Abstract

The commensal bacterium Lactobacillus reuteri inhabits the human gastrointestinal tract and possesses putative probiotic, i.e. health-promoting, properties. In this thesis, features important for the ecological performance of L. reuteri and for interactions between the bacterium, its host and the environment were characterised. Extracellular proteins were identified in two L. reuteri strains. Firstly, fifty-three proteins were revealed in strain DSM 20016 using signal sequence phage display. Secondly, a draft genome sequence of strain ATCC 55730 was screened with bioinformatics tools and 126 genes encoding extracellular proteins were identified. Few obvious adhesion or colonisation factors were found, although an R28-like proteins putatively involved in adherence to mucosal surfaces was detected. The early response to sudden acid shock was studied with gene expression analyses using microarrays. When L. reuteri was exposed to an acidic pH, similar to the conditions in the human stomach, the chaperone-encoding clpA and genes putatively involved in cell envelope biogenesis were induced. Inactivation of clpA resulted in an acidic-sensitive phenotype. Seven complete two-component systems consisting of a histidine kinase and a response regulator were identified in the genome of L. reuteri ATCC 55730. Sequence analysis indicated that all seven systems belonged to the OmpR family. In order to expose the function of one of these systems, the response regulator was inactivated. The resulting mutant displayed increased sensitivity to NaCl and the antibiotic ampicillin. Results from gene expression analyses indicate that this two-component system regulates genes involved in cell envelope alterations in relation to stress. Therefore, this system was designated Lea (L. reuteri Envelope Altering). Taken together, these studies demonstrate the importance of the cell envelope for interactions between L. reuteri and its environment. The findings provide a basic insight in the adaptation mechanisms and the life strategies of this species. In addition, this work provides a foundation for further investigations and characterisation of the commensal bacterium L. reuteri.