chapter 17

An equally valuable contribution of the rumen microorganisms is their ability to convert the nitrogen compounds urea and ammonia ___________.

(NH3) into amino acids

This is due to the rapid fermentation of readily digested carbohydrates, leading to the overproduction of organic acids. Negative impacts of this process include the following:

-Increased acid is absorbed across the rumen wall into the bloodstream, exceeding the buffering capacity of blood and other body fluids, which can lead to death. -Released acids exceed the absorption capacity of the rumen wall. The rumen begins to acidify below its normal pH of 6 to 7. This encourages the growth of acid-tolerant lactic acid bacteria such as Streptococcus bovis. Under these conditions, S. bovis grows quickly and produces large quantities of lactic acid, further acidifying the rumen.

How does this relationship benefit humans?

-Some microbes produce vitamins that we absorb (Vitamin K). -Some microbes assist in digestion of materials in food prior to absorption. -Proper immune function is also a benefit of their presence. -Intestinal microbes can crowd out pathogenic microbes. -More benign members of the intestinal flora can also prime the immune system, keeping it ready in case we ingest a more dangerous microbe related to our normal intestinal flora.

All primary endosymbionts of insects have a number of common characteristics that are evidence of evolutionary events that occur during co-speciation. These characteristics include the following:

-They have highly reduced genomes. -They are found exclusively in specialized host cells, called bacteriocytes. -They are maternally transmitted through generations from mother to offspring. -They are required by the host for survival and/or fertility.

This arrangement allows ruminants to benefit nutritionally from their symbionts in two ways:

-They receive fermentation and other products produced by the microbes, such as vitamins. -They digest rumen microbes as a proportion of them pass into the stomach and intestine along with the fermented slurry. Hindgut fermenters cannot digest their own intestinal microbes and so miss this potential source of nutrition.

A lichen is composed of two organisms, one a _____ and the other a _____. 1. mycobiont;photobiont 2. mycobiont;autobiont 3. phytobiont;photobiont 4. phytobiont;autobiont

1

The Human Microbe Project has shown: 1. individualseachhaveuniquemicrobial species compositions 2. individualswithinafamilyhavesimilar microbial species compositions that are different from other families 3. allhumanshavebasicallythesamemicrobial species composition 4. humanmicrobialspeciescompositionvary geographically

1

formation of root nodules

1) A free-living, potential symbiotic bacterium is attracted to the root hair. 2) The first visible outcome of this association is curling of the root hair around the cell to form a structure called a "shepherd's crook" 3)Attachment induces a series of changes in the root and the bacterium, producing a shepherd's crook and a plant-wall-bound infection thread from which the replicating bacteria enter the root cells to form an enlarged root nodule. 4)Infection thread extends into root cortex as root cells divide 5) Infection thread branches; bacteria enter cells, lose thier cell was and become N2-fixing bacteroids 6) Nodule consisting of plant cells in stimulated to grow by infecting bacteriods

As unpalatable as it seems, coprophagia serves several important purposes for these animals:

1)Microbes in feces are a source of nutrients, especially nitrogenous compounds like amino acids, which may be lacking in a plant-based diet. 2)Ingestion by young herbivores of their mother's or herd mates' feces quickly provides a mature community of intestinal symbiotic microbiota, allowing them to utilize a diet of plants. 3)Feces passed the first time is rich in partially digested food and water and consuming it a second time provides more calories and an extra opportunity for water and nutrient extraction. This second feces is then expelled as a hard, dry pellet

Nodule metabolism

1)Sucrose(product of carbon fixation is transported from the leaves to the roots, but this is not utilized by bacteria 2) Bacteriautilize C4-dicarboxylic acids from plants to use in the TCA cycle 3)Inexchange for C4-dicarboxylic acids bacteria provide the plants with nitrogen in as usable form (Ammonia/ Amino acids)

The relationship between a plant host and Asperisporium caricae, a recognized plant pathogen, is an example of 1. mutualism 2. parasitism 3. commensalism 4. endosymbiosis

