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Microbial
Interactions
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Microbial
Interactions…
• Symbiosis = an association
of two or more different
species of organisms
• relationships can be
intermittent and cyclic or
permanent
• Types of interactions
include
– mutualism, cooperation,
predation,
commensalism,
parasitism, amensalism,
and competition
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Mutualism
• Some reciprocal benefit to both partners
• Relationship with some degree of obligation
– partners cannot live separately
• Mutualist and host are dependent on each
other
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Commensalism
• One organism benefits and the other is
neither harmed nor helped (neutral)
• Commensal - organism that benefits
• Often syntrophic -
growth of one organism depends on or
is improved by growth factors, nutrients, or substrates provided
by another organism growing nearby
• Can also involve modification of environment
by one organism, making it more suited for
another organism
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Examples of
Commensalism
• Microbial succession during spoilage of milk
– fermenting bacteria promote growth of acid tolerant species
• Formation of biofilms
– initial colonizer helps other microorganisms attach
• Skin or surface microbes on plants or animals
– host plant or animal releases volatile, soluble, and
particulate organic compounds used by commensals
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Cooperation
• Like commensalism, a positive (not obligate)
symbiosis which involves syntrophic (one
organism lives off the byproducts of another)
relationships
• Benefits both organisms in relationship
• Differs from mutualism because
cooperative relationship is not obligatory
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Predation
• Among microbes
involves a predator
species that attacks,
usually killing its prey
• Bdellovibrio penetrates
cell wall, grows outside
plasma membrane
• Benefits by providing
nutrients for primary
producers
Parasitism
• One organism gains
(parasite) and the other
is harmed (host)
• Always some coexistence between host
and parasite
• Successful parasites
have evolved to co-exist
in equilibrium with their
hosts
– if balance upset, host
or parasite may die
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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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Competition
• Occurs when two organisms try to acquire or
use the same resource
• 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
– dynamic relationships exist
• Microbiome
– all the genes of the host and the microbiota
– goal is to determine the impact that microbial gene function
has on human health
• Pathogenicity
– ability to produce pathological change or disease
• Pathogen
– any disease-producing microorganism
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Normal Microbiota
of the Human Body
• Normal microbiota or microflora
– microbes regularly found at an anatomical site
• Relationship begins at birth
– varies with environment and food source
– Bifidobacteria
• found in breast fed babies
• protrophic – can synthesize all amino acids and
growth factors from simple carbohydrates
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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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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
– constant sloughing of skin cells
• 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 numbers of
potentially pathogenic microbes
– e.g., Streptococcus pneumoniae, Neisseria
meningitidis, and Haemophilus influenzae
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Oropharynx
• Division of the pharynx lying between the soft
palate and the upper edge of the epiglottis
– alpha-hemolytic streptococci
– diphtheroids
– Gram-negative cocci
– anaerobes in tonsillar crypts
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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 and External Ear
• 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
• External ear
– similar to skin flora as well as fungi
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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
Small Intestine
• Most microbes killed • Divided into three
areas
by acidic conditions
– some survive if pass
through stomach
very quickly
– duodenum
– some can survive if
ingested in food
particles
– jejunum
• contains few
organisms
– 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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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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