Microbial Interactions 32 Microbial Interactions Copyright © McGraw-Hill Global Education Holdings, LLC. Permission required for reproduction or display.

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

Eye and External Ear Eye External ear 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

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

Stomach Small Intestine Divided into three areas duodenum contains few organisms jejunum ileum flora present becoming similar to that in colon pH becomes more alkaline Most microbes killed by acidic conditions some survive if pass through stomach very quickly some can survive if ingested in food particles

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

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