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
Types of Bacterial infections (5)
-3 things that determine the type of infection that develops
- Purulent infections
- Bacteraemic infections
- Toxaemic infections
- Granulomatous infections
- local infection
- Type of infection that develops depends on properties of pathogen, methods of pathogen to infect & invade host and health or susceptibility of host
Types of Bacterial Infections
- Purulent (2 other names)
- what is it
- Produce pus at site of inflammation
- formed from the accumulation of dead phagocytes, bacteria and host tissue
- AKA suppurative, pyogenic or purulent
- can be caused by many species part of normal skin flora if they enter through damaged skin
e.g. stahylococcal, streptococcal, corynebacterium
Types of Bacterial Infections
Bacteraemic
e.g.
- Occur when bacteria enter the blood stream
- may occur following a wound, surgery or injection
- may lead to septic shock
e.g. Staphylococcus aureus can infect wounds and cause blood poisoning
Types of Bacterial Infections
Toxaemic
- Bacterial toxins in blood (may be endo or exotoxic)
- Toxins interfere w/ cellcular mechanisms and pathoways
- is often fatal
Types of Bacterial Infections
Granulomatous
e.g. of mycobacteria
- Usually caused by slow growing, intracellular organisms
- lesions become walled off due to host response
e. g. mycobacteria causing tuberculosis
- one feature of immune response is formation of a granuloma -> first acts to contain infection, but some bacilli survive inside granulomas for long time in dormant state
Types of Bacterial Infections
Localised
e.g. salmonella enterica
- bacteria that are adapated to infecting one site (i.e. gut, respiratory, urogenital or integument)
e. g. salmonella enterica (is infection of gut)
