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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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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