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Chapter 30
Microbial Interactions
1
Microbial Interactions
• Physical associations
– ectosymbiont
• organism located on surface of another organism
(usually larger)
– consortium
• physical contact between dissimilar organisms of
similar size
– endosymbiont
• organism located within another organism
– There are also examples of hosts that have
more than one symbiont associated with it
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Figure 30.1
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Mutualism
• Some reciprocal benefit to both
partners
• Relationship with some degree of
obligation
– often partners cannot live separately
• mutualist and host are dependent on
each other
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Microorganism-Insect Mutualisms
• Endosymbiotic microbe provides
needed vitamins and amino acids
• Insect host provides secure habitat
and nutrients
• e.g., aphid-Buchnera aphidicola
interaction
• e.g., insect-Wolbachia interactions
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e.g., protozoan-termite
relationship
• termite provides food for protozoan
• protozoan digests cellulose in wood
particles, providing nutrients for termite
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Figure 30.2
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e.g., zooxanthellae-marine invertebrate
interactions
• zooxanthellae
– dinoflagellates harbored by marine invertebrates
such as reef-building corals
– provide organic carbon to host
• coral has pigments that protect algae from UV
radiation
– also provide nitrogenous compounds, phosphates
and CO2 to endosymbionts
• coral bleaching
– caused by as little as a 2o C temperature increase
– results from loss of either photosynthetic pigments
or expulsion of the zooxanthellae
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Figure 30.3
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Hydrothermal vents and
Related Geological Activity
Figure 30.4
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Tube Worm-Bacterial
Relationships
• Exist thousands of meters below
ocean surface
• Chemolithotrophic bacteria live
within specialized organ
(trophosome) of host tube worm
– fix CO2 with electrons provided by H2S
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Figure 30.5
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The rumen ecosystem
• ruminants
– animals that have stomach divided into
four compartments and chew a cud
• rumen
– upper part of the ruminant stomach
– contains large, diverse population of
microbes
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Ruminant
stomach
Figure 30.6
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Ruminants
• Ruminant and microbial community
have a mutualistic relationship
– specific interactions occur within the
microbial community
• In cows, acetate, CO2, and H2 are used by
methanogenic archaea to generate
methane (CH4), a greenhouse gas
– methane released by belching
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Cooperation
• Benefits both organisms in relationship
• Differs from mutualism because cooperative
relationship is not obligatory
• Syntrophism
– growth of one organism depends on or is improved
by growth factors, nutrients, or substrates provided
by another organism growing nearby
– also called crossfeeding or satellite phenomenon
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Examples of
cooperation
OM =
organic
material
Figure 30.7
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Another
example
if any of three
members is
missing or
removed,
degradation will
not take place
Figure 30.8
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More examples
• Relationships between sulfideoxidizing bacteria and a variety of
animals
– bacterium used as food source
– e.g., Pompeii worm and Palm worm
– e.g., shrimp
– e.g., nematodes
– e.g., gastropods
– e.g., sponges
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Figure 30.9
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Figure 30.10
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Figure 30.11
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Figure 30.12 (a)
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Commensalism
• One organism benefits and the other is
neither harmed nor helped
• Commensal
– organism that benefits
• Often syntrophic
• Can also involve modification of
environment by one organism, making it
more suited for another organism
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An Example of Commensalism
• nitrification
NH3NO2 NO3
– carried out by two different bacteria
• e.g., Nitrosomonas carries out first step
• e.g., Nitrobacter carries out second step
(i.e., it benefits from its association with
Nitrosomonas)
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Another Example
• Anaerobic methanogenic ecosystems
– fermentative bacteria produce fatty acids
– fatty acids catabolized by anaerobic bacteria
(e.g., Syntrophobacter), producing H2
– methanogen uses H2 for methanogenesis
• interspecies hydrogen transfer
• by utilizing H2, methanogen lowers [H2],
promoting activity of H2-producing bacterium
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More examples
• nonpathogenic E. coli in human
colon
• E. coli-Bacteroides interaction
– E. coli consumes oxygen creating
suitable anaerobic environment for
Bacteroides
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More Examples
• Microbial succession during spoilage of milk
– fermentative bacteria produce acids that promote
growth of acid tolerant species
• Formation of biofilms
– initial colonizer makes it possible for other
microorganisms to attach
• Skin or surface microbes on plants or animals
– host plant or animal releases volatile, soluble, and
particulate organic compounds, which are used by
commensals
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Predation
• Among microbes involves a predator
species that attacks, usually killing
its prey
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Microbial predators
•
Bdellovibrio penetrates
cell wall, grows outside
plasma membrane
• Vampirococcus epibiotic
mode of attacking prey
• Daptobacter penetrates
prey then directly
consumes the
cytoplasmic contents
Figure 30.13
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Table 30.3
• microbial loop
— organic matter produced by autotrophs is mineralized by
microbial predators (e.g., ciliates) before reaching higher consumers
— provides nutrients for primary producers
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Parasitism
• One organism gains (parasite) and the
other is harmed (host)
• Always some co-existence between host
and parasite
