Abstract
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 cells 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.
Highlights
Bacteria are remarkably successful organisms because they can effectively sense and acclimatize to a wide variety of environments
V. fischeri cells secrete molecules, known as microbe-associated molecular patterns (MAMPs), that induce morphological changes and alterations in gene expression in the squid, thereby resulting in a host environment actively shaped by the symbiont
It was proposed that early nitric oxide (NO) sensing through H-NOX “primes” V. fischeri for the crypt environment, where survival may depend upon the ability of the bacterium to combat the formation of these hydroxyl radicals by controlling the levels of free iron (McCormick et al, 1998; Semsey et al, 2006; Wang et al, 2010a; Figure 5)
Summary
Bacteria are remarkably successful organisms because they can effectively sense and acclimatize to a wide variety of environments. In as little as 1 h, V. fischeri and other Gram-negative bacteria make contact with cilia and form biofilm-like aggregates around the cilia and within mucous shed by the host in response to bacterial peptidoglycan (Nyholm et al, 2000; Altura et al, 2013) During these early processes, V. fischeri cells secrete molecules, known as microbe-associated molecular patterns (MAMPs), that induce morphological changes and alterations in gene expression in the squid, thereby resulting in a host environment actively shaped by the symbiont (for reviews, see Nyholm and McFall-Ngai, 2004; Visick and Ruby, 2006; McFall-Ngai et al, 2012) V. fischeri cells dominate over other bacteria within the aggregate www.frontiersin.org
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