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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 2
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 Parasitism • 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
• One organism gains (parasite) and the other is harmed (host)
• Always some co-existence 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 9
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 by acidic conditions – some survive if pass
through stomach very quickly
– some can survive if ingested in food particles
• 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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