I. General: Interactions can beA. mutualistic microbe provides nutrients

- digestion of substances difficult for animal to degrade- fixed carbon- vitamins, cofactors

animal provides habitatB. commensal

animal provides habitatC. predatory animals preying on microbes - grazing, filter feeding  microbes preying on animals - disease (parasitism),

true predation

Animal-Microbe Interactions

II. Animal consumption of microorganismsA. Grazing 1. scrape film of microorganisms (may be decomposers, may be autotrophs, especially cyanobacteria) off of surfaces  e.g., snails, urchins 2. consumption of microbes on decomposing feces - incomplete digestion, especially of cellulose - microbial breakdown of these undigested compounds continues after excretion

- reingestion allows more efficient use of food- microbes also provide some vitamins

 coprophagy (soil microarthropods, rodents (rabbits), snails)

3. consumption of microbes on decomposing detritus

microbes convert cellulose and other refractory compounds into microbial biomass

also immobilize N in the process higher quality food source (from cellulose, low N:C, to microbial

protein, high N:C) 

undigestible detritus is not consumed, microbes recolonize

e.g., earthworms, many aquatic invertebrates (snails, midges)

 – Daphnia eats Sphaerocystis, but most of the Sphaerocystis cells actually survive passage through the gut of Daphia– other algae, such as Chlamydomonas, don’t survive and are digested and assimilated – when in the Daphnia gut, Sphaerocystis actually absorbs nutrients (such as phosphorus) from the remains of other algaeThus, grazing actually enhances the growth of Sphaerocystis in the community; without Daphnia, cyanobacteria outcompete Sphaerocystis

4. Effects on communities: grazing by invertebrates can affect microbial community structure

5. Grazing, Pollution, and Evolution

c. They hatched the eggs and tested their abilities to eat the toxic cyanobacteria.

Daphnia swimming amid toxic cyanobacteria in Lake Constance 

b. Nelson Hairston et al. collected Daphnia eggs in a state of diapause from the sediments, layer by layer

a. P pollution over the past 30 years has caused a proliferation of toxic cyanobacteria in Lake Constance

d. Daphnia with 1960s genes were unable to eat the cyanobacteria, while modern Daphnia were e. The crustaceans adapted to the less nutritious diet, which not only ensured their survival, but has also served as a natural control for the cyanobacteria in the lake.  f. DNA tests further showed that Daphnia evolved from a species that could not cope with the toxic bacteria to a species that could.

"It appears that ecological events that we think of as occurring relatively quickly — such as nutrient enrichment of a lake — can be influenced by the rapid evolution of the animals that are affected... Strong natural selection can lead to rapid changes in organisms, which can, in turn, influence ecosystem processes" Nelson Hairston.

 

B. Filter Feedingaquatic animals, sessile benthic invertebrates: sponges, barnacles, polychaete worms, bivalves, etc. constant flow of water through organism – constant supply of microbes suspended in the water column microorganisms filtered through gills, tentacles, mucous nets microbes are autotrophs, heterotrophs, detritus  sea squirts (tunicates, ascidians) and bivalves can capture particles as small as viruses

tunicates, so named because the outer layer of the body wall is a tough "tunic", made of a substance that is almost identical to cellulose  animal consists of a double sac with two siphons: Sea water is pumped slowly (by cilia) through one siphon, sieved through the inner sac for plankton and organic detritus, and the filtered seawater is pumped out again through the second siphon, the atrium.  Ascidian Tunicates are called sea squirts because when taken out of the water they squirt the water inside their body with force through the atrium

Clavelina, a group of transparent sea squirts

Ciona, with two siphons

cellulose monomer animal biomass

III. Cellulose digestion – key process in animal/microbe mutualisms cellulose – most abundant carbohydrate in the biosphere– chain of glucose molecules linked together – most animals can’t degrade it– rely on cellulolytic microorganisms

microbial biomass

degradation products

1. coprophagous and detritivorous animals (above) 

2. animals that cultivate microbes externally

 

3. intestinal symbionts

IV. Cultivation of Microorganisms (External), plant-eating insects:  

A. leaf-cutting ants 

mutualism – ants – excavate cavity in soil bring leaves to fungus inoculate leaves w/fungus fungus grows by decomposing cellulose ants eat fungus cellulose --> fungal biomass --> ant biomass

also, when ants eat fungi, the acquire cellulase, so they are able to continue degrading cellulose in their guts, using enzyme produced by the fungus

obligate mutualismfungal garden breaks down without ants ants protect fungus from competitorsfungus requires ants for dispersalants require fungus for food

 

Apterostigma garden

Worker ant carryinga piece of fungus

Atta – basidiomycete (many are Lepiota) relationshipfungus is deficient in proteases (enzymes that degrade protein)thus, poor competitor w/other fungi

