Social and antisocial microbes

What did all life evolve from

LUCA - inferred cellular organism

Inferred because of the shared fundamental biochemical and genetic characteristics of all life

Features of LUCA (and what setting are they consistent with)

Anaerobic, CO₂ and nitrogen fixing, hydrogen dependent, thermophilic, dependent on transition metals

Current theories about the location of the origin of life

Surface origin hypothesis - but problems of UV light, drying up, not having the high temperature and pressure needed

Subsurface origin hypothesis - seems inhospitable but has high temperature and pressure, and gradients of temperature and minerals

Most popular theory for the origin of life

RNA world theory - first replicating systems were RNA based

Then DNA became the genetic repository as it’s more stable, and RNA developed a more transitory role in genetic inheritance

Three part system (central dogma) of RNA, DNA, and proteins evolved and became universal

History of life on earth (summary)

World was hot, anoxic, and inorganic

Life formed and changed the earth through oxygenic photosynthesis, CO₂ fixation etc.

O₂ based respiration became possible and more energy became available

Oxygen concentration increased as cyanobacteria developed photosystem that replaced H₂S with H₂O

Great oxidation event - oxygen reacted spontaneously with ocean iron minerals, accumulated in the atmosphere forming the ozone layer.

Multicellular and more complex life now possible

How did eukaryotes arise

Endosymbiosis - incorporation of a bacterium capable of aerobic respiration into early eukaryotic cells, forming cells w/ mitochondria

How did endosymbiosis benefit cells

Increased respiratory capacity

Could carry out oxygen-based metabolism

What did chloroplasts and plant cells arise from

Second endosymbiosis event - bacterium capable of harnessing light energy (photosynthesis) incorporated to become chloroplasts, plant cells formed

Could be even more energy efficient

Problems with the endosymbiotic model

Assumes complex cellular development already - e.g. ability to carry out endocytosis to acquire the mitochondria/chloroplast

Why is endosymbiosis widely supported

Similarities between mitochondria/chloroplasts and prokaryotic cells - smaller ribosomes, mainly circular chromosomes, mainly lack membrane-bound organelles

Two different endosymbiotic models

Mitochondria early and mitochondria late model

Features of the mitochondria late model

Phagocytosis evolved first, mitochondria came from endosymbiotic uptake through phagocytosis

Features of the mitochondria early model

Complex structures and phagocytosis evolved after endosymbiosis

E.g. the ‘hydrogen hypothesis’ - metabolic symbiosis between a facultative anaerobic bacterium (pre-mitochondria) and an hydrogen-consuming anaerobe, which eventually enclosed the bacterium

Evidence for mitochondria early model

Syntrophy → nutritionally interdependent communities of eubacteria and archaea living in anoxic environments, one species relies on the products of the other

There are patterns of gene exchange between these communities

The close interactions may have been able to lead to an early eukaryotic cell, before phagocytosis etc. evolved

Evidence for mitochondria late model

Prokaryotes within prokaryotes: one nitrogen-fixing, spore-forming cyanobacterium has intracellular bacteria

What alternatives to mitochondria exist

Hydrogenosomes - generate energy via the partial oxidation of pyruvate to acetate. Found in some anaerobic single-celled eukaryotes, thought to have evolved from mitochondria. Enveloped by a double membrane and contain their own DNA (though thought to be reduced due to gene loss)

Mitosomes - found in some unicellular anaerobic eukaryotes which haven’t developed mitochondria. Have a double membrane but lack their own DNA and have reduced metabolic capacity (e.g. lack electron transport chain)

How have anaerobic eukaryotes evolved

Eukaryotes losing mitochondria and mitochondrion related organelles

Some evidence of eukaryotes being able to utilise the bacterial sulphur energy pathway through lateral gene transfer of sulphur mobilisation genes

What interactions show similarities to the believed primitive beginnings

Interactions between bacteria and diatoms (microscopic algae)

Diatoms associate with proteobacteria and bacteroidetes

Bacteria have contributed to diatom genomes via HGT, increased their metabolic capacity

Understanding increases as what improves

Molecular techniques

Range of relationships that microbes have with macrobes (host-organisms) - and how soil amoeba exhibit them w/ bacteria

Predator-prey (soil amoeba prey on bacteria as a food source) → parasitism (bacteria can parasitise them) → mutualistic (they have endosymbiotic mutualists)

Definition of commensal

Organism attached to/on another.