2

Most human symbiotic organisms are found on the skin. 1. True 2. False

2. False

A long-standing and intimate relationship between species where one species lives within the body tissues of a host is called 1. mutualism 2. parasitism 3. commensalism 4. endosymbiosis

3

cecal fermentation

A cecal fermentation system is really just an elaboration of the colonic fermentation system (see Figure 17.20). To accommodate cecal fermentation, the cecum is greatly enlarged to become a fermentation vessel for extracting nutrition from plant material.

bacteroids

A form of rhizobium contained within the vesicles formed by the root cells of a root nodule.

leghemoglobin

An O2-binding protein produced in root nodules by the plant cells, which protects the nitrogenase enzyme of bacteroids from O2− inactivation

Symbiotic N2 fixation

Association between Rhizobia bacteria and a legume

Symbionts of herbivores

Both cellulose and starch are carbohydrates made by plants, composed of units of glucose. Starch is an energy storage polymer with glucose units joined by α-1,4-glycosidic linkages, which are easily hydrolyzed by the enzyme amylase produced by animals. Cellulose is an extracellular structural polymer with glucose units linked by β-1,4-glycosidic linkages. The tight packing of linear, not helical, molecules contributes to their strength but impedes their enzymatic digestion. With a few exceptions, animals do not possess the enzyme cellulase to hydrolyze this bond.

Contributions of rumen symbionts in cellulose conversion

Cellulose, or similar polymers, are degraded into glucose by rumen microbes. Glucose is taken up by microbes and fermented. The end products of some microbes are converted by others.

What are the roles of symbionts in herbivorous vertebrates?

Herbivorous vertebrates, like cattle, zebras, geese, and rabbits, depend on the consumption of leafy plants for their energy needs. However, like humans, they do not possess the genes that produce the cellulase enzymes necessary to break down cellulose, the major material of plants. Cellulose, like starch, is composed of long chains of glucose molecules. Starch consists of glucose units joined by α-1,4-glycosidic linkages that are easily hydrolyzed by the enzyme amylase produced by many animals, whereas cellulose is composed of β-1,4-glycosidic linkages that require cellulase for hydrolysis (

Both the cecal and colonic fermentation systems are referred to as hindgut fermentation systems. ___________________relies on host digestive processes in the stomach and small intestine first, and microbial fermentation processes last. The fermentation products of the cecum and colon that are not used by the microbes themselves are then available to be absorbed by the host, along with microbial products such as vitamins.

Hindgut fermentation

Endophytes:

However, microbes are not limited to life on the outside of plants. Some actually live within the tissues of plants. These organisms are called endophytes, a specific term that refers to plant endosymbionts. Modern molecular tools confirm that a large number of different types of bacteria and fungi live within the tissues of various plant species.

Secondary Endosymbionts

Many insect species harbor one or sometimes two primary endosymbionts, plus one or several other secondary endosymbionts. It is possible that some secondary endosymbionts may eventually become primary endosymbionts.

These products are quickly further metabolized by other rumen microbes :

Methanogens can reduce excess acetic acid to methane (CH4) and convert CO2 and H2 into CH4. -You may recall from our investigation of fermentation in the human colon, the removal of H2 by methanogens helps fermentation efficiency by keeping this rate-limiting end product in low concentration.

Symbionts of herbivores: Rumen fermentation

Microbes in the host break down food first, then the hosts absorb fermentation products and microbes (foregut fermentation) • Can digest the rumen microbes as secondary form of nutrition

Biofilm of the intestinal mucus

Most colonizing symbiotic microorganisms are not attached to the epithelial cells lining the lumen of the intestine. Instead, they form a biofilm on the outer edge of the mucous layer that extends into the lumen of the intestine. Colonization of the epithelium would result in infection and subsequent inflammation and damage. The ability to colonize and/or invade the intestinal epithelium is largely a feature of specialized intestinal pathogens.

