While sterility implies the destruction of free-living organisms which may grow within a sample, sterilisation does not necessarily entail destruction of infectious matter.
Evidence of a deletional bias is present in the respective genome sizes of free-living bacteria, facultative and recently derived parasites and obligate parasites and symbionts.
According to this theory, mitochondria, chloroplasts, and possibly other organelles, represent formerly free-living bacteria that were taken inside another cell as an endosymbiont.
