BIOL371 Microbiology Lecture 15 PDF

BIOL371: Microbiology Lecture 15 – Microbial symbioses with microbes, plants, and animals 1 Topics of today 1. 2. 3. 4. 5. Symbioses between microorganisms Plants as microbial habitats Insects as microbial habitats Other vertebrates as microbial habitats Mammalian gut systems as microbial habit...

BIOL371: Microbiology Lecture 15 – Microbial symbioses with microbes, plants, and animals 1 Topics of today 1. 2. 3. 4. 5. Symbioses between microorganisms Plants as microbial habitats Insects as microbial habitats Other vertebrates as microbial habitats Mammalian gut systems as microbial habitats Materials covered:  Chapters 23.1-23.5, 23.7-23.10, 23.14, 23.15  Figures 23.1, 23.5-23.12, 23.16, 23.20-23.23, 23.27, 23.28, 23.31, 23.33, 23.43-23.45 2 Symbiosis  Symbiosis: close, prolonged association between two or more organisms from different species  Mutualism: relationship where both organisms interact to the benefit of both; most mutualistic organisms have coevolved over millions of years  Commensalism: relationship where on organism benefits and the other is not significantly harmed or helped  Parasitism: relationship where the parasite benefits while the host is harmed 3 Symbioses between microorganisms 1. Lichens 2. “Chlorochromatium aggregatum” 3. Methanotrophic consortia 4 Lichens  Lichens: mutualistic relationship between a fungus and an alga (or cyanobacterium)  Alga is photosynthetic and produces organic matter; many algae are nitrogen-fixing as well  The fungus provides a structure for the phototrophic partner to grow protected from erosion as well as providing dissolve inorganic nutrients  Lichens may contain bacterial and archaeal microbiota Lichen on living tree branch Lichen on dead tree branch Lichen on rock surface 5 Bacterial mutualism  Some mutualistic bacterial relationships are given “genus and species” names  “Chlorochromatium aggregatum” consists of green sulfur bacteria and a flagellated rod-shaped bacterium  Found in stratified sulfidic lakes  The green sulfur bacteria are obligate anaerobic phototrophs 6 Methanotrophic consortia  Methanotrophic consortia: couple the activities of two anaerobes to effectively oxidizing methane to carbon dioxide in anoxic marine sediments  Use “nanowires” for direct interspecies electron transfer Electron micrograph of a thin section through a. anaerobic methane-oxidizing consortium, showing the electrically conductive “nanowires” produced by the sulfate reducer (H), connecting it electrically to cytochrome-rich proteins on the surface of the methanotroph (A). 7 Plants as microbial habitats 1. The legume-root nodule symbiosis 2. Mycorrhizae 8 The legume-root nodule symbiosis  The mutualistic relationship between legumes and nitrogen-fixing bacteria is one of the most important symbiosis known  Legume: plants with seeds in pods; e.g., soybeans, clover, alfalfa, beans, and peas  Rhizobia: the best known nitrogen-fixing bacteria engaged in legume-root nodule symbioses  Infection of legume roots by nitrogen-fixing bacteria leads to formation of root nodules that fix nitrogen The nodules developed from infection by Bradyrhizobium japonicum. The main stem of this soybean plant is about 0.5 centimeter in diameter. 9 Features of the legume-root nodule symbiosis  The symbiosis leads to significant increases in combined nitrogen in soil  Nodulated legumes grow well in areas where other plants would not  In general, different rhizobia infect different species of legumes, so the bacteria that infect peas are different than those that infect clover  Cross-inoculation group: group of related legumes that can be infected by a particular species of rhizobia A field of unnodulated (left) and nodulated (right) soybean plants growing in nitrogenpoor soil. The yellow color is typical of chlorosis, the result of nitrogen starvation. 10 Leghemoglobin serves as “oxygen buffer”  Nitrogen-fixing bacteria need oxygen to generate energy for nitrogen fixation  Nitrogenase: the enzyme that fixes nitrogen are inactivated by high oxygen level  Leghemoglobin: oxygen-binding protein in the nodule binds the free oxygen and protect nitrogenase from free oxygen  The heme group of leghemoglobin cycles between the oxidized (Fe3+) and reduced (Fe2+) forms to supply enough oxygen for bacterial respiration while keeping free oxygen within the nodule low Sections of root nodules from the legume Coronilla varia, showing the reddish pigment leghemoglobin. 