oral cavities

Most of the research conducted on oral cavity microbiota has focused on pathogens, particularly those involved in dental diseases. Almost all oral microbiota adhere to surfaces in some way. Many readily adhere to other cells, leading to biofilm formation . Most of us are familiar with plaque biofilm -Plaque formation and cavities are two negative outcomes of the activities of our oral symbiotic bacteria. -Plaque buildup is a major cause of periodontal disease that can result in a loss of gum and bone tissue, infection, and eventual tooth loss. Gingivitis, or inflammation of gingiva, is usually the first sign of trouble. When plaque below the gingiva builds up, bacterial components, such as enzymes and cell wall material, cause inflammation and damage to the epithelial cells surrounding the tooth.

Microbial symbionts of plants

Most plants have symbiotic relationships with soil microbes. • Endophytes: Plant endosymbionts. • Rhizobia: Endophytes that can infect plant root cells, fixing nitrogen and forming root nodules.

Bacteroids can fix____________________ protects nitrogenase from the inhibitory effects of free O2, and the cells are given the necessary metabolic resources from the plant. The ability to fix N2 is coupled with an inability to divide further; bacteroids cannot proliferate.

N2 because leghemoglobin

. During the differentiation process, the bacteroids become larger, lose their flagella, modify their cell wall, and sometimes change from rod shaped to multilobed. The fully differentiated ________________are unable to divide; they are terminally differentiated.

N2-fixing bacteroids

As mentioned earlier, termites need nitrogen compounds for biosynthesis, and these are not found in their cellulose-rich diet. __________________can convert N2 into nitrogenous compounds, such as ammonia, for termite use. These bacteria utilize the acetate released from protozoan fermentation as an energy source for this process. ____________________are also contributed by fermentative bacteria that break down the termite waste product uric acid and convert it to ammonia, which can be released to the termite. Whereas released ammonia can be directly assimilated by the termite, the symbionts can convert the ammonia into other nitrogenous compounds, such as amino acids for their own growth

Nitrogen-fixing bacteria Nitrogen compounds

mutualism, commensalism, parasitism

Parasitism: One species does obvious harm to the other. • Commensalism: One species benefits, but nothing happens to the other species. • Mutualism: Both species are getting some benefit from the arrangement.

Legumes

Plants with seed pods

Studies that compare structure, biochemistry, and genetic information of related modern insect species and their symbionts can be used to trace the evolutionary origins of the symbiosis events. Based on these comparisons, scientists group endosymbionts into two major types. ______________________have evolved along with their hosts over millennia; secondary endosymbionts are more recently acquired. As we will see, the nature of the relationship between a host and its_______________________is quite different from the relationship between a host and its secondary endosymbionts.

Primary endosymbionts primary endosymbionts

Primary Endosymbionts

Primary endosymbionts show evidence of co-speciation with their hosts, whereby an ancestral host/microbe association subsequently diversified into a number of different but related host-symbiont species. Almost all species of modern-day aphids are associated with primary endosymbionts belonging to the genus Buchnera. The few aphid species that lack a Buchnera species have a yeast primary endosymbiont instead. The aphid-Buchnera symbioses that we see today appear to have arisen from a single event approximately 200 million years ago. Development of symbiosis and diversification of both host and bacteria is thought to have evolved by the following scenario.

Are probiotics really good for you?

Probiotics are live microbes that, when ingested, may provide a beneficial effect to the human body. Results of studies on their effectiveness have varied widely. • They may be beneficial in lactose intolerance, antibiotic-induced diarrhea, and childhood illness-related diarrhea. • But can a small addition to a very large number already in the intestines actually have an effect? • Can they survive in the stomach to even reach the intestines?

rumen fermentation

Rumen fermentation provides a more efficient way for herbivores to extract nutrients from plant material.

