Phillip Plourde. Symbiosis The formation of an often long-term association or alliance between two...
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Transcript of Phillip Plourde. Symbiosis The formation of an often long-term association or alliance between two...
Phillip Plourde
Symbiosis
The formation of an often long-term association or alliance between two or more individual organisms
Symbiont Transmission Classified as either Vertical or Horizontal
Vertical Transmission – The microbial partner is passed directly from the parent to the offspring via the egg.
Example – Coral and Zooxanthella
Symbiont Transmission Horizontal Transmission – each new generation
of the host acquires the Symbiont directly from the surrounding environment.
Example – Mammalian intestinal epithelial and Bacteria
Study Organism
Hawaiian bobtail squid Euprymna scolopes
Symbiont – Vibrio fischeri bacteria
Vibro Symbiosis The squid establishes a very tight horizontally
transmitted symbiotic relationship with the bioluminescent bacteria
Extreme specificity between host and symbiont
Light produced by the bacterial facilitates counter illumination
This illumination helps the host avoid predators during their nocturnal activities.
Predator Avoidance Mechanism
At night Symbiotic bacteria are housed in
a “light organ” deep within the organism
Host emits luminescence from its ventral surface
Emitted light mimics downwelling moon and star light thereby obscuring its silhouette
Similar purpose as dual dorsal ventral coloration of many animal in nature. ie sharks, birds etc.
Colonization of Host Problems:
The symbiont is relatively rare in sea water (<0.1% of the bacterioplankton population)
How do you bring the two together?
How do you make sure only the intended microbe enters the host when there is such a plethora of potential invaders in the environment?
Host and bacteria must solve these problems if they are to be successful
Transmission At dawn colonized squid expel about 95% of V. fischeri prior
to burrowing into the sand
Behaviour serves as a means of local enhancement, seeding the environment with more symbionts
Enhancement helps juveniles during colonization which begins immediately after hatching
1. Water brought into mantel cavity and drawn across the light organ.
Mechanisms of Transmission
Pores about 15µm across
Light organ
Bacteria must enter the pores to access the crypts where permanent colonization can occur
Mechanisms of Transmission
Water being drawn across the light organ alone is not enough.
Interesting fact:
1µl / ventilation
2 ventilations / second
500 bacteria / ml sea water
Works out to < 1 bacteria / ventilation
Not Very good odds
Mechanisms of Transmission
2. Facilitated active capture Host has evolved structures that assist in bringing V. fischeri
into the light organ.
• Structure located on inside of mantel cavity
• Ciliated epithelia bring materials into the vicinity of the pores
Mechanisms of Transmission
3. Enriching symbiont via Mucosal aggregation
Before bacteria enters the pore leading to the light organ it must first become the dominant microbe
Cilia cells secrete a mucus in which the symbionts gather
By some as yet unknown mechanism the mucus composition helps to filter out microbes other then the desired V. fischeri
Possibly due to chemotaxis towards N-acetylneuraminic acid, a component of squid mucus
At this stage V. fischeri becomes the dominant microbe in the mucus
Mechanisms of Transmission
After some time clusters migrate into the ducts and eventually into the deep crypts within the light organ
V. fischeri must overcome a number of difficult obstacles while migrating into the light organ including:
High concentrations of nitric oxide synthase
Presence of halide peroxidase – hypohalous acid
Dense cilia that beat in an outward direction
All probably important in keeping out harmful microbes and help to control the symbionts
Mechanisms of Transmission
At each important step the luminescent bacteria have been concentrated and finally undergone winnowing resulting in selection for the specific symbiont
Mechanisms of Transmission
The whole process of colonization is very time sensitive, requiring certain things to happen within a constricted time frame
Light Organ Development
Once V. fischeri colonize the light organ they initiate some reversible and permanent changes in the host morphology!
Light Organ Development
Changes include:
Initiation of bioluminescence Swelling of light organ cells Constriction of the ducts
(reversible) Cessation of mucus shedding
(reversible) Loss of surface epithelium
(permanent)
Likely that these changes would help to limit other microbes from infecting the host
Symbiont Adaptations Number of mutants have been identified that lack
the ability to colonize the squid
These shed some light on the important features of the bacteria
Motility – mutants that lack motility can not fight the microcurrents created by cilia at the pores and as such can not colonize the host
See that V. fischeri has evolved to be highly motile
Symbiont Adaptations
Oxidative stress defences – mutants that have defective putative aerobic and anaerobic NO-inactivating genes can not deal with the various oxidative stresses imposed by the host
See that V. fischeri has evolved enzymatic mechanism to deal with the oxidative stress
Light Production
If bacteria colonize the squid that are unable to produce light they are eliminated and out competed by wild type bacteria
Likely due to monitoring of the luciferase metabolic pathway by the host
Conclusions
Colonization occurs in a series of stages
Each step confers greater specificity between the host and the symbiont
V. fischeri is not a passive player in successful establishment of symbiosis
References Nyholm, S., McFall-Ngai, M. 2004. The Winnowing: Establishing The Squid-
Vibrio Symbiosis, Nature Reviews Microbiology. 2.
Questions?Questions?