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Prescott’s Microbiology, 9th Edition
32
Microbial Interactions
CHAPTER OVERVIEW
This chapter focuses on the relationships microorganisms have with other organisms and each other. The
chapter begins with a discussion of symbiotic relationships, and examples of each type of relationship are
presented. This chapter then focuses on the interactions of the human body with microorganisms. It begins by
discussing the normal microbiota—those microorganisms that have established residence in or on the body.
The relationship between humans and their normal microbiota is usually either mutualistic or commensal. On
occasion, the interaction can shift to parasitism (a pathogenic relationship). In addition, microorganisms that
are not part of the normal microbiota can be pathogenic.
LEARNING OUTCOMES
After reading this chapter you should be able to:
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compare and contrast different types of microbial interactions, including mutualism, cooperation,
commensalism, predation, parasitism, amensalism, and competition
provide at least one specific example of each of these types of interactions
explain the role of germfree mice in evaluating how microorganisms regulate health and disease
summarize the importance of the human microbiome in maintaining homeostasis
infer how data from the Human Microbiome Project might contribute to the development of new disease
treatments
report the common culturable organisms associated with various body sites
predict the environmental host conditions that favor the association of acidophilic, anaerobic, lactic acid
fermenting, and halophilic bacteria
discuss the mechanism by which normal microbiota mediates resistance to pathogens
CHAPTER OUTLINE
I.
Microbial Interactions
A. Microorganisms can be physically associated with other organisms in a number of ways; the
physical associations can be intermittent and cyclic, or they can be permanent
1. Ectosymbiosis—microorganism remains outside the other organism
2. Endosymbiosis—microorganism is found within the other organism
3. Consortium—physical interactions of different organisms of similar size; the interaction is
usually mutually beneficial
B. Mutualism
1. An association with some degree of obligation that provides some reciprocal benefit to both
partners; when separated, the individual organisms will not survive (some examples are given
below)
2. Microorganism-insect mutualisms—bacterial endosymbionts often provide essential vitamins
and amino acids to host insects, while the insect provides a secure physical habitat and ample
nutrients to the bacteria
a. Buchnera-aphid relationship—bacteria live within insect host cells as obligate mutualists
that provide the aphids with essential amino acids not found in plant sap; studied as an
example of coevolution
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in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
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Microbe-termite relationship—protozoa, bacteria, and archaea live in the guts of insects
that ingest but cannot metabolize cellulose; the microbes secrete cellulases, which
metabolize cellulose, releasing nutrients that the insects can use
3. Zooxanthellae—algae harbored by marine invertebrates; reef-building (hermatypic) corals use
zooxanthellae to satisfy most of their energy needs; the coral pigments protect the algae from
ultraviolet radiation
4. Sulfide-based mutualisms
a. Bacteria-tube worm (Riftia) systems use anoxic hydrothermal vent fluids rich in hydrogen
sulfide; endosymbiotic chemolithotrophic bacteria in specialized cells (trophosomes)
oxidize hydrogen sulfide for energy and to fix carbon supplied to the worm; hydrogen
sulfide and oxygen are brought by worm hemoglobin to the bacteria
b. Another gutless marine oligochaete (Olavius) harbors chemoautolithotrophic and sulfatereducing bacterial endosymbionts that digest waste products and provide fixed carbon
5. Methane-based mutualisms—methanotrophs are intracellular symbionts of methane-vent
mussels and sponges that use the bacteria to support their nutritional needs; methanotrophs
within sphagnum moss stems oxidize bog methane to carbon dioxide, that is then fixed by the
plant, providing for efficient carbon cycling
6. The rumen ecosystem—bacteria in the rumen anaerobically metabolize cellulose to smaller
molecules that can be digested by the ruminant; microorganisms produce the majority of
vitamins that are needed by the ruminant; methane also is produced in the process and this
contributes to global climate change
Cooperation
1. A mutually beneficial relationship that is not obligatory, distinguishing it from mutualism
2. Linkage of the carbon cycle and the sulfur cycle by the relationship of sulfide-oxidizing
autotrophic bacteria and heterotrophic organisms (e.g., other bacteria, crustaceans, nematodes,
sponges, gastropods)
3. Linkage of the carbon cycle and the nitrogen cycle by the relationship of cellulolytic microbes
and nitrogen-fixing bacteria
4. Syntrophism is an association in which the growth of one organism either depends on or is
improved by growth factors, nutrients, or substrates provided by another organism growing
nearby
5. In anaerobic environments, interspecies hydrogen transfer occurs; this involves the activities of
fermentative bacteria, which produce low-molecular-weight fatty acids; the fatty acids are
degraded by anaerobes (e.g., Syntrophobacter), producing hydrogen gas; methanogens
