Regulatory networks of c-di-GMP signalling involved in biofilm formation, motility and host pathogen interactions in Salmonella typhimurium

摘要

Adaptation to altered environmental conditions is one of the fundamental bacterial characteristics. The ubiquitous second messenger c-di-GMP plays a key role to adopt bacterial behaviour. Transition from motility to sessility and from acute infection to chronic infection isregulated by c-di-GMP signalling in many bacteria. C-di-GMP is synthesised by the diguanylate cyclase activity of GGDEF domains and hydrolysed by the phosphodiesterase activity of EAL or HD-GYP domainsrespectively.In this thesis we investigated the role of individual GGDEF/EAL domain proteinsencoded by the genome of Salmonella typhimurium UMR1 in regulation of key virulence phenotypes such as invasion into HT-29 epithelial cells, induction of a pro-inflammatory immune response and colonization of the gastrointestinal tract of streptomycin treated mice, a model of human gastroenteritis (Paper I). We identified distinct panels of GGDEF/EAL proteins associated with the regulation of each virulence phenotype. We characterized the networks of corresponding GGDEF/EAL domain proteins regulating invasion and induction of interleukin-8 secretion. Our results revealed that GGDEF/EAL domain proteins regulate invasion of S. typhimurium into HT-29 cells through biofilm dependent and independent pathways. The major biofilm regulator CsgD is involved in c-di-GMP dependent inhibition of invasion and pro-inflammatory response.Interestingly, EAL domain proteins STM1344 and STM1697 behaved unconventional compared to phosphodiesterase EAL proteins with respect to regulation of motility, biofilm formation and the virulence phenotype invasion (Paper IIand Paper III). By using genetic and biochemical approaches, we demonstrate that STM1697 and STM1344 inhibit motility and invasion by binding the master regulator of flagella regulon FlhD4C2. Upon binding to FlhD4C2,STM1697 suppresses the functionality of FlhD4C2 to affect expression of downstream genes. Bioinformatic analysis revealed that the EAL domain proteins STM1344, STM1697 and STM3611 belongtoasub-familyofstand-aloneEALdomainproteinsfromdiversespeciesofEnterobacteria,whichputativelyregulatemotilityandsubsequentlyotherphenotypessuchasbiofilmformationandinvasion.Interestingly, the unconventional EAL domain proteins STM1344, STM1697,STM3375 and the phosphodiesterase STM3611 are all regulated on theposttranscriptional levelby the carbon storage regulator CsrA, a global RNA binding protein(Paper IV).CsgD is a major regulator of biofilm formation and virulence. We characterized the regulatory networks of c-di-GMP metabolizing proteins regulating CsgD expression (PaperV). GGDEF/EALdomainproteins affectrdarmorphotype formationandCsgDexpressionatmultiple levels.TheEALphosphodiesterase STM4264targets a globalpool ofc-di-GMPto inhibit rdar morphotype formation and CsgD expression whereas the GGDEF-EALphosphodiesteraseSTM1703is hypothesized to act locally to inhibit rdar morphotypeformationandCsgDexpressioninS.typhimurium.In conclusion, in this thesis, the c-di-GMP signalling networks of biofilm formation, virulence phenotypes and motility of S. typhimurium were significantly extended.