One benefits and the other is unaffected, not a parasitic relationships.

Definition of mutualism

Both organisms benefit from one another

Definition of symbiosis

General term for close, prolonged association between organisms of different species

What does it mean if the relationship is obligate

One or both organisms cannot survive without the other

Evolution pathway for microbe-host associations

Free-living bacteria → host-associated (living in other organisms) bacteria

Could become: commensal bacteria → symbiotic/mutualistic bacteria

OR

pathogenic bacterial → obligate pathogens

Stages of host-adaptation

Free-living and extracellular → facultative intracellular → obligate intracellular → obligate intracellular mutualist → organelle

What have genomic studies of host-associated bacteria phylogeny revealed

Evolved independently many times

Host adaptation not evenly distributed between bacterial groups, obligate mutualism only in alpha and gamma proteobacteria

Mutualism is rare in vertebrates, and obligate mutualism is unknown in plants

What is host-adaptation

Become more intracellular

Examples of social microbe interactions

Legumes & Rhizobium

Earthworms and Verminephrobacter

Aphid and Buchnera endosymbionts

Insects and Wolbachia

Ruminants

Legumes and Rhizobium interactions

Rhizobium provides a source of fixed nitrogen for the plant, while the plant provides nutrition, protection and low oxygen tension

Very important association for agriculture

This is species-specific for both bacteria and plant, co-evolution of symbiosis

Formation of Rhizobium root nodule is similar to that of disease - recognises, attaches, invades, and forms bacteroid. Plant and bacteria both grow to form the nodule

Earthworms and Verminephrobacter interactions

Most earthworms have specific Verminephrobacter symbionts

Bacteria live off the host waste products and provide nutrients that help the earthworms’ reproduction

The bacteria are vertically transmitted, their genome has been subject to reductive evolution, and they can carry out HGT

The bacteria experience two different environments: nephridia and the cocoon

Aphids and Buchnera interactions

Buchnera is an obligate intracellular endosymbiont. The aphid supplies energy, carbon and nitrogen, while the bacteria produces amino acids w/o which the aphid would starve. Especially tryptophan, bacteria genome changed to have 16 copies of trpEG to help the host.

They have co-evolved with the aphid and their genes undergone reductive evolution.

Bacteria live in specialised insect cells called bacteriocytes, surrounded by the membrane and vertically transmitted by the ovary.

Insects and Wolbachia interactions

Wolbachia are intracellular endosymbionts present in 66% of insects - different interactions in each e.g. in nematodes they seem to be mutualists, essential to the nematode survival and reproduction

They infect the germ line and often manipulate host sex ratio, showing pathogenic capacity

Four different reproductive phenotypes that Wolbachia can cause

1. Feminisation of genetic males

2. Parthenogenic elimination of males from reproduction (only get females - Wolbachia only spread through eggs so want more females)

3. Male killing of infected males

4. Cytoplasmic incompatibility - preventing infected males mating with females without the same Wolbachia

All ensure as many females in the population as possible

Use of Wolbachia for disease vector control

Release infected males into wild populations - offspring die due to reproductive incompatibility

Release infected females - offspring has reduced competence as pathogen vectors

Release of females infected w/ a different Wolbachia strain - stops spread via cytoplasmic incompatibility, reduces insect lifespan

Ruminants and microorganism interactions

Use communities of microorganisms to digest cellulose. Digest the plant material for 9-12 hours, convert it into glucose and then to fatty acids.

Cellulose is hard to break down due to insoluble crystalline microfibrils

What are holobionts

Dynamic relationships of macrobes plus their microbiomes

Holobiont concept - organisms are an expression of their genome and their microbiota

Functions of microbiotas

Make nutrients available to hosts through catabolism and bioconversion of compounds

Synthesise important cofactors or bioactive signalling molecules

Trigger alterations in host function (like altered expression of mucus or immune system

Why is the microbiome important

Essential for human response to infection

Dynamic and major role in health and disease, problems with the microbiome can cause ‘dysbiosis’

Points about disease as a major mortality source past and present

Oldest treatment of gynaecology found on papyrus from around 4000 years ago

TB still infects around ¼ of the population

Diversity of pathogenic microbes

Can be viruses, prokaryotes, single-celled + and multi-celled eukarya, host derived (prions and infectious cancers)

