Regulation of bacterial adaptive behavior by the second messenger cyclic-di-GMP and host components

Abstract

To optimize survival and growth, bacteria have evolved adaptive behaviors that respond to relevant environmental signals. A switch from the motile to the sessile lifestyle is probably the most ancient behavioral transition of microorganisms. Gram-negative bacteria such as Salmonella species and Pseudomonas aeruginosa have a set of extracellular appendages involved in motility and biofilm formation, but also in interaction with the host. These appendages can be regulated by the bacterial second messenger cyclic di-GMP, which allows a millisecond fast response.

The bacterial second messenger c-di-GMP regulates the transition between sessility and motility and between acute and chronic infection. In this work, the signaling pathway involved in motility in Salmonella enterica serovar Typhimurium has been investigated in detail. The phosphodiesterase YhjH specifically downregulates motility by interfering with the flagellar functionality. Three diguanylate cyclases inhibit motility in the yhjH background and interact specifically with one of two c-di-GMP receptors affecting motility (Paper 1). Also non-canonical EAL domain proteins such as STM1697 unconventionally inhibit motility by post-transcriptionally interfering with the major flagellar regulator FlhD4C2 (Paper 2), which downregulates flagellin expression as one final outcome (Paper 2 and Paper 3). STM1697 has also an unconventional phenotype compared to EAL phosphodiesterases with respect to biofilm formation and invasion of the colon adenocarcinoma cell line HT-29 (Paper 2, Paper 4) and affects virulence mediated through the FlhD4C2 interaction (Paper 2). In general, c-di-GMP metabolizing proteins regulate virulence properties of S. Typhimurium such as invasion and production of the pro-inflammatory cytokine interleukin 8 by HT-29, but also secretion of the effector protein SipA from the invasion related type three secretion system and colonization of gut and organs in the streptomycin treated mouse (Paper 4). Surprisingly, c-di-GMP signaling inhibits virulence properties through biofilm components such as the major biofilm regulator CsgD and the cellulose synthase BcsA (Paper 4). These studies show that the c-di-GMP signaling network is involved in virulence in S. Typhimurium. In the last study, the human surfactant protein C an innate immune component of the lung, did not have an effect on bacterial growth, but affected biofilm formation and swarming motility of P. aeruginosa PAO1 (Paper 5).

In conclusion, this thesis sheds light on how the c-di-GMP signaling network and the surfactant protein C regulate the adaptive behavior of S. Typhimurium and P. aeruginosa, respectively.