consume the hydrogen gas during methanogenesis, and this promotes further production of
fatty acids and hydrogen gas
Commensalism
1. A syntrophic relationship in which the microorganism (commensal) benefits, while the host is
neither harmed nor helped; in many cases, the microorganism shares the same food source with
the host
2. Can occur in situations in which waste products of one microorganism serve as the substrate
for another; nitrification requires the activity of two different species; one oxidizes ammonia to
nitrite and the other oxidizes nitrite to nitrate
3. Can occur in situations where one microorganism modifies the environment, making it better
suited for another microorganism
a. Succession of microorganisms in an environment—during milk spoilage, synthesis of
acidic fermentation products by one population stimulates proliferation of acid-tolerant
microorganisms; during biofilm formation, the first colonizer makes it possible for others
to colonize
b. Colonization of surfaces of plants and animals by normal flora—plant or animal produces
organic substances, which are used by the normal flora of the host organism
Predation
1. Predator organism engulfs or attacks a prey organism; prey can be larger or smaller than
predator; normally results in death of prey
2. Predatory microbes include the bacteria, Bdellovibrio, Vampirococcus, and Daptobacter
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in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
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Positive outcomes of predation
a. Microbial loop—microbial predators mineralize the organic matter produced by
autotrophs (primary producers) before it reaches the higher consumers; this returns
nutrients to the primary producers and promotes their activity
b. Ingestion of prey provides protective environment for the prey
Parasitism
1. One organism (parasite) benefits from another (host); there is a degree of coexistence between
the host and parasite that can shift to a pathogenic relationship (a type of predation); once a
parasite establishes the interaction with its host, over time it often loses unnecessary genetic
information in a process called genomic reduction
2. Although once considered an example of a mutualistic interaction, lichens are now believed to
be an example of a controlled parasitism between a fungus (ascomycete) and an alga (green
alga) or cyanobacterium that only occurs when the two potential partners are nutritionally
deprived
a. Fungal partner (mycobiont) obtains nutrients from alga by hyphal projections (haustoria)
that penetrate the algal cell wall, as well as oxygen for respiration
b. 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
Ammensalism—one organism releases a specific compound that harms another organism (e.g.,
antibiotics, bacteriocins, antibacterial peptides, and acidic fermentation products); attine ants are
involved in a complex amensalistic process with bacteria and fungi
Competition
1. Different organisms within a population or community try to acquire the same resources (e.g.,
nutrients, location)
2. If one of the two competing organisms can dominate, it will outgrow the other organism
(competitive exclusion principle); in some cases the competing organisms share the limiting
resource and co-exist at lower population levels
Human-Microbe Interactions
The normal microbial flora or microbiota are microorganisms normally associated with the human
body; acquisition of normal flora is an adaptive process and the composition of the flora is dynamic
The microbiome includes all of the genes of the host and its microbiota; these intimate relationships
define the superorganism, a blend of microbial and human metabolisms
Pathogenicity—the ability to produce pathological changes (disease) as the result of a parasitic
symbiosis between a microorganism and a host; a pathogen is any disease-producing microorganism
Gnotobiotic Animals
1. Gnotobiotic—an environment or animal in which all microbial species present are known or
that is germ-free (e.g., mammalian fetuses in utero are free from microorganisms)
2. Gnotobiotic animals allow investigation of the interactions of animals with specific
microorganisms that are deliberately introduced into the animal
3. Gnotobiotic animals are more susceptible to pathogens, but may be resistant to diseases caused
by protozoa that use bacteria as a food source (e.g. Entamoeba histolytica)
Normal Microbiota of the Human Body
Internal tissues are normally free of microorganisms; however, many other sites are colonized;
normal microbiota are the microorganisms regularly found at any anatomical site; while the fetus is
free of microorganisms, newborns quickly acquire microbiota from the environment
1. Reasons to acquire knowledge of normal human microbiota
a. It provides greater insight into possible infections resulting from injury to these sites
b. It increases understanding of the causes and consequences of growth by microorganisms
normally absent from a specific body site
c. It aids awareness of the role these indigenous microorganisms play in stimulating the
immune response
The relationship between normal microbiota and the host
1. Relationship with normal microbiota is usually mutually beneficial
2. Normal microbiota help repel invading pathogens by a number of mechanisms (e.g.,
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© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution
in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
3.
