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28 Symbiotic Associations: Commensalism, Mutualism, and Normal Microbiota of the Human Body



This chapter discusses the relationships between populations of microorganisms living in close association with each other. The discussion focuses on commensalism and mutualism. Examples of each type of relationship are presented. Additionally, a monitored or gnotobiotic condition in which the identities of all microorganisms present are known is discussed. Finally, the chapter presents an overview of the normal microorganisms associated with the human body.



After reading this chapter you should be able to:

! discuss symbiosis, commensalism, and mutualism

! give some examples of endosymbiosis and ectosymbiosis

! discuss the establishment and maintenance of microbially monitored (gnotobiotic) animal colonies

! discuss the type and distribution of the microbiota (normal microflora) of the human body



I. Introduction

A. A symbiont is any microorganism that spends all or a portion of its life associated with another organism of a different species

B. Symbiosis is the living together in close physical association of two or more different organisms

1. EctosymbiosisCorganisms remain outside each other

2. EndosymbiosisCone organism is found within the other

II. Types of Symbiosis, Functions, and Examples

A. CommensalismCthe microorganism (commensal) benefits, while the host is neither harmed nor helped

1. The microorganism shares the same food source with the host

2. The microorganism is not directly dependent on the metabolism of the host

3. The microorganism causes no particular harm to host

4. ExampleCthe common nonpathogenic strain of Escherichia coli lives in the human colon; this facultative anaerobe uses oxygen creating an anaerobic environment in which obligate anaerobes (e.g. bacteroids) can grow. The bacteroids benefit but the E. coli derives no obvious benefit or harm.

B. Mutualism

1. There is some reciprocal benefit to both partners

2. The microorganism (mutualist) and host are metabolically dependent upon each other

3. Syntropism is a mutually beneficial relationship in which each organism provides one or more growth factors, nutrients or substrates for the other organism; also referred to as cross-feeding or the satellite phenomenon

4. Examples

a. Protozoan-termite relationshipCprotozoa live in the guts of insects which ingest but cannot metabolize cellulose; the protozoa secrete cellulases, which metabolize cellulose, releasing nutrients that the insects can use

b. LichensCan association between a fungus (ascomycetes) and an alga (green algae) or cyanobacteria

(1) Fungal partner (mycobiont) obtains nutrients from alga by hyphal projections (haustoria) that penetrate the algal cell wall

(2) Algal partner (phycobiont) is protected from excess light intensity and is provided with water, minerals, and a firm substratum in which it can grow protected from environmental stress

(3) Lichens have one of three characteristic morphologies

(a) CrustoseCcompact and appressed to a firm substratum

(b) FolioseCleaflike appearance

(c) FruticoseCshrubby shape

c. ZooxanthellaeCalgae harbored by marine invertebrates; reef-building (hermatypic) corals are unable to utilize zooplankton as food and rely heavily on endosymbiotic zooxanthellae; the coral pigments protect the algae from ultraviolet radiation

d. The tube worm-bacterial relationship

(1) Exist in hydrothermal vent communities where the vent fluids are anoxic, with high concentrations of hydrogen sulfide and temperatures up to 350EC

(2) The surrounding seawater reaches temperatures 10E to 20EC above the normal temperature of 2.1EC

(3) Free living and endosymbiotic chemolithotrophic bacteria provide the main energy source in the community through the oxidation of hydrogen sulfide

(4) Chemolithotrophic endosymbiotic bacteria are maintained in a specialized tissue (trophosome) of giant (less than one meter in length), red, gutless tube worms

(5) The tube worm binds hydrogen sulfide to hemoglobin and transports it to the bacteria; the bacteria use the energy from hydrogen sulfide oxidation to synthesize reduced organic material that is supplied to the tube worms

e. Rumen ectosymbiosisCbacteria in the rumen anaerobically metabolize cellulose, and then normal digestion occurs; microorganisms produce the majority of vitamins that are needed by the ruminant; methane is also produced in the process

