Lecture 6 (Linda Stweart) - Microbial Cell Structure 2

Question 1

Identify the components of the bacterial cytoplasm and describe their structure, molecular makeup, and functions

Answer 1

Cytoskeleton:

• FtsZ homologue is a tubulin homologue which forms a ring during septum formation in cell division.
• Mreb is an actin homologue which helps determine cell shape, may be involved in chromosome segregation, and localises proteins.
• CreS maintains curvature in curved rods.

Intracytoplasmic Membranes:
- Plasma membrane infoldings observed in many photosynthetic bacteria and bacteria with high respiratory activity.

Inclusions:

• Granules of organic or inorganic material that are stockpiled by the cell for future use.
• So e are enclosed by a single-layered membrane which can be made of proteins or lipids.

Storage Inclusions:

• Storage of glycogen, amino acids (cyanophycin granules) and sulfur globules (composed of elemental sulfur).
• Carbon storage - Poly-beta-hydroxybutyrate (PHB) is synthesised by cells when there is excess carbon and broken down when needed.
• Phosphate storage - Polyphosphate (volutin). Phosphate is often a limiting nutrient so it is advantageous to be able to store it as polyphosphate for future use.

Microcompartments:

• Not bound by membranes but compartmentalised for a specific function.
• Carboxysomes (in CO2-fixing bacteria) contain the enzyme ribulose-1,5-bisphosphate carboxylate which is used for CO2 fixation.

Gas Vacuoles:

• Composed of protein.
• Forms a cylinder impermeable to water but permeable to gas to provide buoyancy in planktonic cells.
• When gas vacuoles collapse, buoyancy is lost.
• Important in phototropic prokaryotes as they can adjust their position in a water column for optimal photosynthesis.

Magnetosomes (magnetic inclusions):

• Found in aquatic bacteria.
• Consist of a network of membrane-enclosed intracellular crystals of a magnetic iron mineral.
• Magnetite particles used for orientation in Earth’s magnetic field (navigational device in magnetotatic bacteria).
• Magnetic field provides depth information (too deep in water not much photosynthesis/lack of nutrients, water too shallow and bacteria are vulnerable to competition and dramatic changes in environmental conditions).

Ribosomes:

• Bacterial rRNA - small subunit = 16S, large subunit = 23S and 5S.
• 55 proteins (21 in small subunit and 34 in large subunit).
• Sites of protein synthesis.
• Bacteria and archaea ribosome = 70S
• Eukaryotic = 80S

Nucleoid:

• One closed circular, double-stranded DNA molecule.
• Bacterial chromosomes are longer than the length of the cell.
• Supercoiling and nucleoid proteins aid in folding e.g. HU protein.

Plasmids:

• Extrachromosomal DNA found in bacteria, archaea and some fungi.
• Small, closed circular DNA molecules.
• Episomes are plasmids which have become integrated into the chromosome.
• Confer selective advantage e.g. drug resistance due to containing very few non-essential genes.

Question 2

Distinguish pili (fimbriae) and flagella

Answer 2

• Pili are hair-like, proteinaceous appendages which mediate attachment to surfaces, motility and DNA uptake.
• Flagella are thread-like, locomotor appendages (much longer than fimbriae) extending outward from the plasma membrane and cell wall. They play a role in motility and swarming behaviour, attachment to surfaces, and may be a virulence factor.

Question 3

Illustrate the various patterns of flagella distribution

Answer 3

• Monotrichous = one flagellum
• Polar flagellum = one flagellum at end of cell
• Amphitrichous = one flagellum at each end of cell
• Lophotrichous = cluster of flagella at one or both ends of cell
• Peritrichous = flagella spread over entire surface of cell

Question 4

Explain why bacterial chemotaxis is referred to as a “biased random walk”

Answer 4

• Referred to as a biased walk because in the presence of a chemical attractant the tumbling frequency of the bacterium is intermittently reduced and runs of the bacterium in the direction of the attractant are longer.
• Referred to as random because in the absence of a chemical attractant the bacterium moves randomly and the tumbling frequency is fairly constant.

Question 5

Describe the structure of the bacterial endospore

Answer 5

• The spore is surrounded by a thin covering called the exosporium.
• Beneath the exosporium is the spore coat which is composed of thick layers of protein.
• Beneath the coat is the outer membrane.
• Beneath the outer membrane is the cortex which is made of thick peptidoglycan.
• Beneath the cortex is the germ cell wall and which lies beneath this is the inner membrane.
• At the centre of the endospore is the core which contains the nucleoid and ribosomes.

Question 6

Explain why the bacterial endospore a are of particular concern to the food industry and why endospore-forming bacteria are important model organisms

Answer 6

• Endospores are resistant to environmental stresses such as heat and chemical disinfectant.
• Some Endospores have remained viable for around 100,000 years.
• Several species of endospore-forming bacteria are dangerous pathogens e.g. Clostridium botulinum, a food-borne disease that results from ingestion of botulinum toxin.
• In order to prevent botulism, food must be prepared and stored properly.
• The extreme heat resistance of C. botulinum’s Endospores is a major concern of the food industry.
• Bacillus anthracis spores can be produced in a laboratory and used as a bioterrorism agent.

Question 7

What word describes the plasma membrane and everything inside it

Answer 7

Protoplast

Question 8

Give two examples of bacteria with sex pili which assist in surface attachment

Answer 8

• Neisseria

- Streptococcus pyogenes

Question 9

Describe the structure of a flagellum

Answer 9

• Three parts.
• The filament is a hollow, rigid cylinder of flagellum protein which extends from the cell surface to the tip.
• The hook links the filament to the basal body.
• The basal body is a series of rings that drives the flagellar motor.

Question 10

How does the filament in a flagellum assemble

Answer 10

Assembled from the tip.