microbio lecture 22 (ecology & symbiosis)

symbiosis

Most of the interactions between species involve food in that they are either:

• competing for the same food supply

• eating one another (predation)

• or avoiding being eaten (avoiding predation)

These interactions are often brief. However, when two species live in close association for long periods it can be classified as this

Often restricted to only mutually beneficial interactions

parasitism

In symbiosis, at least one member of the pair benefits from the relationship. The other member may be:

• injured

commensalism

In symbiosis, at least one member of the pair benefits from the relationship. The other member may be:

• relatively unaffected

mutualism

In symbiosis, at least one member of the pair benefits from the relationship. The other member may be:

• may also benefit

mutualism examples

• Wolbachia and insects

• Vibrio and squid

• Rhizobium and nitrogen fixation

wolbachia

• Insect symbionts - A classic example of intracellular animal-microbe symbiosis

• cosmpolitan - intracellular parasite that colonizes over half of the world’s insect species

• vertical transmission - transmitted from mother to offspring through the germ line

feminization

genetic males that develop as females

parthenogenesis

development of unfertilized eggs

male killing

male offspring of infected females are killed

cytoplasmic incompatibility

an infected male and uninfected female cannot produce offspring

wolbachia

Releasing _____-infected mosquitos in Indonesia reduced dengue infection rates from 9.4% to 2.3%

• ____ obtain nutrients from their host through specialized transporters

• Genes for the biosynthesis of purines and pyrimidines are conserved across _____

• _____-infected insects have changes in the expression of nucleotide metabolism genes

• When nutrients are limited, ______ infection is beneficial

vibrio

• Bioluminescent bacteria - A classic example of extracellular animal-microbe symbiosis

• bobtailed squid (euprymna scolopes) have light organs that project light at night but the bacteria is producing the light

horizontal transmission

• Squid acquire Vibrio from their environment through ____ ______

• Vibrio colonize crypts inside a light organ

• Squid eject the Vibrio from the crypts in the light organ every morning and reacquire bacteria throughout the day

factors that control who colonizes and persists

Dispersal:

• The movement of microbes between environments determines which microbes are available to colonize

• Host interactions contribute to microbial dispersal

Selection:

• Host filtering excludes or recruits microbes through the immune system and changes to the host environment

Diversification:

• Once they colonize, microbes can evolve into new species

Ecological drift:

• Random processes may cause some microbial populations to vanish

Priority effects:

• The first microbe to colonize influences subsequent colonization

host specificity and host filtering

• Squid secretes chitobiose, a chemoattractant, in mucus at the entrance to the light organ

• Host immune system excludes other bacteria

• Presence of bacteria initiates cellular changes in the host

phylosymbiosis

“microbial community relationships that recapitulate the phylogeny of their host”"

• Microbial dispersal, microbial selection/host filtering, microbial diversification, ecological drift, and priority effects can lead to ______

MAMPs

• Vibrio luminescence peaks at night, when squid are hunting

• Vibrio _____ (the friendly version of PAMPs) drive expression of the cry blue light receptor gene in the squid (a clock gene)

quorum sensing

regulates bioluminescence

vibrio

• ______ presence is necessary for light organ development

• ______ secretes TCT (tracheal cytotoxin), which triggers tissue reorganization

cell surface fraction, LPS, peptidoglycan

how does Vibrio contribute to host development?

• Fractionated culture supernatant of Vibrio by reverse-phase HPLC found a familiar molecule

• Trachael cytotoxin (TCT) is a toxin secreted by Bordella pertussis and Neisseria gonorrhoeae

TCT

is a toxin secreted by Bordella pertussis and Neisseria gonorrhoeae

• adding this to developing squid resulted in full development of light organ

rhizobium

Nitrogen fixing bacteria:

• Perhaps the most agriculturally important mutualistic interaction between bacteria and plants

• These bacteria are essential for the cultivation of leguminous plants such as peas, beans, and alfalfa

nitrogen fixation

• Normally, plants and water leach nitrogen from the soil.

• To compensate, farmers fertilize or rotate crops like corn with legumes (alfalfa, soybeans, peas, etc.). Legumes associate with bacteria that fix nitrogen from the air.

rhizobium

• Gram-negative motile rods

• Associate with the roots (and sometimes stems) of leguminous plants in a species specific manner

• Enter the root via the root hairs

• Invade plant cells and differentiate into semi-dormant nitrogen fixing state (bacteroids).

• Interaction between bacteria and root leads to the formation of nodules-groups of plant cells containing high numbers of bacteroids

nodule formation: recognition

rhicadhesin

a calcium binding protein in the rhizosphere that helps Rhizobium attach to the root hairs during recognition (nodule formation)

Nodule Formation: Invasion

rod factors

excretion of these by bacterium causes root hair curling during invasion (nodule formation)

nod factors

A region of the sym plasmid of Rhizobium leguminosarum biovar viciae, which nodulates peas

The sym plasmid also contains genes that restrict infection to a particular host plant

• nodA, nodB, and nodC are responsible for producing ____ ____, chitin-like molecules that induce root hair curling and nodule formation.

• The addition of exogenous ___ ____ is sufficent to induce nodule formation in the absence of bacteria.

nodD

this product controls transcription of the nod genes, which mediate interaction with the plant root hairs

→ activates transcription of the nod genes by bending DNA at the promoter and thus enhancing RNAP binding and transcription

this is activated by activated by inducers including plant flavonoids

flavonoids

____ are complex organic molecules that are secreted in large quantities by the roots of leguminous plants. They are involved in growth regulation and attracting pollinating animals among other activities

rhizobium-infection

nodule formation

nodule formation

1. adhesion of Rhizobium bacteria to root hair

2. Infection: invagination and curling of root hair

3. Bacteriod development in vesicles in root cortex

4. Bacteroid-induced cell division of cortex cells forming a nodule

leghemoglobin

Nodules are red due to _____:

• In culture Rhizobium are only able to fix N2 under microaerophilic conditions

• Although the bacterium needs some O2 for nitrogen fixation, its nitrogenase is inactivated by high levels of O2

• O2 levels are kept low by the O2 binding protein _____, a plant produced, iron containing protein found in healthy nodules

microaerphilic

In culture Rhizobium are only able to fix N2 under ______ conditions

bacterium, plant

What does each partner get?

• _____: plant provides a steady source of carbohydrates and organic acids

• ____: gets nitrogen in a form it can use (Amides and Ureides) from the bacterium. This gives the plant a significant growth advantage in nutrient poor soils. This advantage is so great, seed companies sell soybean seeds already innoculated with Rhizobium