Echerichia coli kasvu heterogeensus ja toksiin-antitoksiini süsteemid

Toxin-antitoxin systems and heterogeneity of growth inEcherichia coli.SummaryToomas MetsThe pre-existing phenotypic variation in bacterial population is important in coping with theconstantly changing environment. Many forms of stress tolerance (e.g. antibiotics tolerance) havebeen linked to this phenomenon and thus this issue is important for medicine and food industry.Lately there has been increasing evidence about the importance of toxin-antitoxin systems ingenerating heterogeneous populations in bacterial culture. Still, we have much to learn about theconnection between toxin-antitoxin systems and the ability of bacteria to cope with stress.The aim of current thesis was to investigate at a sinlge cell level the relationship between toxinantitoxinsystems and the bacterial growth. For monitoring of the cell growth GFP dilutionmethod was used (Roostalu et.al., 2008). Studying the growth of cold sensitive E. coli strainHM22 (hipA7) (Korch et.al., 2003) it was shown that in the given bacterial strain incubation onlow temperature gives rise to two distinct subpopulations: dividing and non-dividing. Driven bythis phenomenon the growth of other cold or temperature sensitive E. coli strains was studied, butno heterogeneity in growth after transfer to and from the nonpermissive temperature wasobserved. In investigating the effect of toxin overexpression to the growth heterogeneity on asingle cell level, it was found that in addition to slowing down the growth, overexpression of twotoxins (RelE and YafQ) generated a dividing and a non-dividing population of cells. Alsostudying the impact to growth heterogeneity of overexpressing some other ectopically growthinhibiting proteins revealed that producing heterogeneity in growth is not something unique toTA system, but also other proteins can induce differentiation of dividing and non-dividingsubpopulations. The transcription of relBE increases during aminoacid starvation (Christensen et.al., 2001). This observation encouraged us to study the effect of relBE to bacterial growthheterogenity while exiting the aminoacid starvation and stationary phase. No differences werefound while comparing the growth of wildtype and ∆relBE under these conditions.38