Biology 350: Microbial Diversity

Transcription

1 Biology 350: Microbial Diversity Microbial Interactions II: Finishing Up Symbiosis, Predation, and Antibiosis Lecture #31 16 November

2 Notice handouts and announcements for today: Your usual outline and sample questions for today s lecture. A Thanksgiving snippet suggesting that wine may inhibit the growth of foodborne bacterial pathogens. A set of papers regarding Wolbachia weirdnesses. A paper describing a very cool symbiosis between ants, fungi, and bacteria! -2-

3 Some announcements: Your glow in the dark wristbands have arrived---i ll pass them out in class (I have your sizes). Your new T-shirts and buttons should arrive next week. I ll send out sample data regarding metagenomics this weekend---the real thing should arrive next week from Korea! -3-

4 Today s lecture agenda We will reinforce some ideas about symbiotic relationships among microbes. We will cover several more specific associations of interest. We will discuss predation among microbes. We will finish up with a few words about antibiosis. -4-

5 Microbial symbioses range from transient to integrated Microsymbiont can live apart from host Loss of autonomy, gene transfer, gene erosion, etc. Microsymbiont fully integrated into host genome/ cell biology Vibrio fischeri - squid symbiosis Mitochondria and other organelles -5-

6 What happens during integration of a symbiont? Jeon, amoebae,and X-bacteria. (VERY IMPORTANT) Peripheral to obligate associations develop over time. Gene flow between symbiont and host over time (example: mitochondria and nucleus). Gene erosion due to host provision of functions (example: amino acid biosynthesis, etc). Nutritional interdependence (sea slug - sea lettuce example versus more integrated associations). Intracellular environment is VERY different from outside---so selection pressures change. Manipulation of the host is a key to endosymbiont transmission. -6-

7 Remembrances of the Weird: Wolbachia symbioses Wolbachia is an obligate endosymbiont of insects, spiders, mites, and some nematodes. It is a rickettsial alpha proteobacter. Symbiosis impacts life history of partner in many ways. Four specific phenotypes associated with Wolbachia infection: male killing, feminization, parthenogenesis, and cytoplasmic incompatibility. Transovarial transmission (vertical). Nature of the mutualism (benefit to host) is unknown. But nematodes require Wolbachia to reproduce! Reproductive isolation in insects. Wolbachia tends to promote its own transfer. Is involved with nematode-related disease: river blindness, elephantiasis, and heartworm in part due to Wolbachia, not just the filarial nematode (host immune response to bacterium). Antibiotic treatment? Wolbachia has a reduced and streamlined genome. -7-

8 More Weirdnesses about Wolbachia Check out your Wolbachia packet! River blindness caused by Wolbachia carried by the parasitic worm! Mostly due to the eye s response to LPS. Antibiotic treatment helps! Phage and Wolbachia battle inside insects, and the winner impacts cytoplasmic incompatibility (more phage, less CI; more Wolbachia, more CI). Wolbachia s genome is small, but has a great deal of junk or repetitive DNA (as much as 14%). Some species show evidence of HGT, some do not. Evidence that GIANT pieces of the Wolbachia genome can become part of the nuclear genome of the insect host! Now THAT is HGT! -8-

9 A Reminder about Buchnera and Coevolution Phylogenies of aphids and Buchnera very closely related. Aphid phylogeny based on fossils. Buchnera phylogeny based on 16s rrna Synchronous evolution of macro and microsymbiont! VERY long term association and definite coevolution. -9-

10 Next. Let s look at some other associations of microbes and eukaryotes! -10-

11 Something useful: Legumes and Rhizobium Figure 19.9 Figure Rhizobium species have a specific association with the roots of legumes (Sinorhizobium meliloti to alfalfa). Gene products from Rhizobium induce nodule formation in plant roots. Bacterial cells convert into nondividing form called bacteroids. Leghemoglobin is made: heme from the bacterium, the globin from the plant. Leghemoglobin binds oxygen tightly, helps create the low oxygen conditions necessary for nitrogenase to work. Nitrogenase in bacteroids provides ammonia to the plant from dinitrogen gas (this is the basis of crop rotation---plants making their own fertilizer!) Bacteroids get specialized niche, nutrients. -9-

