This summary describes the various symbiotic interactions that microbes have with other
organisms in a multitude of environments. Although ‘symbiosis’ usually invokes an idea
of happy communal living, the term here is simply descriptive of microbes living with
other life, whether they benefit, cause damage to, or have no effect on their partners. The
primary focus here is on human-microbe interaction, including the human normal flora
and human pathogens.
After reading this chapter and attending lecture, you should be able to:
1. Differentiate among various types of symbiotic interactions.
2. Describe at least one example of each type of symbiotic association.
3. Understand the difference between beneficial microbial interactions and harmful
interactions with the host organism.
4. Describe the different areas of the human body colonized by normal flora, and
how normal flora benefits the host
5. Describe the different components of microbe-host interaction: adherence,
invasion, and colonization and growth
6. Understand mechanisms of action of various toxins
7. Define innate resistance and specific immunity, and define some ways our bodies
defend against pathogenic attack.
8. Understand why pathogens are virulent, how they damage the host, how they are
transmitted, and provide various examples of each.
9. Explain how immunization works.
I. GENERAL MICROBIAL INTERACTIONS
A. Symbiosis is defined as an association of two or more species of organisms.
Symbionts can exist on surface of another organism, an ectosymbiont, or within another
organism, and endosymbiont. Consortium is a more general term that describes any
physical contact between two or more organisms.
B. Mutualism is a type of symbiotic relationship where both partners benefit from the
relationship and are obligated to stay together. An example of a mutualistic relationship is
the microorganisms living in the rumen of cattle. The rumen is the upper stomach of the
animal that contains a large, diverse, ecosystem of microorganisms. These organisms
digest the cellulose from grass and provide organics and vitamins that are absorbed
through the rumen lining and into the animal’s bloodstream, and in turn are provided with a constant source of food and a good place to live. In cattle, the fermenting microbes
produce acetate, CO , 2nd H th2t are used as nutrients for methanogenic archaea. The
resulting methane is burped into the atmosphere by the cows and is a large source of
methane, the second most important greenhouse gas in concentration next to CO . Some 2
buildings now exist that pen cows to collect methane for powering vehicles and homes!
A second example of mutualism is found in deep sea hydrothermal vent ecosystems.
Here, vast populations of worms and clams grow by cultivating chemolithotrophic
sulfide-oxidizing bacteria in a special organ called a trophosome. The bacteria oxidize
H 2, fix CO i2to glucose, and provide nutrients to the animals. The animals concentrate
H 2 and O t2 keep the bacteria growing at a swift rate.
A third example of mutualism is the mycorrhizal fungi. Mycorrhizal fungi are found in
soil and form a close relationship with plant roots, sometimes even invading the plant
cells. The mycorrhizal fungi provide the plant with water and nutrients from the soil,
while the plant provides photosynthate (organic carbon produced by photosynthesis in
the plant). This is one of the most successful symbioses in the world—it is estimated that
>80% of all plants form mutualistic interactions with mycorrhizal fungi.
These relationships are positive, but not obligate, symbioses. Cooperative relationships
benefit both organisms, like mutualistic ones, but the relationship is not obligatory. Many
cooperative relationships involved syntrophy, which literally means “eating together.” In
syntrophism, the growth of one organism depends on (or is improved by) growth factors,
nutrients, or substrates that are provided by another organism growing very closely
nearby. Syntrophy is also called cross feeding or the “satellite phenomenon” as the waste
products of one organism feed the next organism or otherwise stimulates its growth.
An example of cooperation is in another hydrothermal vent worm called Alvinella
pompejana. The bacteria attach themselves to the worm’s body in long filaments to gain
access to nutrients coming from the vents. In return, the bacteria protect the worm form
high levels of toxic metals and heat. Shrimp growing in hydrothermal vents also use these
sulfur-oxidizing chemolithotrophs as a food source – the bacteria fix carbon into sugar
for the shrimp, and the shrimp provides a growth surface and access to nutrients from the
D. Pathogenesis and parasitism
In a parasitic relationship, one organism gains (parasite) and the other is harmed (host).
This relationship can also be called pathogenic when the parasite is a microorganism and
causes disease in the host. Successful parasites have evolved to co-exist in equilibrium
with their hosts, such that if the balance between the organisms is upset, the host or
parasite or both may die.
One example of a pathogenic relationship is crown gall disease of plants. The organism
Agrobacterium spp. (either tumefaciens or rhizogenes) is the causative agent of this disease, which is essentially a cancerous growth on the stem or root of a plant. The
Agrobacterium has a plasmid, known as the T plasiid. The T plasmid iarries
virulence (vir) genes that are involved in recognition of phenolic compounds released by
plant wounds (virG) and attachment as well as genes for opine catabolism (opines are a
class of low molecular weight organic molecules). It also has the T-DNA, a section of
DNA encoding oncogenes (genes for uncontrolled growth) and opine synthesis genes.
The T-DNA is directly transferred from the Agrobacterium into the plant cell through a
pore made by the virB gene product. The T-DNA then is inserted into the plant genome,
where the oncogenes lead to tumorigenesis (creating many cells that have the T-DNA)
and synthesis of opines, which can be catabolized by the invading Agrobacterium.
