell with more chromosomes or a cell with one chromosome and one or more megaplasmids) a high surface to volume ratio can be maintained. The great metabolic differences between phylogenetically strongly related bacteria are due to loss of genetic information for complex metabolic characters in order to maintain a small genome size and a favourable surface to volume ratio.