M1: How bacteria cause disease and infective spread Flashcards
What is a commensal organism
One which colonises the healthy host and doesn’t cause disease
What is an opportunistic pathogen
One which causes disease when the opertunitry arises due to disturbance in the homeostasis between host and commensal organisms e.g. in
- immunosuppression
- when they get to sites they don’t normally live in
- antibiotics and other therapies
- dietary imbalances
How does the normal flora protect against pathogenic invasion
It competes with the pathogen for colonisation sites and so acts as a barrier against invasion by undesirable exogenous organisms
What is virulence
The quantitative ability of a pathogenic micro-organism to cause disease - virulence of microbe depend on microbial and host factors
List the microbial factors influencing virulence
- transmissibility
- infectivity
- invasiveness/toxicity
- ability to evade host defences
List the host factors influencing virulence
- age
- genetic factors
- general host defences and local host defences
- immunodeficiency
Outline the exogenous sources of infection
- person to person contact
- zoonosis (animals to humans)
- environmental e.g. water and soil
Outline the endogenous sources of infection
Normal flora can invade causing an endogenous infection = opportunistic pathogens
What is an endemic
An infection that is always present in the population at more or less constant levels
What is an epidemic
An infection that is at higher than normal levels within the population which is usually much higher than the endemic level
What is an outbreak
A localised increase in the disease incidence
What is a pandemic
An epidemic spreading between continents
What are the two modes of transmissions for infections
- Horizontal = transmission of organism between individuals
- Vertical = transmission from mother to offspring in utero/around birth
How do congenital infections occur
Via mothers bloodstream and crossing into placenta or can be acquired shortly before or during delivery
What are the modes of transmission
- respiratory spread
- faecal-oral spread
- via the skin or mucous membranes
- blood
- fomites (inanimate object carrying transmit microorganisms)
- zoonoses
- sexual transmission
How do skin to skin transmissions of infection occur
The skin is usually a good barrier to infection, but transfer can occur via direct contact or via fomites (shared towels); this usually occurs through abrasions in the epidermis
What is cellulitis
A spreading infection of the soft tissues commonly caused by streptococcus progenies getting into small abrasions in the skin and it presents as red, swollen, painful and hot (erythema, oedema = cardinal signs of inflammation)
Give an example of infections that can be spread via saliva
Herpes simplex
What are nosocomial infections and give the most common types of nosocomial infections
Those that are hospital acquired
- UTIs
- wound/skin and soft tissue infections
- respiratory tract infections
How can the spread of infection within hospitals be limited
- Isolating patients with known resistant organism
- Strict adherence to sensible infection control policies
- Hand washing between patients
Which factors determine how easily an infection can spread
- ability to survive
- ability to find alternative host
- shedding capacity
- infectivity
- virulence
- ability to evade immune response
- damaging the host : exotoxins
- producing enzymes
How does neisseria gonorrhoea attach and invade host tissues
Adheres to genital mucosa by fimbriae
How does giardiasis laamblia attach and invade host tissues
Attaches to jejunal mucosa by special sucking disk
How does influenza virus attach and invade host tissues
By its hemagglutinin antigen
How do respiratory bacteria partake in immune evasion
They secrete an IgA protease which degrades the hosts immunoglobulin
How do streptococcus pyogenes evade host immune defences
Express protein A which binds host immunoglobulin preventing opsonisation and complement activation
How does streptococcus pneumoniae evade host immune defences
Has a polysaccharide capsule which inhibits uptake by polymorphs
How are infections diagnosed
- History
- Clinical examination
- Laboratory investigations
- Culture; pathogen detection
What do serological diagnosis look for
A four-fold rise in antibody titre in acute infection or the presence of IgM; this is useful in the diagnosis of viral infections or when it is hard to grow the presumptive bacteria