More examples of pathogenic relationships are given below.
II. HUMAN MICROBE INTERACTIONS
The human body is a diverse environment that encompasses several specific niches.
Application of the symbiotic relationships discussed above helps us understand the many
interactions that occur between us and our normal microbial flora and how pathogens can
invade us to cause disease.
A. Pathogenicity is defined as the ability of an organism to produce pathological change
or disease. A pathogen is any disease-causing organism, which in most cases is a
microorganism or virus. Opportunistic pathogens are those that cause disease only in
people who lack normal host resistance responses. For example, AIDS victims are
highly susceptible to certain fungal infections that would otherwise be destroyed by a
normally functioning immune system. Virulence is the quantitative measure of
pathogenicity. The more virulent a pathogen, the more easily they cause disease.
This is measured by the LD-50, or the number of bacteria required to kill 50% of a
group of infected animals in a test group. The lower the LD-50 is, the more virulent
the pathogen is. In many highly virulent organisms, there is little difference between
the LD-50 and the LD-100 (i.e. the number of pathogens needed to kill all test
animals). In less virulent organisms, there is a larger gap between these two
numbers. Infections occur when any microorganism is established and growing.
Many infections are completely natural, normal, and beneficial. Thus, an infection
per se is not synonymous with disease as only some microbes that colonize the
human body are pathogens. As mentioned above, pathogens can be classified as
C. Interactions between humans and their normal flora
Normal human flora is the collections of microorganisms that have commensal,
mutualistic, or parasitic relationships with us. As described below, our bodies harbor both
ectosymbionts (those on our skin surface) and endosymbionts (those within our bodies).
The average adult human has ~10 bacteria and archaea living in or on their bodies (and
has about 10 human cells!). Studies of our normal microbial flora has allowed us to
gain insight into possible infections resulting from bodily injury, the causes and
consequences of massive growth of microbes where they otherwise shouldn’t grow, and the awareness that human microbial symbionts are extremely important to establishing
and maintaining our immune responses to pathogens.
Normal flora is primarily associated with mucous membranes, regions of the body
that interact directly with the environment and produce mucous.
D. Microbes in and on the body
Microbes in the mouth survive mechanical removal by adhering to gums and teeth by
attaching to glycoproteins deposited by saliva. The acid produced by microbes, especially
upon eating a high sugar diet, contributes to formation of dental plaque, dental caries,
gingivitis, and periodontal disease. The acid is produced by fermentation of sugars to
lactic acid and results in decalcification of enamel, eventually resulting in dental caries.
The sugar is also used by some microorganisms to make dextran, a strong, sticky
material that aids in attachment and serves as a food source. Intriguingly, the oral cavity
is colonized by microorganisms from the surrounding environment within hours
following birth. Products are under study that would be applied to an infant’s mouth soon
after birth that contain commensal microbes that prevent colonization by high acid
producers. It is thought that this treatment may prevent dental diseases altogether!
In the gastrointestinal tract, most microbes are quickly killed by acidic conditions in the
stomach, or they pass through very quickly. Organisms can also be shielded from the
harsh conditions of the stomach by living in or on food particles. Likewise, few
organisms are found in the small intestine, although the ileum (the region between small
and large intestine) has microbial flora that begin to resemble those in the large intestine.
Of all the niches in the human body, the large intestine has the richest diversity and
greatest population. The elimination of these microbes occurs frequently by peristalsis,
desquamation, and mov11ent of mucus through the GI tract. About 7.5% of fecal matter
is bacterial cells (10 cells per mL!). However, since these organisms grow so rapidly
due to the nutrient rich environment, they replace themselves as quickly as they are
Most of the large intestine microbes are anaerobes, such as Bacteroides
thetaiontaomicron. This bacterium is vital to our digestive system as it consumes
complex carbohydrates into simple carbohydrates (e.g. glucose) for absorption into the
bloodstream. The bacteria adhering to the particles in the gut, e.g. exfoliated host cells,
food particles, mucous, are rapidly eliminated. Aside from Bacteroides, other microbes
called Firmicutes also degrade complex carbohydrates, but with far greater efficiency
(i.e. they release more sugar per g of digested food). In other words, more calories are
extracted from the identical amount of food by Firmicutes than by Bacteroidetes. Obese
people tend to have less Bacteroides and more Firmicutes in their guts than slim people.
When these obese people lose weight, their Bacteroides populations increase. Similar
results were obtained with controlling the ratios of Bacteroides to Firmicutes in guts of
mice. These studies show how important our microbial flora is for controlling such vital
processes as sugar release from our diets!
Microbial flora of the healthy human body is in balance with us and provides a mutually
beneficial relationship, such as providing us with growth factors like vitamins B and K. Our microbial flora help prevent colonization of our bodies by opportunistic pathogens.
For example, the microbes in our mouths prevent the colonization of organisms like
Streptococcus due to competition for resources. But when our immune systems are
challenged, then these opportunistic pathogens can flourish and we succumb to diseases
like strep throat. Whenever the host organism (e.g. us) is in anyway compromised by an
autoimmune disease, an illness, or an injury, this is when we’re most susceptible