Bacterial Pathogenesis and Transmission Flashcards
5 ways pathogens are transmitted
- Airbourne
- Water & food
- Contact
- Vehicle (aka fomites)
- Vector (living transmitter such as insects)
Epidemiology - definition
-6 factors that interact in the chain of infection
-is the study of factors that influence the frequency and distribution of infectious diseases
factors that interact in the chain of infection;
-host susceptibility
-means of entry
-pathogenic microorganism
-mode of transmission
-means of escape
-reservoirProcess of infection (6 steps)
- Enter the host
- Adhere to cell surfaces
- Colonise cell surfaces
- invade tissues
- Evade host defenses
- Produce toxins and other harmful products
Process of Infection;
-Entering the host -> ways it is achieved (5)
- penetrate skin or mucous membranes
- ingestion with food
- inhalation in aerosols
- sexual transmission
- transmission on an object (fomites)
*pathogenic organisms have to enter the body
Process of Infection;
-Adhesion to cell surfaces (2)
-Mechanisms;
- Pili or fimbriae - organelles that project from the cell surface and attach to host cells
- Adhesins - attachment molecules that permit the bacterium to adhere to receptors on the membranes of certain cells or tissues
*need to adhere otherwise bacteria will be washed/brushed off
Process of Infection;
Colonise cell surfaces - why must colonise
-e.g.
*bacteria must colonise host tissues -> a single bacterium on its own will never cause any harm (need a population)
e. g. intestinal surface is continually replenished
- to est. an infection at such a site, need to adhere to epithelium and multiply before the mucus and extruded epithelial cells are swept away
Colonisation = when bacteria grow on and within host tissues
Process of Infection;
Invasion of tissues
-Aggressins (2 e.g.)
- Bacteria invade tissues using aggressins (enzymes from bacteria)
- have proteolytic or lipolytic activity to assist bacteria in invading tissues and intercellular spaces
e.g. Neuraminidase degrades neuraminic acid (the intercellular cement of epithelial cells of the intestinal mucosa) -> vibrio cholerae and shigella dysenteriae
also, collagenase (produced by clostridium difficile) -> breaks down collagen which connect cells to form tissues
Process of Infection;
Evasion of host defenses
- Some pathogenic bacteria able to resist bactericidal components of host tissues
- encapsulated strains are more virulent and resistant to phagocytosis (e.g. pneumococci)
- some bacteria can survive inside phagocytic cells (i.e. legionella pneumophila, brucella abortus)
Process of Infection;
Production of toxins (2 types)
-what toxins can do
- Bacterial toxins are soluble substances that alter normal metabolism of host cells and damage host
- are the key of pathogenicity
- 2 types of toxins;
1. Endotoxins: Lipid A of gram -ve cell wall toxin
2. Exotoxins: Protein toxins - toxins may act on cells in number of ways (i.e. damage cell membranes or inhibit protein synthesis)
- also can interfere with cell signalling, inhibit release of neurotransmitter and activate host immune responses
Properties of endotoxins
- what are they
- what makes them toxic
- when released by bacteria
- they are; Lipopolysaccharides that are structural components of cell walls of gram neg. bacteria
- Lipid A component is responsible for toxicity of endotoxin
- is released from growing bacterial cells or from dead cells when they are lysed
Actions of Endotoxins
- comparison to exotoxins
- what it can cause
- Are less potent and less specific in their actions than exotoxins
- are heat stable
- effects on such wide variety of host cells result in host responses such as inflammation and fever than can cause gram-negative sepsis which leads to shock and death
Properties of Exotoxins
- denatured by heat, acid & proteolytic enzymes
- have high biological activity
- potent, have specific poisons
- site of action is more localized and confined to particular cell types or cell receptors
- among the most potenet per unit weight, of all toxic substances
Categories of Exotoxins (3)
- categories based on biological effect
1. Neurotoxins: act on neurons & release of neurotransmitters -> interfere w/ muscle excitation & producing paralysis
2. Cytotoxins: disrupts host cells - have a wide array of host cell specificities & toxic manifestations
3. Enterotoxins: stimulate hypersecretion of water and electrolytes from intestinal epithelium and thus produce watery diarrhea.
Vibrio cholerae toxin
-what toxin does
- when toxin released from bacteria, binds to intestinal cell and triggers endocytosis of toxin
- inside cytoplasm, the toxin A subunit activates the G protein to continually stimulate adenylate cyclase to produce cAMP
- high cAMP levels activate cystic fibrosis transmembrane conductance regulator (CFTR) causing a dramatic efflux of Cl ions and water, leading to water diarrhea
Botulinum toxin
-what it does and leads to
- Botulism due to formation of potent exotoxins by large, spore-forming gram-positive bacterial rods
- caused by the neurotoxins Clostridium botulinum
- toxins block transmission of signal between nerves and muscles -> causes paralysis
