Lecture 1 - Bacterial Cell Structure Flashcards
Describe the structure and function of plasmids.
Plasmids contain genetic material above that needed to survive (superpower) such as antibiotic resistance.
* They are usually small, closed circular DNA molecules (they can be linear).
* The copy number can vary (1-50); the higher the copy number the greater the metabolic burden however the information is more likely to be passed onto daughter cells.
* They exist independently of the bacteria’s chromosome and are not required for growth and reproduction
* Most carry genes that confer selective advantage
They are an important tool in molecular biology as they can be transferred between species.
Describe the structure and function of the nucleoid.
The nucleoid contains the majority of bacterial DNA in particular the DNA essential for life (cell division, structure etc.). It is irregularly shaped (Undefined) and there is usually only one per cell. The size of the nucleoid varies between bacterial species.
What are inclusions and what is their function?
Inclusions are granules of organic or inorganic material (storage) that are used to keep the cell alive when entering the stationary phase where nutrients are low. The nutrients are reserved for future use
* Glycogen
* Poly-β-hydroxybutyrate (PHB) - PHB is a 100% biodegradable plastic it is however very expensive to produce so it is currently only used for stiches for internal surgery.
* Polyphosphate granules
Sulphur granules - important in anaerobic photosynthetic bacteria which use hydrogen sulphide as an electron donor.
Describe the structure of the cytoplasmic membrane of bacteria.
- Phospholipid bilayer
- Integral and peripheral membrane proteins
How does the cell wall vary in bacteria, describe the method of staining used to identify the type of cell wall?
The structure of the cell wall varies between Gram +ve and -ve bacteria.
Gram staining can be used to identify Gram +ve and -ve bacteria
1. Crystal violet stains binds to the peptidoglycan in the cell wall
2. Ethanol removes any unbound crystal violet
In Gram +ve bacteria the crystal violet is retained
In Gram -ve bacteria only some is retained
3. The sample is then counterstained to dye the Gram -ve bacteria pink
When doing Gram staining it is important to look at the edge of the sample for individual cells.
Describe the structure of the Gram +ve cell wall
The Gram-positive bacterial cell wall
90% of the Gram-positive cell wall is composed of peptidoglycan a polymer of N-Acetylglucosamine (NAG/GlcNAc) and N-Acetylmuramic acid (NAM/ManNAc) which form chains linked via peptide bridges (Targets for lysozyme enzyme part of innate immunity).
Within the peptidoglycan there are wall associated proteins and teichoic acids (carbohydrates with a similar structure to peptidoglycan) sometimes they go through the peptidoglycan layer and into the cytoplasmic membrane and are known as lipoteichoic acids. Lipoteichoic acids are important for fluidity (target/ligand for macrophage)
Describe the structure of the Gram -ve cell wall.
Gram-negative cell wall
The gram negative cell wall has a thin layer of peptidoglycan which is in the periplasm (gap) between the outer and cytoplasmic membrane. The outer membrane has a number of structures within it:
Porins - tube filled with water that allows small hydrophilic water to enter the cell and waste to leave. (target for development of new antibiotics)
LPS (Lipopoly-saccharide) - composed of a lipid (allowing the structure to embed in to the membrane), a core polysaccharide that doesn’t change and a variable polysaccharide region which changes between bacteria known as the O-antigen which can be recognised by phagocytes.
Describe the structure and function of the bacterial capsule
The bacterial capsule is composed of extracellular polysaccharides. It is not present in all bacteria but often in the most virulent and its extent depends on species.
It protects from host defences as it increases the bacterial size making phagocytosis harder and also masks ligands/antigens on the bacterial surface.
It also protects from harsh environmental conditions as it is very hydrated and therefore a water source in times of scarcity.
It can also help with the initial attachment to surfaces.
Describe the structure and function of the bacterial S-layer.
The S-layer is present in all archaea and some bacteria however its exact function is not fully known.
It is a monolayer of identical proteins or glycoproteins.
The function is thought to be anti-phagocytic and for adhesion based on adhesion etc. of mutans without the S-layer. The mechanism for this however is not yet known
What is the function of flagella and define the terms:
Polar flagellum
Monotrichous
Amphitrichous
Lophotrichous
Peritrichous
Flagella are used for swimming and attachment to surfaces.
Polar flagellum - Flagellum at end of cell
Monotrichous - One flagellum
Amphitrichous - One flagellum at each end of cell
Lophotrichous - Cluster of flagella at one or both ends
Peritrichous - Spread over entire surface of cell
What are the differences and similarities in structure and function of Fimbrae and Pili?
Fimbriae and pili are surface proteins (therefore ligands for phagocytes).
Fimbriae are short, thin, hair-like, proteinaceous appendages (up to 1000 per cell) important in recognition and attachment to surfaces.
Pili (sex pili) are longer, thicker and less numerous (1-10 per cell) required for mating (transfer of plasmids) and attachment to surfaces.
Why do so many of the features of bacterial cells aidd attachment to surfaces?
It is important for bacteria to have structure that aid initial attachment to surfaces as bacteria have a -ve charge as do most surfaces therefore they need to overcome this repulsion to attach.
Give some feaures of the natural human flora. (attachment)
The majority of bacteria within our body are attached
* Must overcome flushing mechanisms
* Surface-associated communities - BIOFILMS
* Large SA for attachment
* Bacterial pathogens will also colonise
* Different surfaces present - soft tissue, enamel etc.
Communities - mixed species.
How are baccteria identified and classified and why is it important to do so?
It is important that bacteria can be identified and classified
* To bring order to the huge diversity of bacteria
* To enhance communication to ensure scientists are working on and discussing the same organism
* To provide the means for accurate identification for example of pathogens for diagnosis of infection and effective treatment.
Bacteria are identified by:
16S rRNA sequence comparisons (molecular technique)
Classical taxonomy
Use of differential and selective media.
How are rRNA sequences used to classify bacteria?
The prokaryotic 70S ribosome is composed of two subunits (one with a sedimentation rate of 50S and the other with a rate of 30S). Together they have a sedimentation rate of 70S.
The 50S subunit is composed of a section of highly conserved RNA (5S and 23S rRNA)
The 30S subunit is composed of a section of 16S rRNA which is very diverse
Bacteria can be classified based on their 16S rRNA sequence. The more closely related the bacteria the more similar their 16S rRNA sequence.
The sequences are aligned and analysed where the number of differences is calculated. The number of differences are then used to calculate the evolutionary distance = number of differences/length of sequence. The evolutionary distance is then used to derive a phylogenetic tree. The phylogenetic tree separates species depending on their percentage difference.
