Principles of Infection Flashcards
What are the 2 types of parasites?
Commensal - can be opportunistic
Pathogen - conditional
Name the sterile parts of the body
Lower respiratory tract
Blood, joint and subcutaneous connective tissue
Female upper genital tract
Urinary tract 9 (not distal urethrae)
CNS including CSF & eye
Other viscera e.g. liver, spleen and pancreas
What is the result of infections?
Infections cause harm to the host causing disease
What are the 4 Koch’s postulates?
Gene theory states that a microorganism must be:
- Present in every case of the infection
- Cultured from cases in vitro
- Reproduce disease in animals
- Isolated from infected animal
Describe a chronic infection
Slower onset / post acute
May still have major locally / systemic symptoms
Has a chronic inflammatory response when host doesn’t succumb immediately to infection
but can’t clear the infection
e.g. Tuberculosis or chronic osteomyelitis
What are pathogens?
Microbes that cause disease often with small numbers ia natural routes, despite natural barriers and immune defences
Describe the key features of an Acute infection
Rapid onset
Major local & systemic symptoms
Acute inflammatory response
e,g. streptococcus polygenes or straphylococccus aureus
What are the host’s defences?
Innate and adaptive immune system
What is a full pathogen?
Initiates infection via neutral route despite immune defences
What microbial and host mechanisms are kept in balance?
Pathogenic mechanisms
- adhesion, toxins, capsule
Host defensive mechanisms
- natural barriers, defensive cells, complement
What are strict pathogens?
Microbes that will always cause disease e.g Bacillus anthracis (anthrax)
What are the body’s natural barriers?
- skin
- eyes
- GI tract
- lungs
- Gut
Give an example of a toxin mediated acute infection
Diphteria (cornyebacterium diphtheriae Tetanus - gram +ve rods - terminal drumstick spores - anaerobe , soil dweller - opisthotorius = severe hyperextension
What is saprophyte?
Saprophyte is a free living organism living on decaying organic matter
How is inflammation regulated?
Inflammation is an ordered process that is regulated via
- Vasodilation
- Oedema
- Complement activation
- Clotting
- PMNs recruitment
- Mast cell degradation
How do host cells respond to pathogens and commensals?
Normally the host cell will manifest an inflammatory response to a pathogen, but not to a coloniser at a normal non sterile site
What is asymptomatic infection?
Pathogenic microbe infection (not commensal or normal flora)
- mild / no inflammatory response
- damage to host is mild / not at all
e.g. chlamydia trachornitis (urethral in males, cervical in women)
50% men asymptomatic
80% women symptomatic
What is inflammation?
Response to tissue injury
Brings serum molecules and cells to infection sites
What are latent infections?
A virus that never leaves the body and can be reactivated
e.g. chocken pox in children causes shingles in adults (varicella zoster -> herpes zoster)
What are conditional pathogens?
Pathogens causing disease only under certain conditions
What is parasitism?
Pathogens that actually cause disease
What is the role of inflammation?
Increases blood supply
Increases capillary permeability
Cell (polymorphs/macrophage) migration from blood to tissue
What are the symptoms of infection?
Local
- redness, swelling, warmth, pain, loss of functions
- pus (pyogenic infection)
Systemic
- Fever, rigors, chills, tachycardia, tachypnoea (increased breathing rate)
What are virulence factors?
Molecules produced by pathogens which add to the effectiveness of the pathogen to:
- evade immune responses
- enter / exit host cell
- immunosuppression
- colonisation
- obtaining nutrients from host
How can commensals cause disease in certain tissues?
In healthy individuals, commensal microbes (normal flora) do no harm
They cause colonisation not infection
But if the host’s defences are weakened (immuno-compromised) infection may occur
-> opportunistic infection
What are the body’s non specific barriers?
Physical conditions , Microflora, Bile & Mucin
Lysozymes, lactoferrin & tight junctions
Cryptidins
Cilliated epithelia
Toxic lipids & lactoperoxidases
Intraepithelial lymphocytes & phagocytes
What are cross reactive antigens or cross immunity?
When immunity to one form of pathogen is effective in protecting against an antigenically
similar but different organism
e.g. Streptococci and Rheumatic heart disease
Name examples of commensals becoming pathogens in different areas of the body
Staphylococcus aureus
- Commensal in nose
- Pathogen in post operative wound infections
Escherichia Coli
- Commensal in GI tract
- Pathogen in Urinary tract causing UTI
Staphylococcus epidermis
- Commensal on skin
- Pathogen in IV lines causes blood stream infections
What are endotoxins?
Components of the bacterial cell wall => drive a massive immune response
What is the consequence of inflammation?
Can cause tissue damage
What are virulent mcirobes?
Highly pathogenic microbes
Give an example of an opportunistic infection
HIV causing a decline of CD4
What are the 3 different types of infection?
Local - surface infection (wounds)
Invasive - Penetrate barriers & spread
Systemic - Via blood to other sites
What does the adaptive immune response involve?
T cells and antibodies
What is commensalism?
No harm done to either side, usually one side benefits - these don’t cause disease
What is virulence?
Virulence is the degree to which a microbe causes disease depending on
- virulent strains
- gene content alters phenotype
- host susceptibility
How are blood and tissue defences initiated?
Defences of blood and tissue involve tissue damage and are controlled by feedback mechanisms e.g inflammation
What are the body’s specific barriers?
Phagocytes: monocytes, macrophages, PMNs & neutrophils
Complement
Acute Phase proteins released by the liver
Transferrin
=> these cause macrophage activation
When do commensals become Pathogens?
At different sites, due to immunosuppression allowing them to bypass defences
What are the stages of infection?
- Acquisition from spread
- Colonisation
- Penetration and spread
- Immune evasion
- Tissue damage
- Shedding & transmission
- Resolution
(not all microbes require all these stages)
How does Whooping cough arise?
Toxin mediated disease (bordetella pertussis)
- invasive adenylate cyclase
- lethal toxin (dermonecrotic) superantigen
- trachaeal cytotoxin
- pertussis Toxin (PTx)
What is the role of virulence factors during infection?
Aid bacterium adhere and colonise in host
Promote tissue damage
e.g. M protein of streptococccus pyogenes acts as
adhesion and structural components of bacterial cell
wall
How do Neisserie Gonorrhae cause gonorrhae?
Fimbrae allow bacteria to adhere to P blood group antigen of uroepithelial cells
What is the net effect of adhesion of pathogens to host cells?
Multiplication at mucosal surface
Localised immune activation
Survival & transmission
How are infections spread?
Direct contact
Indirect Contact
Airborne
Blood products
How does pathogen adhesion occur?
Via surface adhesion structures of bacteria/virus
Attach to mucosal surfaces via specific receptors on host cells
e.g. Influenza virus A - haemoglutinin & sialyoligosaccharides
HIV & CD4 + CXRC5 surface proteins of CD4 cells
Allows specific cell entry
