M7+8 Maiden- The social microbe I: symbioses and origin of eukaryotes + II – interactions of microbes and macrobes + The antisocial microbe – introduction to pathogens

what do we predict luca was like?

• anaerobic- all oxygen was reduced into water in the early world

• CO₂ fixing

• H₂ dependent

• N₂ fixing

• thermophilic

• dependent on transition metals

these traits are all consistent with a hydrothermal setting

why is the surface origin of life hypothesis not valid?

• UV light, low pressures and the possibility of drying up makes them unstable and unfavourable environments for the origin of life

• subsurface hydrothermal vents are actually ideal, they produce large temperature gradients, mineral gradients etc

what are the mitochondria early and mitochondria late hypotheses?

• these are two hypotheses describing the endosymbiotic event introducing an aerobic bacterium into another host, resulting in a eukaryotic cell containing a mitochondrion

• the mitochondrion late hypothesis (A) assumes that the host, a eukaryote, was already quite developed (endosymbiotic event occurred late in its development), as it had evolved the process of phagocytosis of the bacteria

  • however, in this theory, the host would have had to exist and evolve for a long period, solely living off of fermentation as its metabolism, which is very weak

• the mitochondrion early hypothesis (B) assumes that the host, an archaeal cell, was not very developed (no nucleus yet), and instead it lived in a metabolic symbiosis with a bacterium

  • this bacterium would have produced CO₂ and H₂ in its metabolism, and this hydrogen provided vital fuel for the metabolism (methane-producing) of the archaea, in a highly dependent relationship, until eventually they merged

what are lichens and what are the roles of the symbionts?

• lichens are mutualistic associations of a fungus with a cyanobacteria or algae

• the mycobiont (fungus) is a macrobe, so it can form a large structure which protects the photobiont and absorbs minerals

• the photobiont (bacteria or algae) photosynthesises, fixes nitrogen and synthesises organic nutrients

what are the roles of legumes and Rhizobium in their symbiosis?

• the bacterium fixes nitrogen from the atmosphere into bioavailable nutrients

• the plant provides the bacterium with carbohydrates, protects it within root nodule structures, and creates a specific microenvironment

  • it produces the leghaemoglobin protein, which buffers the free oxygen concentration- this keeps it high enough to allow for aerobic respiration, but low enough to allow the oxygen-sensitive nitrogenase enzyme to work

what are the three types of insect endosymbionts?

obligate mutualists:

• these are domesticated within the host and can’t survive outside it (next best thing to an organelle/organ)

• they are restricted in bacteriomes produced by the host

• dependent on host-based mechanisms for transmission

facultative symbionts:

• resemble pathogens as they invade cells in uninfected hosts (and can establish maternal inheritance) and are erratically distributed

• not required for host reproduction but may confer benefits

reproductive manipulators:

• parasites that spread by maternal inheritance, and manipulate proportions of female offspring to spread through a population

• eg. infected males sterilise uninfected females, so that offspring from infected females individuals are more successful

what are two examples of a intracellular insect endosymbionts?

• Buchnera are an obligate intracellular endosymbiont of aphids (mutualist)

• these are vertically transmitted through the ovary cells

• they live in specialised bacteriocyte cells (obligate- they are unculturable)

• the host aphid supplies energy, carbon and nitrogen

• the symbiont produces amino acids, especially tryptophan

• Wolbachia is a large and highly prevalent group of intracellular endosymbionts

• in some cases (eg. nematodes) they are mutualists

• often they are reproductive manipulators, altering host sex ratios

how do Wolbachia alter host sex ratios?

1. feminisation of genetic males from infected mothers

2. causing parthenogenesis (where only female offspring can be produced)

3. killing of infected males

4. cytoplasmic incompatibility so that infected males can’t mate with uninfected females

these methods all increase the proportion of infected females in the population, because vertical transmission can only occur through infected females

what are holobionts?

• the macrobe and its microbiome combined comprise the holobiont (metaorganism)

how do the measles and varicella zoster viruses behave differently?

measles is an acute infection:

• hosts infected with the measles virus develop life-long immunity to it, so it can only survive in large populations

• otherwise it runs out of susceptible hosts and dies out as it has no other reservoir

chickenpox/shingles is a chronic infection:

• hosts infected with the varicella zoster virus (VZV) develop chickenpox

• the pathogen then remains in a quiescent state in the host

• it can be reactivated to cause shingles

how can microbes cause cancers?

• certain viruses and bacteria can cause cancers directly or indirectly eg. by the alteration of host cells or the production of toxic substances

• eg. human papillomaviruses (HPV), which cause cervical cancer

• eg. devil facial tumour disease (DFTD) and canine transmissible venereal tumour (CTVT)

what are prions?