Quantification of absolute and relative fitness in Acinetobacter baylyi under varying growth conditions

Abstract

Natural transformation is one of three ways for bacteria to acquire genetic material (DNA) horizontally. Several hypotheses exist for why some bacteria have this ability to take up DNA, and what the incoming DNA is used for. These include use of DNA for recombination, repair or as nutrients.

This master study focuses on the “food hypothesis”. This hypothesis implies that the exogenous DNA is either used in the synthesis of new DNA and RNA molecules, or broken down to carbon, nitrogen and phosphate to use as energy and in synthesis of new nucleotides. The naturally transformable Acinetobacter baylyi is used as a model organism to test this hypothesis. By removing the comFECB operon from the wild type bacteria a non-transformable strain is produced. Comparisons of growth rates between these two strains in monocultures give the absolute fitness, whereas co-cultures give the relative fitness. If the comFECB operon has evolved as a means to use DNA as food, than the transformable wild type would have a fitness advantage over the non-transformable strain when DNA is the only nutrient in the growth media.

Although the addition of DNA to the growth media resulted in increased growth of A. baylyi, this effect was seen in both strains, independently of functional com-genes. This implies that DNA is in fact used as nutrients by this species, but not only by natural transformable bacteria with intact comFECB operons. The experiments done in this thesis do therefore not offer support for the “food hypothesis”.