12 Something gross: Ruminant Symbioses Figure Figure Amazing if gassy subject! Cellulose can ONLY be broken down by prokaryotes. Thus cows eat grass, and microbes in the rumen digest cellulose for the cows. COMPLEX microbiota involved! Cellulose converted to sugars, then sugars converted to volatile fatty acids (propionic acid, for example). VFAs go into the bloodstream of the cow--- feeding them! Rumen is very anaerobic. Much hydrogen is generated, and then archaeans use the hydrogen and carbon dioxide to make methane---significant source of methane on Earth. The rumen is a complex zoo of symbiotic bacteria, archaea, and protists. -12-

13 Something Deep: Thiotrophic Symbiosis in Riftia Hydrothermal vents in the deep ocean, driven by geochemical processes. Vent water is hot and rich in hydrogen sulfide. Riftia tube worms exist there in large numbers. Worms can be two meters in length, and grow extremely rapidly. Adapted to high sulfide conditions. No mouth, no anus but does have a trophosome. Trophosome is packed with bacteria. Bacteria oxidize hydrogen sulfide to elemental sulfur to generate energy. Bacteria also have Rubisco, and fix carbon dioxide to sugars. Establishment seems to be horizontal: early stages of worm development have a pore through which the chemoauthotrophic bacteria enter. Bacteria have functional flagellar and chemotaxis genes, though they cannot be cultivated in the lab. Remarkable thiotrophic symbiosis! -13-

14 Ant Farming and Microbial Symbioses! Leafcutter ants chew up leaves, keep the mass in their nest, and farm fungi to eat on that mass. Ant farmers! Only the fungi seem to grow. Why? Long story short: the ants carry an species of actinomycete on their bodies. This actinomycete has antibacterial and antifungal properties (except for the food fungus ). A tripartite symbiosis: ant, fungus, and bacterium. Originally thought to be a member of Streptomyces, the bacterial ant symbiont appears to be Pseudonorcardia. Strong antimicrobial action! Lots of coevolutionary action going on here! -14-

15 Next. Some thoughts about predation and antibiosis -15-

16 Does predation have a role in the microbial world? Protists preying upon prokaryotes is common. In seawater, bacteriophage attacking prokaryotes common as well (10 7 phages/ml but great white sharks/ml so which is the more important predator?). Prokaryotic predators do exist, and may be more common than expected. Bdellovibrio is the best understood, with a biphasic life cycle and periplasmic growth phase. Predators like Bdellovibrio are found everywhere, from the Great Salt Lake to the human intestine. Other approaches exist: wolf pack group predation (Myxobacter), epibiotic (Vampirococcus), and direct invasion (Daptobacter). Prokaryotic predation could play a role in microbial ecology, altering community structure and overall population sizes. More research is needed! -16-

17 Just a word about antibiosis You know a bit about how antibiotics work from an earlier lecture. Concentration of antibiotics in nature well below effectiveness ---so what is the role in nature? Some prokaryotes make bacteriocins. Bacteriocins are short peptides with strong antimicrobial properties (many act as proton pores). Bacteriocin producing bacteria ALSO make proteins that inhibit the action of that bacteriocin! Toxin+immunity genes linked. Many lactic acid bacteria make bacteriocins and these antimicrobials are part of the appeal of probiotics, and are currently used in the dairy industry to inhibit other bacteria. -17-

18 Modes of action of antibiotics and bacteriocins -18-

19 Summing up today s lecture Symbiotic associations can be transitory or obligate. Many symbiotic associations integrate the microsymbiont into the cellular architecture of the host. Genes are lost, and functions narrowed. The association become obligate. Several specific associations were discussed, including Buchnera, Wolbachia, Riftia, Rhizobium, rumen-like symbioses, and ant-microbe interactions. There are four basic strategies of prokaryotic predation: wolfpack, epibiotic, direct invasion, and periplasmic. Predation may be important in regulating prokaryotic population structure and density. Antibiotics definitely can inhibit bacterial growth in the lab, and at high doses, but their role in nature is unclear. Bacteriocins are antimicrobial proteins that may be as interesting and important as antibiotics---and appear to be relevant in nature, as well. -19-

20 Topic(s) for next lecture Happy Thanksgiving! Microbiology Under the Microscope, Part III: Tales of Bacterial Food Pathogens! No reading, other than handouts provided! -20-