Bacterial Sporulation and Inclusions Flashcards
(29 cards)
CELL INCLUSIONS
BACTERIAL CELL RESERVES
What would happen if a bacterium was growing in a medium containing an
excess of a particularly nutrient(s)?
➢ The medium that a bacterium (or a community of bacteria), is growing in,
may be “unbalanced”.
➢ In the environment there is not a steady stream of nutrients. And therefore a
“feast or famine” scenario may be common.
➢ In such cases it may be possible for the organism to store an excess nutrient
Poly-β-hydroxyalkanoates (PHAs
lipids made of PHA monomers
Poly-β-hydroxybutyrate (PHB)
lipids made of PHB monomers.
LIPID INCLUSIONS
➢ Monomers can vary from C4 to C18.
➢ Lipid inclusions are synthesised when carbon is in excess
➢ Lipid inclusions are broken down for use in biosynthesis or energy
generation when required.
PHA inclusion structure
Inclusions are not simply granules
containing a particular molecule.
Enzymes are involved in their
formation and a specific metabolism is
often associated with them
Cyanophycin
An amino acid polymer
composed of an aspartic acid backbone
and arginine side groups
Accumulates in the form of
granules in the cytoplasm
during phosphate or sulfur
starvation, generally in the
early and mid-stationary phase.
Used as a nitrogen, and possibly, carbon-storage compound.
Mostly found in cyanobacteria and some heterotrophic bacteria
Volutin (polyphosphate granules)
Initially discovered in Spirullum
volutans.
Occurs in many bacterial species.
Granules composed of polyphosphate
.
Granules can be degraded and used as
sources of phosphate for nucleic acid
and phospholipid
Sulfur Inclusions
➢ Many Bacteria (and Archaea) are capable of oxidizing reduced sulfur
compounds such as H2S (hydrogen sulfide).
➢ These oxidations are linked to reactions of energy metabolism and
biosynthesis.
➢ In either case elemental sulfur may accumulate in the cell as visible
globules.
➢ Globules reside in the periplasmic space.
➢ Ultimately granules are oxidized to sulfate and disappear
Polysaccharide granules
➢ Many bacteria form glycogen granules.
➢ Glycogen is produced when carbon is in excess and is consumed when
carbon is limiting.
➢ Glycogen is a polymer of glucose with 1,4-α-glycosidic and 1,6-glycosidic
linkages
Gas vesicles
A gas vesicle is a spindle shaped structure (cylindrical tube closed by
conical end caps) made of protein.
Main function:
Gas vesicles confer buoyancy
to cells by decreasing their
density.
Gas vesicles occur in five phyla of the Bacteria and two groups
of the Archaea, but they are mostly restricted to planktonic
microorganisms, in which they provide buoyancy.
Main protein: (GvpA)
It is a small and highly hydrophobic
protein which is aligned as parallel
ribs
The ribs are clamped together by a
second protein (GvpC)
Magnetosome
➢ Inclusion composed of magnetite, Fe3O4.
➢ Bacteria orientate and migrate along the earth’s magnetic field.
➢ Function is unclear but thought to guide cells towards sediments
(field points downward) where O2 levels are lower.
Carboxysome
A carboxysome is a polyhedral shaped bacterial microcompartment
surrounded by a protein shell.
Inside the shell
Ribulose-1,5-bisphosphate
carboxylase/oxygenase (RuBisCO).
there is the enzyme
RuBisCO can act as a carboxylase
and an oxygenase.
So, CO2 and O2 compete for the
RuBisCO reaction.
How do the carboxysomes work?
Carboxylase function: To capture/store/trap/fix CO2
1- There is a plenty of bicarbonate (HCO3- )in the
cytoplasm of the bacteria.
2- Bicarbonate enters the carboxysome
3- The carbonic anhydrase enzyme in the
carboxysome converts bicarbonate into CO2.
4- In the presence of CO2, RuBisCO fixes CO2 into
PGA (phosphoglycerate).
in other words: CO2 is trapped into PGA.
Where are Carboxysomes found?
Carboxysomes are found in CO2 fixing bacteria such as cyanobacteria.
