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
Mechanisms of Transmission
1.
Water brought into mantel cavity and
drawn across the light organ.
Light organ
Bacteria must enter the
pores to access the crypts
where permanent
colonization can occur
Pores about 15µm across
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
Facilitated active capture
2.

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 SquidVibrio Symbiosis, Nature Reviews Microbiology. 2.
Questions?
Questions?