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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 3

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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 5

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 6

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 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 8

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 10

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 11

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 12

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 13

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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) 15

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 16

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 18

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 19

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 20

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 21

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 22

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 23

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 24