Ruminants benefit from this microbial activity in several ways, including:

Supplying cellulolytic microbes with these nitrogen compounds facilitates their growth increasing the digestion of cellulose and enhancing fermentation. This increases the harvest of fermentation products by the host. Digesting rumen microbes provides a source of protein for the ruminant. This allows ruminants to exist on poor-quality food, such as grass, which is low in protein and amino acids.

What are the roles of symbionts in invertebrates?

Symbiotic microbial relationships occur widely within invertebrates. Housing for these symbionts ranges from colonization in the gut lumen to the formation of specialized cells, and even organs, whose exclusive function is cultivating endosymbionts in the host.

_____________________ between termites, their protozoa, and bacteria are quite different from those found in herbivores. The major fermenting microbes in the termite gut are protozoa, not bacteria. Wood particles are first chewed into small pieces by the termite. Termites are one of a very few animals capable of producing their own cellulase. ___________________is secreted by the termite salivary glands and attaches to the ingested cellulose particles. These move through the midgut and into the anaerobic hindgut where most of the microbial metabolic action occurs.

Symbiotic relationships Cellulase

The microbiota of skin varies locally, depending on: • Oiliness • Sweatiness • Hairiness • Environmental exposure T/F

T

Process Diagram: Metabolic activities of symbiotic microbes of lower termites

Termite salivary glands produce cellulase that coats chewed cellulose particles. The particles move into the anaerobic hindgut (size exaggerated) and are engulfed by protozoa. This begins the protozoan conversion of cellulose to acetate, CO2, and H2. Acetate is an energy and carbon source for the termite. H2 and CO2 are converted by bacterial symbionts to more acetate, and to CH4 by archaeons. Nitrogen compounds are synthesized by bacteria for termite and protozoan use.

Termites

Termites fall into two evolutionary groups. "Higher" termites primarily feed on plant materials found in soil or on fungus. "Lower" termites feed primarily on wood.

mycobiont

The fungal component of a lichen

______________The total of all microbes living on or in the human body

The human microbiome

Symbiotic N2 fixation

The nitrogenase enzyme, found only in bacteria and archaea, is irreversibly inactivated by O2. Nitrogen-fixing organisms can therefore only carry out this process under anoxic conditions, or they require some way to prevent the detrimental effects of oxygen on the nitrogen fixation pathway.

Vagina

The vagina is in contact with skin and is situated near the anus. As a result, it is not uncommon to find bacteria present from both surfaces. -Favorable conditions for the growth of Candida include a moist environment and a near neutral pH. The composition of vaginal microbiota is affected by sexual practices, diabetes, antibiotic treatment, age, and hormone levels that affect pH.

Secondary endosymbionts show much more diversity in terms of their association with the host. Their characteristics include the following:

They are more recently acquired and do not show evidence of co-speciation with their host. They can sometimes be found in the environment as free-living forms. They are not restricted to bacteriocytes and can sometimes be found in a variety of cells. They may live extracellularly in the insect gut or the body cavity. They are not always present or essential. They are not always maternally transmitted.

What roles do microbial symbionts play in plants?

We have defined a symbiont as an organism that has evolved a long-standing and intimate relationship with another species. Not all relationships are symbiotic because many microbes can form transient and loose associations with other species, including plants. Soil bacteria can obtain numerous advantages by living near, on, or in living plants. As we explored in Section 15.4, saprophytic soil microbes can convert dead plant material to organic nutrients and form loose but important relationships with plants. These microbes take advantage of dead plant material, and nearby living plants use the resulting organic nutrients for growth.

Why would intestinal microbes be needed for proper immune function?

When these molecules come in contact with immune cells, they can stimulate an immune response that can also protect against related pathogenic strains. This important routine contact stimulates immune responses on a continuous basis, keeping it "primed" and ready for rapid defense against an invading pathogenic microorganism. In this way, commonly benign normal microbiota provide practice for building immune defenses.

