Bacterial structure Flashcards
What types of cell are bacteria?
Prokaryotic cells
What are prokaryotic cells?
Cells that have a simple structure and lack a nucleus or membrane-bound organelles
Are bacteria multi or uni-cellular organisms?
How big are they?
Unicellular organisms
Variable length (0.1-5um)
What are the 2 main shapes of bacteria?
- Cocci (round)
- Bacilli (rod-shaped
What determines the shapes of bacteria?
The cell wall
What components make up a bacterial cell?
- Granules
- Cell wall
- Capsule
- Cytoplasmic membrane
- Flagella
- Single coiled chromosome
- Fimbriae
- Cytoplasm
- Ribosomes
What is the cytoplasm?
Jelly-like aqueous solution (80% water)
What does the cytoplasm contain?
- One nucleoid
- One or several plasmids
- Many ribosomes
- Granules
- Various types of solute
What is a nucleoid?
The bacterial chromosome is one large circular molecule composed of DNA (60%) with small amounts of RNA and proteins that regulate transcription (transcription factors) or carry out dynamic spatial organisation of the nucleic acid
What is a plasmid?
A large circular molecule of double stranded DNA that can replicate autonomously from the chromosome
What are granules?
Concentrated deposits of various nutrients or waste such as starch or glycogen
What are the various types of solutes found in the cytoplasm?
- Macromolecules such as proteins (enzymes) and nucleic acids (DNA and RNA)
- Small molecules that are energy sources, precursors of macromolecules, metabolites or vitamins
- Inorganic ions required for enzymatic activity
What is the structure of the nucleoid?
1000x longer than bacteria and extensively folded to form a dense body that can be visualised by electron microscopy
Proteins encoded by plasmid genes are not essential under normal conditions but are important in what?
Important to cause disease (virulence factors) or to survive to antibiotics and heavy metals (resistance genes)
What are ribosomes the site of?
These organelles are the site of protein synthesis
How is ribosome size measured?
Their size is measured by the Svendberg, which is a measure of sedimentation rate by centrifugation
What S is the ribosome in a eukaryotic cell vs bacterial cell?
Bacteria = 70S
Eukaryotic cell = 80S
Ribosomes consist of 2 subunits - what are they?
1) A small subunit (30S) that reads the mRNA
2) A large subunit (50S) that assembles amino acids into the peptide chain
What is the cytoplasmic membrane?
Flexible structure composed of a phospholipid bilayer and proteins (e.g., membrane receptors, transport protein and enzymes)
What are the three main functions of the cytoplasmic membrane?
1) Permeability barrier containing a variety of different transport systems that selectively mediate passage of substances into and out of the cell (selective permeability)
2) It plays a key role in bacterial respiration since ATP is synthesised using electron transport systems located on the membrane
3) Anchor for external structures such as flagella and fimbriae
Describe the phospholipid bilayer
- Each phospholipid molecule has one head that ‘likes’ water (hydrophilic) and two tails that ‘hate’ water (hydrophobic)
- The inner double layer of tails forms a barrier to most hydrophilic molecules
- Only small molecules (H2O, O2 & CO2) and some lipid-soluble compiunds can enter the bacterial cell by passive diffusion.
What is the cell wall or envelope?
Rigid structure that protects the cell from osmotic lysis and provides mechanical protection
What is the main component of the cell wall or envelope?
- Peptidoglycan, a polymer of alternating polysaccharide subunits (NAG and NAM) cross-linked by short chains of peptides
What is the structure of gram +ve bacteria (cell wall)
Thick peptidoglycan layer and tectonic acids
What is the structure of gram -ve bacteria (cell wall)
Thin layer of peptidoglycan surrounded by an outer membrane. Outer surface contains lipopolysaccharides (LPS) and open channels (porins) that allow diffusion of small hydrophilic molecules into the periplasmic space.
LPS structure has 3 regions - what are these?
1) An external polysaccharide chain
2) A core oligosaccharide that contains unusual sugar residues
3) An internal lipid region responsible for the toxicity of gram -ve bactera
What are 2 types of atypical bacteria?
1) Mycoplasma
2) Mycobacteria
Mycoplasma do not have a cell wall - which means what?
They cannot be visualised by gram stain and are resistant to antimicrobials targeting the cell wall such as penicillin
What are mycobacteria?
Gram +ve bacteria with a wax-like outer membrane composed of mycolic acids that prevent uptake of the dyes used in the gram stain.
What is a capsule or glycocalyx?
A polysaccharide layer outside of the cell wall, which confers mucoid appearance to bacterial colonies
What do capsules mediate?
Mediate attachment to particular surfaces that protect bacteria from phagocytosis by protozoa or WBC (phagocytes) and from antimicrobial agents
Why are surface components important in bacterial ecology?
They mediate contact of the bacterial cell with the surrounding environment.
What are the 3 main types of surface components?
1) Flagella
2) Fimbriae
3) Endospores
What are flagella?
Filamentous protein structures that allow the bacterial cell to move/ swim and have antigenic properties (H antigen of salmonella)
What are fimbriae?
Shorter and stiffer than flagella and are usually involved in adherence to surfaces (virulence factors)
Bacteria can have one or multiple flagella that can be arranged in 4 different ways - what are these?
1) Monotrichous
2) Lophotrichous
3) Amphitrichous
4) Peritrichous
What is monotrichous?
Bacteria have a single flagellum
What is lophotrichous?
Bacteria have multiple flagella located at the same end of the bacterial surface
What is amphitrichous?
Bacteria have a single flagellum on each of two opposite ends (only one flagellum operates at a time)
What are peritrichous?
Bacteria have flagella projecting in all directions
Name 4 examples of flagella-possessing veterinary pathogens
1) Vibrio Cholera
2) Pseudomonas
3) Salmonella
4) Campylobacter
Some bacteria produce endospores - what are they?
Dormant, highly resistant bodies that ensure survival during adverse environmental conditions (e.g., heat, dessication and toxic chemicals)
Spore structure - what does the core contain?
Core contains genomic DNA and is surrounded by the inner membrane and a ‘normal’ peptidoglycan layer (germ cell wall)
What can be used for classification of endospore-forming species?
Shape, size and position within mother cell
What are the non-bulging classes of endospores?
- Oval central
- Spherical central
- Oval sub terminal
What are the bulging classes of endospore?
- Oval sub terminal
- Oval terminal
- Spherical terminal
- Free spore (not bulging)
What are the spore-forming bacteria of veterinary interest?
Bacillus
Clostridium
What are examples of bacillus?
B anthracis
B larvae
What are examples of clostridium?
C. difficile
C. tetani
What is sporulation and germination?
The process leading to spore formation is called sporulation
The reverse process leading to the vegetative cycle is called germination
How is sporulation triggered?
Triggered by unfavourable conditions
Germination occurs in response to what?
Germination occurs in response to specific factors such as heat, abrasion of the spore coat or environmental acidity.
