Lecture notes

Lecture notes Microbial Physiology And Growth (BIO231) Week 6

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Module
Microbial Physiology And Growth (BIO231)

Institution
Queen Mary, University Of London (QMUL)

Complete lecture notes of BIO231 Microbial Physiology and Growth week 6 on Symbiosis & Infection from Plants to Humans. Includes all the lectures slides content along with word-for-word commentary from lecturer and in red is my personal commentary that explains some concepts in easier terms.

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Uploaded on May 22, 2023
Number of pages 24
Written in 2022/2023
Type Lecture notes
Professor(s) Christoph engl
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Week 6 - Symbiosis & Infection from Plants
to Humans
Wednesday, 29 March 2023 13:14

LO1: differentiate between different types of symbiosis
LO2: describe the basic principles of root nodule formation and nitrogen
fixation
LO3: describe the role of the microbes in the gut of animals
LO4: identify the causative agent of an infection via Koch's postulates
LO5: differentiate between incidence and prevalence and define other technical
terms associated with infection
LO6: describe the stages of infection and how infections spread
LO7: differentiate between exotoxins and endotoxins and associate pathogens
with these pathogenic mechanisms
LO8: understand how agar plates can be used to distinguish and identify
pathogens

- This week we discuss the outcomes of biofilm formation, because the biofilm
formation is the prerequisite of host microbe interactions
- Microbes can inhabit pretty much any ecological niche, they can interact with
plants, animals, humans, interact between themselves independent of any
other additional host
- These interactions are necessary because

Symbiosis

- Some symbionts can become opportunistic pathogens
- Opportunistic pathogens: symbiotic relationships can become harmful when
symbiotic microbes colonise the “wrong” habitat
- E.g. If Staphylococcus epidermis bacteria that's normally on the skin in a
healthy symbiotic relationship, moves in the bloodstream that can cause
a severe infection

,- Some symbionts can become opportunistic pathogens
- Opportunistic pathogens: symbiotic relationships can become harmful when
symbiotic microbes colonise the “wrong” habitat
- E.g. If Staphylococcus epidermis bacteria that's normally on the skin in a
healthy symbiotic relationship, moves in the bloodstream that can cause
a severe infection

Consortia of Microbes
- Symbiosis of bacteria/microbes isn't only associated with hosts such as plants or
animals, there can also be symbiosis between microbes
- E.g. Chlorochromatium aggregatum Consortium

Chlorochromatium aggregatum

= motile phototrophic consortium
- A central flagellated rod-shaped bacterium surrounded by and attached to
13-69 green sulphur bacteria (epibionts)
Green sulphur bacteria:
- Obligate anaerobe, photolithoautotrophic, contain bacteriochlorophyll,
anoxic photosynthesis with sulfide as electron donor

- Central bacterium isn't photosynthetic while the epibionts are (they’re
photolithoautotrophic)

- Chlorobium chlorochromatii (the epibiont here) can be grown in pure culture,
no naturally free living variant has been observed, supporting the view that in
nature, a symbiotic lifestyle is obligate for epibionts - Brock textbook
- The bacterium (central one) is called Chlorochromatium aggregatum and it
requires an alpha-ketoglutarate, an intermediate of the citric acid cycle, which
is supplied to it by the epibionts

- Works by having one species of bacterium in the centre of the consortia, and it
is directly connected via extrusions from the outer membrane of the central
bacteria, fusion between the central bacterium & epibionts (bacteria that are
on the outside)
→ shared periplasm, this is where the transfer of nutrients from the
phototroph (epibionts) to the chemotroph (central bacterium) happen
- They exchange of amino acids
- The central bacteria is motile, which provides the consortium a means of
moving forward (e.g. through the water column because they're typically found
in aquatic environments)
- The epibionts themselves are NOT motile, so without the bacterium in the

, phototroph (epibionts) to the chemotroph (central bacterium) happen
- They exchange of amino acids
- The central bacteria is motile, which provides the consortium a means of
moving forward (e.g. through the water column because they're typically found
in aquatic environments)
- The epibionts themselves are NOT motile, so without the bacterium in the
centre, they cant move
- But the epibionts CAN do photosynthesis!
- The epibionts are green sulphur bacteria which do anoxic photosynthesis,
e.g. they can be found in the black sea (where there's no oxygen) and use
sulfide as electron donors
- SO epibionts provide the nutrients via photosynthesis for the central
bacterium and the central bacterium in return provides amino acids and
locomotion in exchange for the epibionts nutrients

Host - Microbe Interactions

- Microbes associate with plant in either the phyllosphere (the leaves) or in the
rhizosphere (roots), and animal hosts (in their gut, teeth, or skin) in diverse
microbial communities
- Approximate ratio 1:1 of bacterial to human cells
- Microbes affect host fitness
- Often the bacteria provide nutrients for the host but they can also cause
diseases (the opportunistic pathogens)

Root Nodules – Symbiotic Nitrogen Fixation

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