 Atta creates high-protease micro-environment

macerates leaves, mixing w/saliva (high in proteases)adds fecal matter (high in proteases)then inoculates w/fungal mycelia

 thus, relationship maintained by complementary enzyme systems regionally significant: introduce large amounts of organic

matter to soil

B. Ambrosia beetles and wood-degrading fungi 

beetles carry fungus in mycetangia (specialized mouth or leg pouch for carrying fungi)

as beetles burrow into wood, fungal spores are dislodged and inoculated (along with nutrients required by the fungus)

beetles maintain appropriate moisture conditions in the tunnel by opening and closing passageways

beetles produce selective antibiotics, keeping away other microbes

fungus digests cellulose (which beetle is unable to do), produces vitamins and growth factors

beetle consumes fungus (high in protein); often, fungus is the sole food source on which the beetle is capable of surviving

C. Many others: bark-feeding beetles, ship timber worms, wood wasps, gall midges

 gall midges: larvae and fungal spores deposited into leaves

fungus grows parasitically on plantlarvae eat fungus

  D. Termites: external and internal cultivation of microbial

mutualists

cultivate fungi in wood degradationingest fungi to obtain cellulasealso fungal gardens

V. Intestinal Symbiontsextremely complex gut microflora present in most

animals monogastric (one gastric chamber)potential benefits to host: 1) growth factors synthesis demonstrated importance of growth factor synthesis e.g., vitamin K (deficiency in

germfree animals) 2) pathogen barrier

 

pathogen

host

mutualist -

B. Ruminants – who are they and why do we care?  deer, moose, antelope, giraffe, caribou, cow, sheep, goat “Ruminants are earth’s dominant herbivores, due in part to the evolution within this group of a mechanism utilizing microorganisms to digest plant components not susceptible to attack by ruminant enzymes.” (Hungate 1975) Thus, we care for two reasons: i. Ruminants are earth’s dominant herbivores in natural ecosystems ii. Human food economy depends on ruminants

C. Ruminant digestion – diet is high in cellulose, but ruminants cannot produce cellulase  i. Food is first chewed, then enters the rumen ii. The rumen is a specialized chamber for microbial fermentation, containing many bacteria and protozoa a. rumen environment is quite uniform - anaerobic, high T (30-40 degrees C), neutral pH (5.5-7), constant substrate supply b. in the rumen, cellulolytic bacteria and protozoa hydrolyze cellulose to produce cellobiose and glucose

c. these sugars are then fermented, producing volatile fatty acids (acetic, propionic, and butyric) and CO2 and CH4

 iii. food passes from the rumen into the reticulum, where it is formed into small portions called ‘cuds’

iv. cuds are regurgitated into the mouth where they are chewed again – ‘rumination’ v. these solids are now finely divided and very well mixed with saliva; they are swallowed again, but this time the material enters the abomasum, an organ more like a true stomach, where ‘true’ digestion begins and continues into the small and large intestine

Diagram of the rumen and gastrointestinal system of a cow, showing the route of passage of food.

D. overall fermentation reaction: cellulose --> acetate, propionate, butyrate, CO2, CH4, H2O 

three main products that benefit the animal

i) Volatile fatty acids: acetate, propionate, butyrate these pass through the rumen wall and are absorbed

propionate – used for carbohydrate biosynthesisacetate, butyrate – used for energy

ii) microbial cells – contributes protein to ruminant’s diet – probably the main source of protein  many rumen bacteria can use urea as a sole N source; often part of cattle feed to promote protein synthesis (cheap meat) iii) Heat – important to the ruminant’s thermoregulation

Biochemical reactions in the rumen – end products shown in red (bold)

E. rumen microflorai. bacteria: bacterial populations: 109 to 1011/mL – very high highly specialized bacterial community all are obligate anaerobesspecific groups specialize in the degradation of cellulose, starch,

hemicellulose, sugar, fatty acids, proteins, fats some autotrophically produce methane, acetate many different bacterial genera

Bacteriodes succinogenes, and Ruminococcus albus

globally among the most important

cellulolytic rumen bacteria 

Methanobacterium ruminantium

many other methanogens 

composition varies

among different ruminants

among different parts of the world

when diet changes

ii. protozoaprimarily ciliatesobligate anaerobes105 to 106 / mLsome degrade cellulose, starch, carbohydrates

(but compared to bacteria, not quantitatively as important)

others are predators ruminant digests protozoa – thus, some protein

contribution to ruminant’s dietprobably easier for ruminant to digest than

bacteria

4. Dynamics of rumen ecosystemSudden switch from dried forage (high cellulose) to diet high in glucose or grain (lower cellulose)- death within 18 hours

 

- Streptococcus bovis explosive growth (dividing time of 20 minutes) – goes from 107 to 1010 cells/mL

 

- lactic acid produces lots of lactic acid, which can’t go through rumen wall

 

- not enough lactic acid degrading bacteria to convert lactate to VFAs

 

- pH drops 

- tissues destroyed

Gradual diet switch to high protein dietprotozoan predators keep pace with S. bovislactic acid degraders also keep pacepossible to maintain balance w/o killing ruminant

