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?