Lecture Sixteen - Bacterial structure and function II Flashcards
What is the role of adhesion in bacterial infection?
To initiate infection or colonisation bacteria must adhere to the mucosal membrane.
They can do this through capsules, glycocalyx, fimbriae/pili and/or adhesions.
Adherence is specific:
Involves interactions between specific macromolecules on the surface of the bactera and the host cells.
The specificity of adherence explains:
- Why a particular bacterial species will preferentially infect particular sites in the body = Tissue specificity.
- Why particular bacterial species infect a particular host = Host specificity.
–> There are excepetions to this called zoonosis (a pathogen may infect multiple hosts).
What are the routes of entry for bacterial infection?
Most infections begin on the mucous membranes of the respiratory, gastro-intestinal or genitourinary tracts.
- Usually act as microbial barrier.
Occasionally the skin is breached via wounds or insect bites allows access to other sites.
Bacteria that initiate infection adhere specifically to epithelial.
Descibe the mouth as an entry point for bacterial infection.
Very complex bacterial habitat (>600 species).
Teeth is a mineral matrix.
Saliva contains microbial nutrients but not sufficient for hrowth (inorganic compounds).
Also contains antibacterial substances.
Presence of food particles makes the oral cavity a favourable bacterial habitat.
Dental plaque is a biofilm of normal flora bacteria that develops on the surface of the teeth.
Initiated by attachement of salivary glycoproteins.
Provides attachement site for growth of mixed microbial community (including highly specific).
Thick bacterial zones (plaque) forms - anaerobic environment that allows some bacteria to florish.
Dental plaque leads to tooth decay:
Streptococcus spp convert sucrose to dextran polysaccharide (strongly adhesive).
Cariogenic bacteria convert carbohydrates to lactic acid, acetic acid and formic acid. Saliva cannot penetrate the plaque biofilm to wash away the acid.
As plaque accumulates, acid attacks the tooth, resulting in tooth decay, opportunistic disease.
DENTAL CARIES:
Presence of carbohydrates (particularly sucrose) predisposes host to dental caries.
People with low sucrose diets have less acid- producing bacteria in their dental plaque, and are therefore less susceptible to dental caries.
What are opportunistic diseases caused by our own normal microbial flora?
Trauma or damage to skin and mucosal layers or use of antibiotics can result in:
OPPORTUNISTIC DISEASE
Occurs due to overgrowth of a particular spp of microbial flora or due to normal flora gaining access to tissue sites that are not their normal niche eg:
– Antibiotic associated colitis: C. difficile – Streptococcal Pneumonia
– Bacterial Vaginosis and Thrush
– Dental Disease: Dental decay (caries)
Some cures may involve replacing your Microbiota with healthy bacteria from another person = FEACAL TRANSPLANT.
What are the major external structures in bacteria?
Flagella - motility.
Fimbriae/pili - adhesion.
Sex pilus - conjugation.
Note that not every bacteria will have all of these structures, some may not have any.
Explain flagella.
Arrangement of flagella:
Depends on the number of flagella and arrangement cells are termed as:
Monotrichous = cells with single flagella.
Lophotrichous = cells with a tuft of flagella at one end.
Amphitrichous - cells have a flagella at both ends.
Peritrichous = flagella that cover the surface of the cell.
Polar = flagella only at the ends.
Motility enables the cell to:
– Flee from a harmful environment.
– Move towards a favourable environment.
• Flagella are long whip like structures that extend beyond the surface of the cell and glycocalyx and help propel the cell.
• Prokaryotic flagella are made up of three parts:
– Long thin filament – Hook
– Basal body
A favourable stimuli:
– Increases the runs.
– Decreases the tumbles.
i.e. cells moves towards the attraction & vice versa.
This movent in response is referred to as Taxis:
– Phototaxis – stimulus due to light.
– Chemotaxis – stimulus due to a chemical.
– Aerotaxis – stimulus due to oxygen level.
– Osmotaxis – stimulus due to solute concentration.
– Positive taxis – towards a favourable stimulus.
– Negative taxis – away from an unfavourable stimulus.
• Egs: Movement towards a nutrient – positive chemotaxis.
Explain pili/fimbriae.
Fine filamentous structures.
Fimbriae are shorter and more numerous than Sex pili.
Mediate adherence to surfaces (e. g. host cells/mucous membranes).
Involved in inter-bacterial interactions.
Some prokaryotes have short fine hairlike extensions.
Fimbriae (sometimes referred to as Pili but distinct from Sex Pili).
– 3-10nm diametre slender tubes, sticky proteinaceous bristle like projections. Up tp approx 1000 per cell.
Some Gram negative bacteria use fimbriae – for twitching motility.
– to adhere to one another.
– to stick to substances in the environment.
- Egs: Fimbriae are important in biofilms by helping the bacteria to adhere to substrates and each other.
- Neisseria gonorrhoeae colonize the mucous membrane of the reproductive tract, Neisseria cells that lack fimbriae are non- pathogenic.
Explain F or sex pili.
Involved in conjugation - the direct transfer of DNA between two bacteria cells.
– 9-10 nm diametre tubules composed of proteins called pilin.
– Sex pili are longer than fimbriae but shorter than flagella.
– typically there are only one to ten sex pili present in bacteria that have them.
- Bacteria that carry specific genes for conjugative transfer of genetic information can extrude the sex pilus.
- This attaches to another bacterium and holds the “mating pair” in place while DNA is transferred.
E.g. Conjugation pilus or the sex pilus.
E.g. Geobacter pilin-like filaments function as nanowires to transfer electrons outside the cell onto insoluble electron acceptors.
Explain capsules.
Involved in:
- Glycocalyx, slime layer.
- Adhesion/immune evasion.
Endospores are:
• Heat resistant.
- Dormant for many years.
- Spore formation is a complex process (8 h, 200 genes).
What are characteristics of gram positive and negitive bacteria?
Gram positive:
- Have a thick peptidoglycan layer.
- Teichoic and lipotechoic acids.
- Generally rods or cocci.
- May form heat-resistant endospores.
e. g. Staphylococcus, Lactococcus, Clostridium, Bacillus.
Gram negitive:
- Thin peptidoglycan layer.
- Lipopolysaccharides in cell wall.
- Rods or cocci.
e. g. Enteric bacteria (Escherichia, Salmonella, Shigella).