VI. Microbial Predators  Cytophaga – bacterium that eats other bacteria Many protist grazers – amoebae, cilliates, flagellates Nematode- and Rotifer-trapping fungi

- fungi that actually prey on nematodes and rotifers- fungi produce ‘traps’

adhesive hyphaeconstrictive rings

- when nematode swims through ring, ring contracts immediately by osmotic expansion, trapping the nematode- hyphae then penetrate inside nematode, degrading it enzymatically from the inside out (spider)- trap construction induced by presence of nematodes

Fungi with adhesive hyphae

Photo by B. A. Jaffee

Adhesive network

Photo by B. A. Jaffee

Constricting ring

Photo by B. A. Jaffee

Rotifer killers, Haptoglossa mirabilis

-infects rotifer host by means of a gun-shaped attack cell

-the anterior end of the cell is elongated to form a barrel

-the wall at the mouth is invaginated deep into the cell to form a bore

-walled chamber at the base of the bore houses a complex, missile-like attack apparatus

-projectile is fired from the gun cell at high speed to accomplish initial penetration of the host

-spore then develops into fungus, killing the rotifer

Electron micrograph of business end of the Haptoglossa Gun Cell. Photograph by Jane Robb, University of Guelph. 

harpoon-shaped projectile

• Epulopiscium fischelsoni • 500 m long, >100x longer than most bacteria• Originally thought to be a protist• Surface:volume problem, solved by membrane invaginations• (The large size of Epulopiscium baffles people because it seems to

break the known rules of diffusion limitation and size. However, Koch calculated with known equations dealing with diffusion limitation and cell size and found that theoretically (according to our known model), the cells could reach a 410 micrometer diameter before diffusion limitation if an extremely high substrate concentration occured (he used 0.1% glucose as the substrate concentration) (Shulz and Jorgensen 2001). Still, the actual substrate concentration the gut of surgeonfish is not known. )

• Even though Epulopiscium is not a Protista, they contain an unusual cortex which seems to be made of vesicles, capsules, and tubules, all of which are structures generally found in protists rather than bacteria. Some suggest that the vesicles play a part in excreting waste products - this, as well as an intracellular system of transport organelles, might be what allows Epulopiscium to overcome the constraints of diffusion limitation with a large cell size (Shulz and Jorgensen 2001).

Maturing daughter cells within the E. fishelsoni parent cell. A) The daughter cell is ~ 390 by 45 micrometers in size and lacks caps - decondensed DNA is dispersed evenly below the cell wall. B) The daughter cell is ~ 350 by 45 micrometers and has two caps. C) The daughter cell is ~ 350 by 45 micrometers and has a single cap. D) The daughter cell is ~ 360 by 45 micrometers and has two caps and almost completely separated DNA. E) The daughter cell is ~360 by 45 micrometers with two caps and completely separated DNA.

Thiomargarista namibiensis can grow to as large as 3/4 of a

millimeter across, which means it is visible to the

naked eye as a speck

 

Epulopiscium fischelsoni is large, but not the largest…

Epulopiscium fishelsoni cell that is 237 micrometers long. The arrows point to two apical nucleoids. From Bresler et al.

Why so big?

• Some think that the large size might correlate with its unique method of reproduction or give it a selective advantage against protozoan predation.

Diurnal Cycle

• In the mornings, E. fishelsoni cells (isolated from a surgeonfish gut, not a culture) were found to contain compact, spherical nucleoids at the apices of the cells which elongated during the day. Throughout the day, the average length of the cells increased witht he nucleoids making up a large percentage of the parent cell volume. In the late afternoons and evenings, these nucleoids reached a maximum of approximately 50 - 75% of the length of the parent cells. During the night, over 70% of the E. fishelsoni cells found in the gut contained two nucleoids; the rest of the cells were smaller and lacked incipient daughter cells. These smaller cells, which were almost always found only in early morning samples, were assumed to be the released daughter cells; the parent cells are destroyed in the process of releasing daughter cells.

• During the day when the nucleoids are elongating and the bacteria are reaching their full size, the surgeonfish would be at its most active feeding frequently and filling its gut with algal food materials. In addition, the metabolism of the E. fishelsoni at this time suppress the pH of the gut fluids. During the night when the fish would be inactive in reef shelters, the modal cell size declines and the bacteria does not suppress the pH (Bresler et. al 1998).

• It is not known if the abnormally large size of Epulopiscium fishelsoni has anything to do with its unique reproduction in which one or two daughter cells form inside of the parent cell. Metabacterium polyspora is phylogenetically related to E. fishelsoni and is thought by some to point towards the evolution of the special reproductive system of Epulopiscium. M. polyspora live in the intestines of rodants, grow to an unusual size (not as large as Epulopiscium, but unusual nontheless), and reproduce by forming two or more refractile endospores per cell (Shulz and Jorgensen 2001).