The digestive tract is a continuous tube that goes from mouth to anus. Compared to the digestive tract in many other species, like ruminants, the human digestive tract is quite simple. The major organs consist of the stomach, the small intestine (consisting of the duodenum, jejunum, and ileum), _____________________. The stomach is a single large chamber, where food is hydrolyzed by hydrochloric acid. The pH of the stomach is approximately 2, and few microbes can withstand this environment. For many intestinal pathogens like Salmonella, Vibrio, and most E. coli strains, ingestion of 10,000 or more bacterial cells is needed to cause disease.

and colon, or large intestine

Once inside the cell, the bacteria differentiate, gain the ability to fix N2, and are now termed_______________. Even though the bacteroids are within the plant host cell, they remain separated from the plant cytoplasm by the surrounding plant cell membrane.

bacteroids

Some types of endosymbiotic nitrogen-fixing bacteria differentiate into specialized cell types, called____________________. These bacteroids use their relationship with the plant to solve the problems of both high-energy requirements and oxygen toxicity by inducing the plant root to produce a protective structure called a ________________, in which the plant provides a continual supply of nutrients to the bacteria and shields them from high concentrations of O2

bacteroids root nodule

. Although colonic fermentation works well for diets that are primarily digestible by the host alone, it is not sufficient for herbivores. To extract nutrients, primarily short-chain fatty acids, microbial fermentation is necessary. The two basic templates for accommodating fermentation in herbivores are ___________________and_______________________.

cecal fermentation rumen fermentation

In cecal fermentation systems, the _______________is the major site of fermentation (see Figure 17.20B). The _______________ is a major digestive organ found in a group of herbivores, called ruminants (see Figure 17.20C). In rumen fermentation, the primary site of fermentation is the rumen

cecum rumen

In cecal fermentation

cecum in major site of fermentation

• Carnivores and omnivore

colon is main fermentation chamber

However, the bacterial symbionts must have oxygen for the tricarboxylic acid (TCA) cycle and electron transport system, which they use to generate energy from carbon nutrients that are provided by the plant. In this metabolic process, leghemoglobin serves a dual purpose:

delivering a steady supply of O2 to the membrane-associated electron transport system while keeping it away from nitrogenase. In doing this, leghemoglobin gives the interior of the nodule its characteristic red color

Rumen: Microbes in the host break down food first, then the hosts absorb fermentation products and microbes (___________________) • Can digest the rumen microbes as secondary form of nutrition

foregut fermentation

In general, experiments using germ-free and___________________animals have demonstrated that symbiotic microbes provide nutritional, immunological, and developmental benefits. _____________ animals are born and raised under completely sterile conditions. They have no symbionts or any other microbial inhabitants. Gnotobiotic animals are initially germ free, but are introduced to a few select or known (gnotos is Greek for "known") species for controlled study.

gnotobiotic Germ-free

symbiont

has developed a longstanding relationship with another organism (symbiosis).

Lichens

have been classified as individual species, each one is actually a composite organism, composed of two or three different microorganisms.

Cecal: Host digestive enzymes used to breakdown food prior to absorption (_______________________) • Some cecal fermenters practice coprophagia (_________________) to increase nutrient uptake

hindgut fermentation consumption of feces

The__________________begins with an invagination of the plant cell wall that forms a narrow tube called the infection thread (Figure 17.9). As the infection thread lengthens, the bacteria divide so that the infection thread becomes filled with dividing cells.

infection process

Inside the root nodule, oxygen, even at low concentration, must be prevented from reaching the nitrogenase enzyme. This is managed through the use of a plant-derived hemoprotein called ________________

leghemoglobin

Throughout recorded human history, cultivation of legume plants has been used to enrich agricultural soils, and crop rotation involving ___________________ is a long-standing practice for maintaining soil fertility.

legumes

Ectosymbionts

live on the surface of the host. • These do not have to be single-celled organisms!

