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Chapter 30 Microbial Interactions 1 Microbial Interactions • Physical associations – ectosymbiont • organism located on surface of another organism (usually larger) – consortium • physical contact between dissimilar organisms of similar size – endosymbiont • organism located within another organism – There are also examples of hosts that have more than one symbiont associated with it 2 Figure 30.1 3 Mutualism • Some reciprocal benefit to both partners • Relationship with some degree of obligation – often partners cannot live separately • mutualist and host are dependent on each other 4 Microorganism-Insect Mutualisms • Endosymbiotic microbe provides needed vitamins and amino acids • Insect host provides secure habitat and nutrients • e.g., aphid-Buchnera aphidicola interaction • e.g., insect-Wolbachia interactions 5 e.g., protozoan-termite relationship • termite provides food for protozoan • protozoan digests cellulose in wood particles, providing nutrients for termite 6 Figure 30.2 7 e.g., zooxanthellae-marine invertebrate interactions • zooxanthellae – dinoflagellates harbored by marine invertebrates such as reef-building corals – provide organic carbon to host • coral has pigments that protect algae from UV radiation – also provide nitrogenous compounds, phosphates and CO2 to endosymbionts • coral bleaching – caused by as little as a 2o C temperature increase – results from loss of either photosynthetic pigments or expulsion of the zooxanthellae 8 Figure 30.3 9 Hydrothermal vents and Related Geological Activity Figure 30.4 10 Tube Worm-Bacterial Relationships • Exist thousands of meters below ocean surface • Chemolithotrophic bacteria live within specialized organ (trophosome) of host tube worm – fix CO2 with electrons provided by H2S 11 Figure 30.5 12 The rumen ecosystem • ruminants – animals that have stomach divided into four compartments and chew a cud • rumen – upper part of the ruminant stomach – contains large, diverse population of microbes 13 Ruminant stomach Figure 30.6 14 Ruminants • Ruminant and microbial community have a mutualistic relationship – specific interactions occur within the microbial community • In cows, acetate, CO2, and H2 are used by methanogenic archaea to generate methane (CH4), a greenhouse gas – methane released by belching 15 Cooperation • Benefits both organisms in relationship • Differs from mutualism because cooperative relationship is not obligatory • Syntrophism – growth of one organism depends on or is improved by growth factors, nutrients, or substrates provided by another organism growing nearby – also called crossfeeding or satellite phenomenon 16 Examples of cooperation OM = organic material Figure 30.7 17 Another example if any of three members is missing or removed, degradation will not take place Figure 30.8 18 More examples • Relationships between sulfideoxidizing bacteria and a variety of animals – bacterium used as food source – e.g., Pompeii worm and Palm worm – e.g., shrimp – e.g., nematodes – e.g., gastropods – e.g., sponges 19 Figure 30.9 20 Figure 30.10 21 Figure 30.11 22 Figure 30.12 (a) 23 Commensalism • One organism benefits and the other is neither harmed nor helped • Commensal – organism that benefits • Often syntrophic • Can also involve modification of environment by one organism, making it more suited for another organism 24 An Example of Commensalism • nitrification NH3NO2 NO3 – carried out by two different bacteria • e.g., Nitrosomonas carries out first step • e.g., Nitrobacter carries out second step (i.e., it benefits from its association with Nitrosomonas) 25 Another Example • Anaerobic methanogenic ecosystems – fermentative bacteria produce fatty acids – fatty acids catabolized by anaerobic bacteria (e.g., Syntrophobacter), producing H2 – methanogen uses H2 for methanogenesis • interspecies hydrogen transfer • by utilizing H2, methanogen lowers [H2], promoting activity of H2-producing bacterium 26 More examples • nonpathogenic E. coli in human colon • E. coli-Bacteroides interaction – E. coli consumes oxygen creating suitable anaerobic environment for Bacteroides 27 More Examples • Microbial succession during spoilage of milk – fermentative bacteria produce acids that promote growth of acid tolerant species • Formation of biofilms – initial colonizer makes it possible for other microorganisms to attach • Skin or surface microbes on plants or animals – host plant or animal releases volatile, soluble, and particulate organic compounds, which are used by commensals 28 Predation • Among microbes involves a predator species that attacks, usually killing its prey 29 Microbial predators • Bdellovibrio penetrates cell wall, grows outside plasma membrane • Vampirococcus epibiotic mode of attacking prey • Daptobacter penetrates prey then directly consumes the cytoplasmic contents Figure 30.13 30 Table 30.3 • microbial loop — organic matter produced by autotrophs is mineralized by microbial predators (e.g., ciliates) before reaching higher consumers — provides nutrients for primary producers 31 Parasitism • One organism gains (parasite) and the other is harmed (host) • Always some co-existence between host and parasite • Successful parasites have evolved to coexist in equilibrium with their hosts – if balance upset, host or parasite may die 32 Balance between Host and Parasite • Example – Typhus – Rickettsia typhi is causative agent • harbored in fleas, lives on rats • transmitted to humans by flea bites – is endemic within population until societal changes, e.g., war or other disruptions occur, and then becomes epidemic 33 Another example • lichens, an example of a controlled parasitism – association only occurs when organisms are nutritionally deprived – mycobiont • fungal partner • provides water, minerals, sheltered environment and firm substratum for growth – phycobiont • alga or cyanobacterium • provides organic carbon and oxygen 34 Figure 30.14 35 Genomic reduction • Outcome of long term parasitic relationship • Parasite loses unused genomic information 36 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 37 Figure 30.15 (a) and (b) 38 Competition • Occurs when two organisms try to acquire or use the same resource 39 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 40 Human-Microbe Interactions • The human body is a diverse environment – specific niches are present • The application of ecological principles will help scientists understand the many interactions that occur between the host and its normal microbial flora 41 Human-Microbe Interactions • Pathogenicity – ability to produce pathological change or disease • Pathogen – any disease-producing microorganism 42 Reasons to study normal human microbiota • To gain insight into possible infections resulting from injury • To understand causes and consequences of overgrowth of microbes normally absent from a body site • To increase awareness of role played by indigenous microbe in stimulating immune response 43 Interactions between a Host and Its Normal Flora • interactions include a broad range of symbiotic interactions including – commensalism – mutualism – parasitism • examples of both ecto- and endosymbiotic relationships are present in the host 44 Copyright © The McGraw-Hill companies, Inc. Permission required for reproduction or display. Figure 30.17 45 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 • Inhibitory substances (e.g., lysozyme, cathelicidins) 46 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 47 Nose and Nasopharynx • Staphylococcus aureus and S. epidermidis – predominant bacteria present – found just inside nostrils • Nasopharynx may contain low numbes of potentially pathogenic microbes – e.g., Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae 48 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 49 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 50 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 51 Stomach • Most microbes killed by acidic conditions – some survive if pass through stomach very quickly – some can survive if ingested in food particles 52 Small Intestine • Divided into three areas – duodenum • contains few organisms – jejunum – ileum • flora present becoming similar to that in colon • pH becomes more alkaline 53 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 54 Copyright © The McGraw-Hill companies, Inc. Permission required for reproduction or display. Figure 30.18 55 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 56 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 57