• Successful parasites have evolved to coexist in equilibrium with their hosts
– if balance upset, host or parasite may die
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Balance between Host and
Parasite
• Example – Typhus
– Rickettsia typhi is causative agent
• harbored in fleas, lives on rats
• transmitted to humans by flea bites
– is endemic within population until
societal changes, e.g., war or other
disruptions occur, and then becomes
epidemic
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Another example
• lichens, an example of a controlled parasitism
– association only occurs when organisms are
nutritionally deprived
– mycobiont
• fungal partner
• provides water, minerals, sheltered
environment and firm substratum for
growth
– phycobiont
• alga or cyanobacterium
• provides organic carbon and oxygen
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Figure 30.14
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Genomic reduction
• Outcome of long term parasitic
relationship
• Parasite loses unused genomic
information
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Ammensalism
• Negative impact of one organism on another
based on release of a specific compound
• Some examples
– antibiotic production by fungi and bacteria
– use of antibiotic-producing streptomycin by ants to
control fungal parasites
– bacteriocin production by bacteria
– production of antibacterial peptides by insects and
mammals
• e.g., cecropins, defensins, and athelicidins
– production of organic acids during fermentation
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Figure 30.15 (a) and (b)
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Competition
• Occurs when two organisms try to
acquire or use the same resource
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Two possible outcomes of
competition
• One organism dominates
– competitive exclusion principal
• two organisms overlap too much in their
resource use, and one population is
excluded
• Two organisms share the resource
– both survive at lower population levels
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Human-Microbe Interactions
• The human body is a diverse
environment
– specific niches are present
• The application of ecological
principles will help scientists
understand the many interactions
that occur between the host and its
normal microbial flora
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Human-Microbe Interactions
• Pathogenicity
– ability to produce pathological change
or disease
• Pathogen
– any disease-producing microorganism
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Reasons to study normal
human microbiota
• To gain insight into possible infections
resulting from injury
• To understand causes and consequences
of overgrowth of microbes normally
absent from a body site
• To increase awareness of role played by
indigenous microbe in stimulating
immune response
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Interactions between a Host
and Its Normal Flora
• interactions include a broad range of
symbiotic interactions including
– commensalism
– mutualism
– parasitism
• examples of both ecto- and
endosymbiotic relationships are
present in the host
44
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Figure 30.17
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Skin
• Commensal microbes include both
resident and transient microbiota
• Mechanically strong barrier
• Inhospitable environment
– slightly acidic pH
– high concentration of NaCl
– many areas low in moisture
• Inhibitory substances (e.g., lysozyme,
cathelicidins)
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Acne vulgaris
• Caused in part by activities of
Propionibacterium acnes
• sebum
– fluid secreted by oil glands
– accumulates, providing hospitable
environment for P. acnes
• comedo
– plug of sebum and keratin in duct of oil
gland
– results from inflammatory response to
sebum accumulation
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Nose and Nasopharynx
• Staphylococcus aureus and S. epidermidis
– predominant bacteria present
– found just inside nostrils
• Nasopharynx may contain low numbes of
potentially pathogenic microbes
– e.g., Streptococcus pneumoniae, Neisseria
meningitidis, and Haemophilus influenzae
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Respiratory tract
• No normal microbiota
• Microbes moved by:
– continuous stream of mucous
generated by ciliated epithelial cells
– phagocytic action of alveolar
macrophages
– lysozyme in mucus
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Eye
• From birth throughout a human life,
small numbers of bacterial
commensals are found on the
conjunctiva of the eye
– the predominant bacterium is
Staphylococcus epidermidis
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Mouth
• Contains organisms that survive
mechanical removal by adhering to gums
and teeth
– contribute to formation of dental plaque,
dental caries, gingivitis, and periodontal
disease
• Within hours of birth, the oral cavity is
colonized by microorganisms from the
surrounding environment
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Stomach
• Most microbes killed by acidic
conditions
– some survive if pass through stomach
very quickly
– some can survive if ingested in food
particles
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Small Intestine
• Divided into three areas
– duodenum
• contains few organisms
– jejunum
– ileum
• flora present becoming similar to that in
colon
• pH becomes more alkaline
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Large intestine (colon)
• Largest microbial population of body
– eliminated from body by peristalsis,
desquamation, and movement of mucus
– replaced rapidly because of their high
reproductive rate
– most of the microbes present are anaerobes
– Bacteroides thetaiontaomicron
• colonizes exfoliated host cells, food particles and
sloughed mucus
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Figure 30.18
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Genitourinary tract
• Kidneys, ureter, and bladder
– normally free of microbes
• Distal portions of urethra
– few microbes found
• Female genital tract
– complex microbiota in a state of flux due to
menstrual cycle
– acid-tolerant lactobacilli predominate
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The Relationship between Normal
Microbiota and the Host
• Usually mutually beneficial
– normal microbiota often prevent colonization by
pathogens
– bacterial produces, e.g., vitamins B and K are
beneficial to the host
• Opportunistic pathogens
– members of normal microbiota that produce disease
under certain circumstances
• Compromised host
– debilitated host with lowered resistance to infection
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