11 Critical steps in root nodule formation  After infection, rhizobia rapidly divide in the root nodule  These bacteria change shape and are called bacteroids that form a symbiosome within the nodule 12 Attachment and infection  Roots of leguminous plants secrete organic compounds that stimulate the growth of diverse rhizosphere microbial community  If suitable rhizobia are present in the soil, they will form large populations and eventually attach to the root hairs  Cell surface proteins of rhizobia and plant, including the adhesion protein rhicadhesin of rhizobia, are involved in the attachment  After attachment, a rhizobial cell penetrates the root hair  Infection thread: a cellulosic tube formed by the plant, induced by the bacterium, that spreads down the root hair  Plant cells divide to form a tumour-like nodule consisting of plant cells filled with bacteroids  Note that some leguminous plants form nodules on their stems instead of roots Light micrograph of an early-stage nodule from a legume infected with a rhizobium strain containing the beta-galactosidase gene. 13 Biochemistry of root nodules  Bacteroids depend on the plant to provide nutrients  Major organic compounds transported to the bacteroids are the C4 organic acids succinate, fumarate and malate; which are used as electron donors in two paths 1. Citric acid cycle leading to electron transport chain to generate ATP 2. Pyruvate as electron donor  Nitrogenase converts nitrogen to ammonia, which is assimilated to glutamine and asparagine and transported throughout the plant N2 + 16 ATP + 8 e– + 8H+ 2NH3 + H2 +16 ADP +16 Pi 14 Mycorrhizae  Mycorrhizae: mutualisms between plant roots and fungi  The fungus transfers inorganic nutrients (nitrogen and phosphorus in particular)  The plant transfer carbohydrates to the fungus  Two classes: ectomycorrhizae and endomycorrhizae Six-month-old pine seedling Pine seedling with fungal development Nonmycorrhizal Mycorrhizal 15 Ectomycorrhizae  Fungus remains outside the plant roots  Fungal cells form an extensive sheath around the outside of the root with only a small penetration into the root tissue  Found primary in forest trees, particularly boreal and temperate forests – almost every root of every tree is mycorrhizal  Example of ectomycorrhizae: truffles  Rarely found in nature except in association with roots  Some trees can form multiple mycorrhizal associations with multiple fungi  Allow exchange of nutrients and carbon between trees of the same and different species  Important for forest ecology and forest health Rootlet (red) and mycorrhizal fungus (green) 16 Endomycorrhizae  Fungal mycelium becomes deeply embed with the root tissues  Five classes, the most common is arbuscular mycorrhizae  More common than ectomycorrhizae, arbuscular mycorrhizae colonize >80% of terrestrial plants including many crop and grassland species  Cannot be cultured in pure culture 17 Arbuscular mycorrhizal root colonization  Plant roots release the hormone strigolactones, which stimulate growth of the root systems as well as germination of fungal spores and mycelial branching  The fungi produce oligosaccharide signaling molecules to initiate formation of the mycorrhizal state  The fungal mycelium forms an attachment structure called the hyphopodium (HP) and penetrates through the epidermal cells and cells of the outer cortex  Mycelium can spread intercellularly or intracellularly in the outer cortex Intracellular spreading of mycelium Structure of strigol, a type of strigolactone Intercellular spreading of mycelium 18 Insects as microbial habitats 1. Heritable symbionts of insects 2. Defensive symbiosis 3. Termites 19 Heritable symbionts of insects  Microbial symbionts can be acquired from:  Environmental reservoirs (horizontal transmission)  Parent (heritable transmission)  Heritable symbionts of insects are obligate, lacking a free-living replicative stage  Divided into two classes based on host dependency  Primary symbionts – required for host reproduction  Secondary symbionts – not required for host reproduction (a) The cedar aphid Cinara cedri, a model organism for studies of symbioses. (b) Transmission electron micrograph of the bacteriome of C. cedri showing two bacteriocytes: Buchnera aphidicola (the primary symbiont) or Serratia symbiotica, the smaller, secondary symbiont. 20 Primary and secondary heritable symbionts of insects  Primary symbionts: found in several insect groups  Restricted to specialized region of the host called bacteriome  Within the bacteriome, bacterial cells reside in specialized cells called bacteriocytes  Secondary symbionts: broadly distributed among insect groups  Can invade different cells or live extracellularly in insect hemolymph (body cavity fluid)  Can co-reside with primary symbionts in bacteriome or displace primary symbionts  Must confer benefits to the host; e.g., protection against pathogen 21 Population control with secondary heritable symbionts  Wolbachia is a secondary endosymbiont of ~50% of insect species  When males of insects infected with Wolbachia mate with uninfected females, progeny are not viable  Large numbers of Wolbachia-infected mosquitoes had been released in Myanmar, Saudi Arabia, and Australia to combat mosquito-borne viral diseases with promising results  In addition, transmission of viruses like dengue and yellow fever appears be reduced when mosquitoes are infected with Wolbachia 22 Defensive symbiosis  