III. Gnotobiotic AnimalsCall microbial species present are known; may even be germ-free

A. Mammalian fetuses (in utero) are free from microorganisms

B. Gnotobiotic colonies are established by cesarean-section delivery in a germfree isolator

C. They are maintained in a sterile environment

D. Normal mating and delivery of gnotobiotic animals maintains the condition

E. Gnotobiotic animals allow investigation of the interactions of animals with specific microorganisms, which are deliberately introduced

F. Gnotobiotic animals are only superficially normal and require many nutritional supplements normally provided by the normal microbiota

G. Gnotobiotic animals are unusually susceptible to pathogens but may be resistant to some diseases caused by protozoa that use bacteria as a food source (e.g. Entamoeba histolytica)

IV. Distribution of the Normal Microbiota of the Human Body

A. Reasons to acquire knowledge of normal human microbiota and its distribution

1. It provides greater insight into possible infections resulting from injury to these areas

2. It gives perspective on the possible sources and significance of microorganisms isolated from an infection site

3. It increases understanding of the causes and consequences of overgrowth of microorganisms normally absent from a specific body site

4. It aids awareness of the role these indigenous microorganisms play in stimulating the immune response that provides protection against potential pathogens

B. Skin

1. Resident microbiota multiply on or in the skin

a. They vary from one part of the body to another

b. They experience periodic drying in a slightly acidic, hypertonic environment

2. Transient microbiota are found on the skin for a short time and do not multiply there; they usually die in a few hours

3. One species, Propionibacterium acnes, is associated with acne vulgaris

C. Nose and nasopharynx

1. NoseCjust inside the nares; contains the same organisms as skin, including Staphylococcus epidermidis and S. aureus

2. NasopharynxCabove the level of the soft palate; contains nonencapsulated strains of some of the same species that may cause clinical infection; other species also are found

D. OropharynxCbetween the soft palate and upper edge of the epiglottis; houses many different species

E. Respiratory tractCno normal microbiota due to mucociliary blanket, the enzyme lysozyme in mucus, and the phagocytic action of alveolar macrophages

F. Oral cavity (mouth)Ccontains those organisms that survive mechanical removal by adhering to gums (anaerobes) and teeth (aerobes); organisms contribute to the formation of dental plaque, dental caries, gingivitis, and periodontal disease

G. EyeCaerobic commensals are found on the conjunctiva

H. External earCresembles microbiota of the skin with some fungi

I. StomachCmost microorganisms are killed by acidic conditions unless they pass through very quickly; the number of microorganisms present increases immediately after a meal, but decreases quickly

J. Small intestine

1. DuodenumCfew microorganisms present because of stomach acidity and inhibitory action of bile and pancreatic secretions

2. JejunumCEnterococcus fecalis, diphtheroids, lactobacilli, and Candida albicans

3. IleumCmicrobiota resemble that of the colon as the pH becomes more alkaline

K. Large intestine (colon)Clargest microbial population of the body

1. Over 300 different species have been isolated from human feces

2. Most are anaerobes or facultative organisms growing anaerobically

3. Ratio of anaerobes to facultatives is approximately 300:1

4. They are excreted by peristalsis, segmentation, desquamation, and movement of mucus

5. They are replaced rapidly because of their high reproductive rate

6. This is a self-balancing (self-regulating) microbial ecology

7. The balance may change with: stress, altitude, starvation, diet, parasite infection, diarrhea, use of antibiotics or probiotics (microorganisms orally administered that promote health)

L. Genitourinary tract

1. Kidneys, ureter, and bladder are normally free of microorganisms

2. MalesCa few microorganisms are found in distal portions of the urethra

3. FemalesCcomplex microbiota in a state of flux due to menstrual cycle; Döderlein=s bacilli are primarily Lactobacillus acidophilus that forms lactic acid and thereby maintains the pH of the vagina and cervical os between 4.4 and 4.6


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