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competition, production of inhibitory chemicals)
Establishment of a stable microflora has been linked to the induction of immune competency
in gnotobiotic organisms
Under some conditions, normal microbiota can become pathogenic; such microorganisms are
referred to as opportunistic
Compromised host—host that is seriously debilitated and has lowered resistance; is often
target of opportunistic microorganisms
Skin
1. Resident microbiota multiply on or in the skin; transients usually are unable to multiply on
skin; most skin bacteria are found on superficial cells, colonizing dead cells, or closely
associated with oil and sweat glands
2. Skin surface varies from one part of the body to another and generally is a hostile
environment; skin surface undergoes periodic drying, is slightly acidic, is salty, and has
antibacterial substances (e.g., lysozyme and cathelicidins)
3. Staphylococcus epidermidis and corynebacteria (dry areas and sweat glands); gram-negative
bacteria (moist areas), yeast (scalp); Propionibacterium acnes is associated with acne vulgaris
and can change sweat gland secretions leading to body odor
Nose and nasopharynx
1. Nose—just inside the nares; Staphylococcus epidermidis and S. aureus are predominant; they
are also found on skin of face
2. Nasopharynx—above the level of the soft palate; contains nonencapsulated strains of some of
the same species that may cause clinical infection (e.g., streptococci and Neisseria); other
species also are found
Oropharynx—between the soft palate and upper edge of the epiglottis; houses many different
species, including staphylococci and streptococci
Respiratory tract—no normal microbiota; microbes are removed by three mechanisms: continuous
stream of mucous and the activity of ciliated epithelial cells that move the microbes out,
phagocytosis of microbes by alveolar macrophages, and the bactericidal effect of lysozyme
Eye and external ear
1. Eye—aerobic commensals are found on the conjunctiva
2. External ear—resembles microbiota of the skin; includes some fungi
Mouth
1. Contains those organisms that survive mechanical removal by adhering to surfaces such as the
gums and teeth
2. Normal microbiota include streptococci, lactobacilli, and actinomycetes
3. Some contribute to the formation of dental plaque, dental caries, gingivitis, and periodontal
disease
Stomach—microorganisms typically are killed by acidic conditions unless they pass through very
quickly or are resistant to gastric pH; a few resistant genera are known
Small intestine
1. Duodenum—few microorganisms present because of stomach acidity and inhibitory action of
bile and pancreatic secretions; those that are found are gram-positive rods and cocci
2. Jejunum—Enterococcus faecalis, diphtheroids, lactobacilli, and Candida albicans are
occasionally found
3. Ileum—microbiota resemble those of the colon (e.g., anaerobic gram-negative rods and
Enterobacteriaceae)
Large intestine (colon)
1. Largest microbial population of the body
2. Over 400 different species have been isolated from human feces; most are anaerobes or
facultative organisms growing anaerobically
3. Normal microbiota are excreted by peristalsis, segmentation, desquamation, and movement of
mucus, but are replaced rapidly because of high reproductive rate; the microbial community is
self-regulating and can be disturbed by stress, altitude, starvation, diet, parasite infection,
diarrhea, and use of antibiotics or probiotics
Genitourinary tract
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in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
1.
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Kidneys, ureter, and bladder are normally free of microorganisms; though in both males and
females a few microorganisms are found in distal portions of the urethra
Female genital tract hosts a complex microbiota in a state of flux due to menstrual cycle;
Lactobacillus acidophilus predominates; it forms lactic acid and thereby maintains an acidic
pH in the vagina
CRITICAL THINKING
1.
Explain the complex endosymbiont-tube worm relationship. Why has hydrogen sulfide been referred to
as the “sunlight” of hydrothermal vent communities?
2.
Some patients who take antibiotics for acne develop yeast infections of the mouth or urinary tract.
Explain why this may happen.
3.
Compare and contrast mutualism, commensalism, and cooperation. Do the same for predation,
competition, parasitism, and amensalism.
4.
Describe why the microenvironment of the skin is both favorable to some microorganisms and
unfavorable to others. Explain your answer.
CONCEPT MAPPING CHALLENGE
Construct a concept map using the words below; provide your own linking words.
Symbioses Phycobiont Mycobiont Mutualism Corals Syntrophobacter
Lichen Parasitism Predation Termites Bdellovibrio
Tube worms Commensalism
Human microflora Methanogens
Actinobacteria
Firmicutes
Bifidobacteria
Bacteroides
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© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution
in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.