Production of toxic and antimicrobial chemicals: a widespread defensive strategy used by insects to deter pathogens and predators  The defensive chemical is most often the product of microorganisms symbiotically associated with the insect  Example: Rove beetle deters predators using the chemical pederin, which is synthesized by an endosymbiont Pseudomonas species  Pedrin is a cytotoxin inhibiting mitosis in eukaryotes that accumulates in the insect’s hemolymph and is deposited in its eggs, deterring insect predation on the eggs Rove beetle Pederin 23 Termites  Termites are classified as higher and lower based on phylogeny  Hind guts of termites are rich in diverse communities of anaerobes capable of digesting cellulose  Contain both acetate and organic acids producers Lower termite Higher termite 24 Bioluminescent symbionts  Bioluminescence: several species of bacteria can emit light  Most bioluminescent bacteria inhabit marine environment  Some bioluminescent bacteria colonize specialized light organs of certain marine fishes and squids  The animals use light to communicate, avoid predators, and attract prey  The bacterial symbionts obtain nutrients from the host to expand populations Colonies of Photobacterium phosphoreum photographed by their own light Flashlight fish; the bright area is the light organ containing bioluminescent bacteria Flashlight fish photographed by its own light Electron micrograph of a section through the light-emitting organ showing the dense array of 25 bioluminescent bacteria (arrows). Mammalian gut systems as microbial habitat 1. Alternative mammalian gut systems 2. The rumen and rumen activities 3. Rumen microbes and their dynamic relationship 26 Evolution of a herbivorous lifestyle  Phylogenetics suggests that different lineages evolved and herbivorous lifestyle  Microbial associations with certain animas led to their ability to catabolize plant fibres (cellulose, the most abundant biomass) 27 Alternative mammalian gut systems  Herbivores have evolved twp digestive plans:  Foregut fermentation: fermentation chamber precedes the small intestine  Hindgut fermentation: uses caecum and/or large intestine 28 Rumen  Ruminant animals: cows, sheep, goat, bison, muskoxen etc  Possess a special digestive organ, the rumen  Cellulose and other plant polysaccharides are digested with the help of microbes (bacteria, anaerobic fungi, protists) Food travels from the esophagus into the reticulo-rumen, consisting of the reticulum and rumen. The reticulum collects smaller digesta particles and moves them into the omasum. Larger particles remain in the rumen and are regurgitated as cud (partially digested fiber). Cud is chewed until food particles are small enough to pass from the reticulum into the omasum, abomasum, and intestines, in that order. The abomasum is an acidic vessel, analogous to the stomach of monogastric animals Fistulated cow for studies on rumen digestion 29 Rumen microbiota  Bacteria: 1010 – 1011/mL of rumen constituents; >300 species identified  Degradation of cellulose and starch  Archaea: 106 – 108/mL; methanogens  Fills a functional niche of consuming the available hydrogen and carbon dioxide to methane  Major environmental impact  Ciliate protozoa: 104 – 106/mL  Degradation of fibres and starch  Ingest bacteria for proteins  Fungi: 103 – 106/mL; obligate anaerobes  Digestion of cellulose and other polysaccharides  Recent studies suggest they play an important role in degradation of large fibrous polysaccharides 30 Polysaccharide-degrading enzymes and cellulosomes  Multiple enzyme activities are required for the complete digestion of polysaccharides; e.g., digestion of cellulose requires a minimum of endoglucanse, cellobiohydrolase, and beta-glucosidase activities  Cellulosomes: protein scaffold containing multiple polysaccharide-degrading enzymes found on the surface of polysaccharide-degrading, anaerobic bacteria and fungi  Efficient digestion of polysaccharides  Main fermentation products of ruminant microbes are volatile fatty acids (acetate, propionate, butyrate) and methane and carbon dioxide  Volatile fatty acids pass through the rumen wall into the bloodstream and used by the animal as its main energy source 31 Dynamics of rumen microbial community  Microbial populations in the rumen change rapidly; e.g., population of anaerobic fungi increases rapidly and transiently following feeding with fibres  Digested microbes provide proteins for the animal  Microbial composition changes with changing diets  The bacterium Streptococcus bovis can increase from 107/mL to 1010/mL when a fibre diet to a grain diet abruptly  Acidosis: inflammation of rumen epithelium caused by pH <5.5 (lower functional limit of rumen); severe cases can cause hemorrhaging and death  Caused by abrupt change from a fibre diet to a grain diet because S. bovis is a lactic acid producer  Prevention includes gradual transition of diets or mixture of fibre and grain diet 32