LO: Explain how Koch's postulates establish a causal link between a microorganism and disease, and outline their limitations
Koch's postulates
Establishing that a specific microbe causes a disease
Microbe present in every case of the disease and absent from healthy hosts
It can be isolated from a diseased host and grown in pure culture
Pure culture causes disease when introduced into a healthy host
The same microbe is re-isolated from new host and matches original agent
Assumes if you get the bug you will get the disease
Disease generally results from an interaction between the host and pathogen and the resulting damage is both direct and indirect
LO: Describe the host, pathogen and environmental factors that influence susceptibility to infection
LO: Describe the principal bacterial virulence factors
Pathogen factors
Proximate - "how"
Evolutionary - "why"
Adhesins and pili: attach to host surfaces, establish colonisation and resist clearance
Virulence can be favoured by selection when disease aids in transmission
Immune evasion: antigenic variation, IgA protease
Much damage is an accidental by-product, not an adaptation as often a transmission dead-end
Toxins: exotoxins target host cells, endotoxins drives the inflammatory cascade
Virulence: sometimes an adaptation, sometimes and accident
Inoculum - "how much"
Route of entry - "which way"
Below threshold - defences clear organism before it establishes
Decides which defences are met or bypassed
Threshold varies hugely
Host factors
Immunity
Physiological reserve - withstanding infection
Barriers
Age, frailty, comorbidity and nutrition
Ciliary escalator
Decides how much insult a patient absorbs before decompensating
Phagocytes
Antibody and T cells
Environmental factors
Physical - how the pathogen survived outside a host
Social - influences exposure and contact
Biological - vectors
LO: Distinguish colonisation from infection, endogenous from exogenous sources, and community acquired from healthcare-associated infection
Is it causing disease?
Colonisation
Infection
Present
Present
No host response or damage
Damage or host response
Positive culture infection
Do not treat colonisation
Where did it come from?
Endogenous
Exogenous
From own flora
From outside
Prevention differs
Endogenous - keep patient upright to reduce aspiration
Exogenous - Transmission precautions
Where was it acquired?
Community acquired
Healthcare associated
Outside healthcare
In care
Different likely organisms and resistance
LO: Describe the chain of infection, including routes of transmission (contact, droplet, airborne, faecal, oral, vector-borne, blood-borne) and the distinction between horizontal and vertical transmission
The chain of infection
Infection needs every link connected, break any link and transmission stops
flowchart LR
A[Infectious agent] --> Reservoir
Reservoir --> B[Portal of exit]
B --> Transmission
Transmission --> C[Portal of entry]
C --> D[Susceptible host]
Reservoir - where the organism normally lives and multiplies: human, animal or environment
Portal of exit - How the organism leaves the reservoir to spread
Transmission
Horizontal - between individuals
Vertical - mother to child
LO: Outline the principles of infection prevention and control, including standard precautions, hand hygiene, PPE, transmission-based (isolation) precautions, and statutory notification of notifiable diseases
Breaking the chain of infection
Infection control: policies, procedures and practices implemented primarily in healthcare settings to prevent and minimise the spread of infections
Hospital and community infection control team
Cleaners
Medical microbiologists
Local health protection team
Environmental health officer
Hand hygiene
WHO (World Health Organisation) 5 moments -applied to every patient as you cannot tell who is infectious-
Before touching patient
Before a clean or aseptic procedure
After body fluid exposure risk
After touching patient
After touching the patient's surroundings
Soap and water when hands are soiled, rather than alcohol gel
PPE
Risk assessed by exposure: gloves, apron, goggles
Transmission based precautions
Contact
Enteric
Respiratory - Droplet
Respiratory - Airborne
Spread
Direct or indirect contact
Faecal/oral
Large droplets, short range (~2m)
Small aerosols that stay suspended and travel further
Precautions
Single room, gloves and apron, hand hygiene, dedicated or decontaminated equipment
Single room (ensuite), gloves and apron, soap and water handwashing
Single room, fluid resistant surgical mask, eye protection
Fit-tested FFP3 mask, negative pressure room
Notifiable diseases
Statutory duty
Who
Attending doctor
To Whom
'Proper officer' of the local authority, in practice the UKHSA health protection team
When
On clinical suspicion. In writing within 3 days or by phone within 24 hours
Why
Public health can act: outbreak detection, contact tracing, control