Signaling between Two Sensor Kinases Controls Biofilms and Host Colonization in a Bacterial Symbiont

Organisms within all domains of life must acclimate to fluctuating environments to survive. To do this, cells utilize sensory circuits, which function to connect environmental stimuli to an intracellular response. One common sensory pathway utilized by bacteria is two-component signaling (TCS), composed of an environmental sensor (the sensor kinase, SK) and a cognate, intracellular effector (the response regulator, RR). The marine bacterium Vibrio fischeri uses an elaborate TCS phosphorelay containing a hybrid SK, RscS, and two RRs, SypE and SypG, to colonize its natural squid host, Euprymna scolopes. This TCS pathway regulates V. fischeri\u27s ability to form a biofilm, or a community of cells encased in an extracellular matrix, a process required to initiate colonization. Between the sypE and sypG genes lies sypF, which encodes another putative hybrid SK. Due to its location and predicted function, I hypothesized that sypF might also regulate biofilms. Indeed, I found that SypF was critical for biofilms by functioning downstream of RscS to directly control SypE and SypG. Surprisingly, although a mutant variant of SypF, SypF*, functioned as an SK both in vitro and in vivo, this did not seem to be the case for wild-type SypF. Specifically, wild-type SypF exhibited SK activity in vitro, but this activity was dispensable for colonization. In fact, only a single non-enzymatic domain within SypF, the HPt domain, was critical in vivo. Remarkably, this domain within SypF directly interacted with RscS, permitting a bypass of RscS\u27s own HPt domain and SypF\u27s enzymatic function. These unique findings represent the first in vivo example of a functional SK that exploits the enzymatic activity of another SK, an adaptation that demonstrates the elegant plasticity in the arrangement of TCS regulators. This flexibility in TCS pathways likely permits bacteria to acutely manage a vast repertoire of different environments thus promoting their survival both inside and outside a hos

Gimme shelter: how Vibrio fischeri successfully navigates an animal’s multiple environments

Bacteria successfully colonize distinct niches because they can sense and appropriately respond to a variety of environmental signals. Of particular interest is how a bacterium negotiates the multiple, complex environments posed during successful infection of an animal host. One tractable model system to study how a bacterium manages a host’s multiple environments is the symbiotic relationship between the marine bacterium, Vibrio fischeri, and its squid host, Euprymna scolopes. V. fischeri encounters many different host surroundings ranging from initial contact with the squid to ultimate colonization of a specialized organ known as the light organ. For example, upon recognition of the squid, V. fischeri forms a biofilm aggregate outside the light organ that is required for efficient colonization. The bacteria then disperse from this biofilm to enter the organ, where they are exposed to nitric oxide, a molecule that can act as both a signal and an antimicrobial. After successfully managing this potentially hostile environment, V. fischeri finally establish their niche in the deep crypts of the light organ where the bacteria bioluminesce in a pheromone-dependent fashion, a phenotype that E. scolopes utilizes for anti-predation purposes. The mechanism by which V. fischeri manages these environments to outcompete all other bacterial species for colonization of E. scolopes is an important and intriguing question that will permit valuable insights into how a bacterium successfully associates with a host. This review focuses on specific molecular pathways that allow V. fischeri to establish this exquisite bacteria-host interaction

MrpH, a new class of metal-binding adhesin, requires zinc to mediate biofilm formation

Author summary Many bacteria use fimbriae to adhere to surfaces, and this function is often essential for pathogens to gain a foothold in the host. In this study, we examine the major virulence-associated fimbrial protein, MrpH, of the bacterial urinary tract pathogenProteus mirabilis. This species is particularly known for causing catheter-associated urinary tract infections, in which it forms damaging urinary stones and crystalline biofilms that can block the flow of urine through indwelling catheters. MrpH resides at the tip of mannose-resistantProteus-like (MR/P) fimbriae and is required for MR/P-dependent adherence to surfaces. Although MR/P belongs to a well-known class of adhesive fimbriae encoded by the chaperone-usher pathway, we found that MrpH has a dramatically different structure compared with other tip-located adhesins in this family. Unexpectedly, MrpH was found to bind a zinc cation, which we show is essential for MR/P-mediated biofilm formation and adherence to red blood cells. Furthermore, MR/P-mediated adherence can be modified by controlling zinc levels. These findings have the potential to aid development of better anti-biofilm urinary catheters or other methods to preventP.mirabilisinfection of the urinary tract. Proteus mirabilis, a Gram-negative uropathogen, is a major causative agent in catheter-associated urinary tract infections (CAUTI). Mannose-resistantProteus-like fimbriae (MR/P) are crucially important forP.mirabilisinfectivity and are required for biofilm formation and auto-aggregation, as well as for bladder and kidney colonization. Here, the X-ray crystal structure of the MR/P tip adhesin, MrpH, is reported. The structure has a fold not previously described and contains a transition metal center with Zn(2+)coordinated by three conserved histidine residues and a ligand. Using biofilm assays, chelation, metal complementation, and site-directed mutagenesis of the three histidines, we show that an intact metal binding site occupied by zinc is essential for MR/P fimbria-mediated biofilm formation, and furthermore, thatP.mirabilisbiofilm formation is reversible in a zinc-dependent manner. Zinc is also required for MR/P-dependent agglutination of erythrocytes, and mutation of the metal binding site rendersP.mirabilisunfit in a mouse model of UTI. The studies presented here provide important clues as to the mechanism of MR/P-mediated biofilm formation and serve as a starting point for identifying the physiological MR/P fimbrial receptor