But no known archaea are pathogens

Predominate pathogenic lifestyle

Infectious - spreading from one host to another

Different modes of transmission

Bacteriophages as infectious agents

Host-to-host transmission

Horizontal and vertical transmission

Zoonoses and species leaps

Multi-host transmission system

Bacteriophage transmission (2 types)

Lytic bacteriophages - absorb to bacterial host, inject their DNA, replicate that DNA at expense of host, translate using host machinery, assemble virions and release them through lysis

Lysogenic phages - integrate their genome into host DNA, activated into lytic cycle

Host-to-host transmission

Requires every infected host to give rise to one other infection to continue transmission

Many cases the infection is not permanent, only transitory. Hosts that are likely to develop the disease after infection are called ‘susceptible hosts’. Infected hosts return to being ‘susceptible hosts’ after having cleared the infection

Many infections require direct transmission

Vertical transmission

Passing of infections from parent to offspring

Horizontal transmission

Passing of infectious agents among different individuals

Zoonoses and species leaps

Pathogens evolved in one host species infecting other/multiple other host species

If the recipient host species cannot transmit the pathogen, what are they known as

‘Dead end’ host

Multi-host transmission system (+ challenges)

Pathogens evolving complex lifestyles involving multiple hosts

Challenges are needing the ability to survive in lots of different environments and evade multiple different immune systems

Benefit of multi-host transmission systems

Pathogens can take advantage of one host to infect another - may be able to cause the infection or promote spread

Examples of zoonoses and species leaps

Rabies - found in many mammal species and spread by biting. Within the host the virus moves through the immune system to the brain where it can cause behavioural changes (e.g. aggression). Virus grows in the salivary glands to promote spread

SARS - severe acute respiratory syndrome, caused by SARS coronaviruses and has crossed species barriers

Examples of vertical transmission

Can occur through many routes: intracellular, transplacetally, via milk, during birth

Common vertical pathogen transmission is from pregnant women to foetus or mother to child during birth and breastfeeding

Types of infection for pregnant women

Maternal infectious = severe disease during pregnant but no transfer to child

Congenital infectious = mild/asymptomatic during pregnancy but can be transferred to child

Neonatal infectious = serious complications after birth to child, sometimes caused by vertical transmission

Example of bacteriophage transmission

E. coli lambda phage - can persist as a episome. Lots of elaborations past the simple lytic lifestyle.

Example of host-to-host transmission

Chlamydia - obligate intracellular pathogen of humans, resides permanently in the host population, spread by sexual transmit. Relatively small genomes (genome reduction because of metabolic specialisation for intracellular life) and isolates are of very low diversity

Example of multi-host transmission system

Schistosomiasis - eggs are deposited and move to the intestine, released into the environment as urine/faeces. Hatch into miracida which infect snails (the intermediate hosts). Sporocysts grow in the snails and release cercaeriae into water which humans encounter, cercaeriae penetrate skin and infect the human.

What is host immunity (+ example)

Host becomes immune to further infection, can vary in length.

Measles - highly infectious disease passed by the respiratory route, but single infection provides ‘lifelong immunity’ so can only persist in a large enough host population, becomes extinct in small islands.

What is an acute infection (+ example)

Occurs over a short period of time, during which passes to another host

E.g. measles

What is a chronic infection

Long-lasting illness persisting for extended periods. To get around host death/immunity pathogens can remain in quiescent states in the host and can be reactivated to become infectious again

Example of how infection can be chronic by remaining in latent state

Chicken pox/shingles - varicella zoster virus enters via epithelial cells in the mucosal lining and spreads via local transmission. Establishes latency in the sensory ganglia around the body, can be reactivated to cause infectious lesions.

What are some diverse affects that pathogens can have on their hosts

Can cause cancer (viruses and bacteria) directly and indirectly

Examples of a microbe causing cancer

Infection of HPV can cause cervical cancer

Devil facial tumour disease and canine transmissible venereal tumour are both cancers that can be infectious, one dog cell becoming replicative and spreading between organisms

What are prions

Infectious protein agents - forms of proteins naturally present in mammalian brains. Converted to pathogenic form autocatalytically (as the product acts as a catalyst and speeds up reaction).

Prion protein is produced by normal cells and therefore always replicated. It has an abnormal 3D shape that turns it into a prion, but the conformation is highly stable so not inactivated by normal sterilisation techniques

What can prions cause

Degenerative brain disorders like kuru, BSE