The pathogenesis of bacterial infections Flashcards
What is pathogenesis?
Pathogenesis involves all the cellular events leading to the development of disease
What is the definition of a pathogen?
an organism that has the ability to cause disease
What causes peptic-ulcer disease?
Stress/Environmental factors
What bacteria causes peptic ulcers?
H.Pylori
What are three reasons why infections are not in all hosts in the population?
- Infection depends on host factors as well as bacterial factors
- Differences in hosts range (wide vs limited)
- Even among the host range some can be resistant whilst others are susceptible
What is pathogenicity?
An ability to cause disease (damage the host)
What is Virulence?
The degree of pathology caused by the organism/ infection
What is infectivity?
The capacity for transmission and spreading to new hosts
What can effect infectivity?
- Type of pathogen
- Dosage/ Inoculum
- Host factors
- Environmental factors
- Virulence: Species, Strain
What is transmissibility?
- Capacity to grow in parts of the body
- Capacity to readily exit an infected host
- Capacity to survive in transition between hosts
(sporulation, environment and arthropod-borne infections)
What are the two different sources of infection?
Exogenous and Endogenous
What are exogenous sources of infections?
Mainly true pathogens
What are endogenous sources of infections?
Opportunistic pathogens and persistent true pathogens
What is the first step of a succesful bacteria pathogen?
Attachment to and entry into the host body
The movement through fluids is mediated by the flagella
What are the helical structures that extend from the bacteria made up from?
They are long helical structures that extend outward from the surface of the cell
composed of polymerisation of flagellin protein
evolutionarily conserved
What are some routes of entry for bacteria?
Skin and extended mucosa (direct contact)
Body orifices e.g eye/ear reproductive/urinary tract
Direct injection into the blood stream (arthropod-borne infections)
What is the second step for a succesful bacterial pathogen?
Bacterial pathogens often bind to a niche to gain entry into tissue and have to evade host defences
Why can it be a struggle for bacteria to enter into a host cell?
Many parts of the body are protected by mucin which acts as a lubricant and also traps bacteria (prevents microbes from gaining access/ binding to the first layer of cells)
mucin= mesh of proteins and polysaccharides
What are some strategies bacteria can use to evade host defences?
- molecular mimicry
- switching off “irritants”
- hiding inside epithelial/ immune cells
- prevention of activity immune response
What is the third step for a succesful bacteria pathogen?
Multiplication and Spread
Once the bacteria have entered/ passed the dermal and mucin layers they can either become an extracellular or intracellular pathogen
What is the 4th step in being a succesful bacterial pathogen?
Damage to the host and therefore inducing disease
What are some of the mechanisms that can be used to induce disease?
- lysis of host cells/tissues via toxins or invasion
- disruption of the immune system
- disruption of the barrier function
- disruption of the local mciroflora
What is the fifth step of a succesful bacterial infection
Transmission to a new susceptible host
What are some ways the immune system can respond after damage through secretion of toxins?
- Creates pus (proteins, dead cells, host cells)
- Spreading factors, proteins like DNases and proteases facilitate spread into neighbouring tissue
What are some common mechanisms of transmission?
- TB- coughing/sneezing
- Anthrax- air inhalation of spores
- Contaminated water- E.coli in fecal material
Vectors: Insects
What are four major issues for bacteria in the “outside world”?
- availability of essential nutrients
- lack of adherence sites
- exposure to noxious chemicals/ predators
- Exposure to sunlight and extreme weather
What are some survival mechanisms of bacteria in the ‘outside world’?
- Form endospores
- “Hibernation”/ changes into metabolism/ slow down or go into the dormant stage
- Genome plasticity
- Parasitize another host
- Form a collective= biofilm
What are four different types of colonisation factors?
- Adhesins
- Bacterial mobility/ Chemotaxis
- Production of enzymes that breakdown mucin or other host tissues
- Metabolism
What are adhesins?
Best mechanism of adhering to host surfaces through specialist attachment proteins known as pilli/ fimbriae
How is metabolism a colonisation factor?
The ability to use specific nutrients to contribute to succesful colonisation
What are four different virulence strategies?
- Colonisation factors
- Invasiveness
- Toxigenicity
- Aggressins
How is ‘invasiveness’ a virulence strategy?
Several species of pathogenic bacteria are capsulated which assists invasion by neutralising, escaping or withstanding phagocytosis
What is an exotoxin?
Exotoxins are proteins produced inside pathogenic bacteria
They are then secreted or released into the surrounding medium following lysis
What is an endotoxin?
The lipid portion of lipopolysaccharides that are part of the outer membrane of the cell wall of gram negative bacteria
What are the key points of exotoxins?
- Mainly produced by Gram + positive bacteria
- They are proteins in nature and a few are enzymes
- fatal in small doses
- thermolabile
- Can cause cytopathic effects
What are the key points about endotoxins?
- Often released after natural autolysis or artificial disruption
- thermostable
- integral part or the outer layer of the bacterial cell wall
- Mainly associated with gram-negative bacteria
- Can cause TSS
How do aggressins work?
They contribute to the ability of capsulate or non-capsulate bacteria to invade and multiply in the hosts tissues
They then enhance the permeability of tissues and facilitate the spread of bacteria
What percentage of peptic ulcers are caused by H.pylori?
85-95%
What cancer is H.pylori linked with?
Gastric cancer
bacterial class I carcinogen
What are some of the weaknesses of kochs postulates?
- Not all infections can be isolated
- Some agents present in both healthy and sick hosts
- may be no animal model for the experimental infection
- may be no characteristic lesions
What is the difference between exogenous and endogenous pathogens?
Endogenous pathogens originate from the hosts own body whereas exogenous pathogens come from outside the hosts body
What can only gram positive bacteria do as a survival mechanism?
Create endopsores
How may some succesful bacteria resist phagocytosis?
- Inhibit opsonisation
- Kill phagocytes
- Inhibit fusion of lysosomes
- escape from phagocytes
- survive the acid environment