Endosymbionts

live within tissues or cells of the host.

root nodule

lump on a plant root that contains nitrogen-fixing bacteria

If primary endosymbionts are not found free-living in the environment, how do insects acquire them? This cannot be left to chance occurrence since survival of the insect species requires possession of the primary endosymbionts. In most cases, the insects inherit them from their mother, a process termed _________________________. For aphids, which can give birth to live offspring, Buchnera migrate from bacteriocytes and infect the embryo as it develops inside the ovary

maternal transmission

The hydrogen and CO2 that are waste products of the protozoa are captured by methanogenic archaea. They use hydrogen as an electron donor to convert CO2 into _________________. Recall that methanogens increase fermentation efficiency by removing the inhibiting _________________. This results in the production of more acetate, and therefore more energy for the termite. As in the rumen, some termite gut bacteria carry out anaerobic acetogenesis by converting CO2 and H2 into acetate.

methane gas (CH4). end product H2

3 types of symbiotic relationships

mutualism, commensalism, parasitism

Types of Lichens

mycobiont photobiont

Rhizobia

nitrogen fixing bacteria

Legume plants of the family Leguminosae, such as beans, peas, alfalfa, clover, and peanuts, are notable for their ability to form root nodules harboring bacteria collectively known as____________ The majority of rhizobia are from the Rhizobiales order of the α-Proteobacteria, and some are from the β-Proteobacteria.

rhizobia.

In ruminants (a group of herbivores)

rumen is major site of fermentation

photobiont

the photosynthetic component of a lichen

All corals possess photosynthetic algal endosymbionts, called __________________________, which are mutualistic partners of the coral polyps. The majority of zooxanthellae are dinoflagellates of a single genus Symbiodunium. In the coral, they are contained within specialized intracellular vacuoles, called "symbiosomes" found in the cells surrounding the gut

zooxanthellae

Symbionts of invertebrates: Secondary endosymbionts

• Can sometimes be found free-living • Aren't restricted to bacteriocytes • May live extracellularly in insect gut or body cavity • Not always present or essential • Not always maternally transmitted

Symbionts of Humans: Digestive Tract

• Could include between 800 and 2,500 (or more) bacteria • Most likely also includes a variety of bacteriophages • Anatomy/diet (and other factors) lead to dynamic bacterial population changes moving along the length of the tract

Cellulose diets don't provide nitrogen. How do termites get it? Symbiotic relationships with nitrogen fixers!

• Even more complex....Protozoa intermites provide fermentation products to bacterial nitrogen fixers— a triangle of symbiosis

Termites

• Higher termites- feed on plant materials in soil or fungus • Lower termites- feed on wood • Both depend on microbes in their gut to break down cellulose material

Symbionts of humans: Digestive tract • Do these microbes attach to the human tissues?

• It doesn't appear so (except for pathogenic microbes). • Instead, they seem to form a biofilm along the mucus that lines the interior of the intestines.

Symbionts of humans: Skin

• Patterns of bacteria types found on skin are still emerging • Swabbing the skin is a commonly used sampling method, but what are we missing? • Pores, hair follicles and sebaceous glands can form microbe habitats • By crowding out harmful competitors, these microbes may help with disease prevention

Nitrogen cycling between microbes and ruminant host

• Rumen microorganisms are able to convert nitrogen compounds (Ammonia and urea) into amino acids • Ruminants the n digest the microorganisms and amino acids are absorbed by the cow

Common areas with symbionts include

• Skin • Vagina • Oral cavities • Digestive tract

Shipworms: "Termites of the sea

• There are 65 different marine species known for eating timber hulls of ships and wood pilings of wharves • They are actually bivalve mollusks related to clams • Cellulose-digesting bacteria in specialized organs on the gills secrete cellulases, running with the ingested wood into the digestive tract • Nitrogen is obtained from nitrogen-fixing endosymbionts (as in termites)

Primary endosymbionts: exhibit co-speciation

• They have highly reduced genomes. • They are found exclusively in specialized host cells, called bacteriocytes. • They are maternally transmitted through generations from mother to offspring. • They are required by the host for survival and/or fertility.

Symbionts of humans

• Vertebrates don't harbor endosymbionts • Many microorganisms colonize internal body cavities and outer surfaces