Bacteria have the ability to effectively sense and adapt to a variety of different environments by using molecular signalling cascades to detect extracellular signals and activate intracellular molecular pathways, which is one of the reasons why they are one of the most successful domains of life. These signalling pathways allow bacteria to recognise environmental changes such as osmolarity, oxygen levels, antimicrobials and nutrient levels so they can establish a suitable niche within host organisms. A common and “simplified” model system that has been used in several studies to understand more about how these signalling pathways work in colonisation and infection is the symbiotic interactions between the bioluminescent marine bacterium, Vibrio fischeri, and its nocturnal squid host, Euprymna scolopes, as V. fischeri is the sole bacterium that can withstand the environmental conditions within the squid to colonise the specialised light organ and produce bioluminescent light.
For successful colonisation to occur, a number of steps are required which are facilitated by signalling pathways, and these can be experimentally tracked within V. fischeri. These include being taken via ventilation from the surrounding seawater by young squid that are initially aposymbiotic, followed by making contact with cilia on epithelial fields on the surface of the light organ to form biofilm-like aggregates around them in response to bacterial peptidoglycan. In the later stages of colonisation, the bacterial cells leave the aggregate, enter the ducts of the light organ, move on through the antechambers (which they are unable to colonise), and finally arrive within the crypts in the light organ.
Some of the responses of the host tissue to the presence of these symbiotic bacteria include the production of ROS and reactive nitrogen species (RNS), as well as certain types of antimicrobials. In all of the stages of colonisation, V. fischeri cells continually interact with antimicrobials, such as during initial biofilm aggregation outside the light organ and persistent colonisation within, so they have evolved specific mechanisms to counteract these molecules to maintain the symbiotic relationship.
To manage and reduce the levels of halide peroxide (HOCl), a toxic ROS to many bacteria, V. fischeri cells may produce a catalase enzyme to convert H2O2 to water and oxygen. The increase in iron levels can also result in a higher production of ROS, so V, fischeri use particular mechanisms to reduce these. In addition, nitric oxide (NO) might be initially toxic for V. fischeri in certain environmental types, but the bacterium may have evolved to sense and resist free NO, as V. fischeri encodes H-NOX. This is a protein that binds to NO and results in the detoxification of the molecule during colonisation. E. scolopes expresses a range of enzymes that are potentially antimicrobial, such as five peptidoglycan-recognition proteins (PGPRs).
They also can actively sense their destination and migrate towards it because of their motility from flagella and chemotaxis. V. fischeri cells are dependent on their flagella to colonise their host in the early steps, but tend to lose these appendages within the light organ, so motility may not be an important factor during all stages of colonisation. The variation in use and presence of these appendages may be because of certain environmental cues. Chemotaxis involves a series of “runs” (smooth swimming) and “tumbles” (for re-orientation). These events depend on a complicated TCS pathway involving methyl-accepting chemotaxis proteins (MCPs), but by controlling the activity of these molecules, V. fischeri is able to continually respond to signals from a chemogradient and increase the amount of “smooth” runs towards the site of colonisation.
Once bacteria reach the crypt space and reach a certain high cell density, they are able to fully establish the symbiosis with the squid host and start to produce light and bioluminesce. This bioluminescence is a critical component of the symbiotic relationship, as it provides a nutrient-rich environment for V. fischeri, and in return they produce counterillumination to the squid so that they can remain hidden from predators in the moonlight while they are hunting for their own food.
The lux operon encodes for the required structural proteins to generating light, and one of the genes in the operon, LuxI makes a pheromone, 3-oxo-C6-HSL that promotes lux transcription by binding to and activating LuxR. These regulators are part of a positive feedback loop, so that the LuxR-3-oxo-C6-HSL encourages the synthesis of both Lux enzymes, and more of 3-oxo-C6-HSL, which ultimately results in higher light generation. Bioluminescence may be affected from detected signals caused by changes in oxygen levels, osmolarity, Mg2+ levels and iron levels.
The review seems to mention how similar processes and mechanisms occur in other bacteria, including the closely related Vibrio cholerae on a few occasions (example), so this review seems to indicate how patchy the current knowledge is on the mechanisms involved in the establishment of the symbiosis between V. fischeri and E. scolopes. It also seems as though there have been many isolated studies that specifically focus on V. fischeri, but the authors of this current review keep raising questions that indicate that many functions of the main signals and mechanisms are still not completely understood, such as the need to identify the functions of MCPS to determine how V. fischeri cells can alter their movement towards particular colonisation sites.
Norsworthy, A.N., and Visick, K.L. (2013) Gimme shelter: how Vibrio fischeri successfully navigates an animal’s multiple environments. Frontiers in Microbiology, 4 (356): 1-14