Thylakoid membranes
➢ A thylakoid is a membrane-bound compartment inside chloroplasts and
cyanobacteria.
➢ They are the site of the light-dependent reactions of photosynthesis.
➢ Cyanobacteria have structures called Phycobilisomes that act as light-
harvesting antennae attached to the membrane
Heterocysts
➢ Heterocysts are specialized nitrogen-fixing cells, produced by some
filamentous cyanobacteria.
➢ They form during nitrogen starvation conditions
➢ Vegetative cell: Vegetative bacteria are the bacterial cells that are metabolizing and undergoing binary fission. They are “alive” and replicating. Can do
photosynthesis.
Heterocyst: A cell that is differentiated and is now specialised to store nitrogen.
Cannot do photosynthesis.
Heterocyst differentiation
➢ An oxygen-free environment is required for nitrogen fixation to occur.
➢ Thick heterocyst cell walls are relatively impervious to oxygen.
➢ Internal membranes are thylakoids that have lost their chlorophyll and provide
sites for nitrogenase.
➢ Other cell contents are generally lost.
➢ Heterocysts are connected to vegetative cells through special pores in their end
walls.
➢ Heterocysts are morphologically and functionally distinct from vegetative cells.
➢ This is an example of cell differentiation.
SPORULATION - ENDOSPORES
➢ What is an endospores: dormant, non-reproductive structure produced
by bacteria (= bacterial resting cells)
➢ They are formed in some species in response to nutrient starvation (e.g.
carbon, nitrogen or phosphorous).
➢ These resting cells are characterised by:
* Low metabolic rates.
* Resistance to desiccation (dehydration), heat, irradiation and chemical
treatments (e.g. H2O2 and acids) compared to the actively growing cells.
➢ This means that endospores can survive for a very long time – even
centuries
HOW LONG CAN ENDOSPORES LAST?
1- Suspension of endospores of the bacterium Clostridium aceticum prepared
in 1947 was transferred to sterile growth medium 34 years later: Growth
commenced within 12 hours.
2- Thermoactinomyces spores recovered from >2000 year old archaeological
sites and >7000 year old sediment cores have been revived (contamination?).
3- Spores from gut of extinct bee trapped in amber for 25-40 million years
have been revived.
4- Spores of a halophile trapped in salt crystals have been revived: Thought
to be 250 million years old
WHY IS SPORULATION IMPORTANT
➢ Knowledge of heat-resistant forms essential in the development of
adequate sterilization methods:
* Culture media sterilization.
* Food and other perishable products.
➢ The most highly resistant spores can survive heating to 150°C.
➢ Most autoclaves operating at 121°C will kill spores of most species.
WHICH BACTERIA CAN FORM ENDOSPORES?
➢ About 20 genera of Bacteria have been shown to form endospores.
➢ Only studied in detail in a few species of Bacillus and Clostridium.
➢ Genes essential for the formation of endospores seem to be universal in
these species.
➢ Phylogenetically the ability to produce endospores appears to be tied to a particular lineage of Gram-positive bacteria
PATHOGENIC SPORE FORMERS
Many spore-formers are pathogenic:
* Bacillus anthracis (anthrax)
* B. cereus (food poisoning)
* B. thuringiensis (pathogenic to insects)
* Clostridium botulinum (botulism)
* C. tetani (tetanus)
* C. perfringens (gangrene and food poisoning)
* C. difficile (diarrhoea).
SPORE STRUCTURE
Exosporium: Thin protein covering.
Spore coat: Layers of spore-specific proteins.
Cortex: Loosely cross-linked peptidoglycan layer.
Core: Core wall, cytoplasmic membrane, cytoplasm,
nucleoid, ribosomes etc
KEY FACTS ABOUT ENDOSPORES
➢ The process of sporulation takes 8-10 h
➢ Endospores:
* 1000 × more resistant to heat.
* 10 × more resistant to UV irradiation.
* Very resistant to chemicals and desiccation.
* Metabolically inactive and are able to survive for long periods of
time.
* Can germinate and outgrow within minutes of being placed in
nutrients.
