Pathology Michaelmas 2013 Notes

Pathology Michaelmas 2013- Experimental and clinical evidence

Info on specific pathogens is all-encompassing

Antibiotics still need to be covered. Perhaps more info on general bacteriology, especially experiments?

Bacteriology- GENERAL:

• CLASSICAL EXPERIMENT demonstrating TRANSFORMATION: mice were infected with both rough (no-capsule) strains and smooth (capsule) strains. When infected with the more virulent smooth strain, the mice died. When infected with the rough strain, the mouse survived, and this also happened when the smooth strain was heat-inactivated before infection. HOWEVER, heat inactivating the smooth strain, infecting the mouse, and then infecting with the rough strain lead to the mouse dying after a while. There must have been exchange (transformation) of genetic information (encoding the capsule) between the strains.

• The INFECTIOUS DOSE of a pathogen can give us an idea of its virulence e.g Shigellosis can caused by as few as 10 bacteria whereas E.coli requires between 10⁶-10⁸.

• Diversifying the flora through FAECAL TRANSPLANTS helps patients with recurrent Clostridium difficile better than antibiotics.

• KNOCKOUT EXPERIMENTS demonstrate the importance of CAPSULES for bacterial virulence as represented by infectious dose. Moreover, the fact that incubation of a non-capsular strain in heat-inactivated serum results in that strain surviving as well as a capsular strain shows that capsules disrupt the complement-mediated opsonisation process (thus also making them anti-phagocytic).

Staphylococcus Aureus:

Usually part of normal human flora. Nose is the main site of colonisation; 30% of people are colonised at any one time. Skin also an important site, especially in hospitals.

• DISEASES: abscesses; various pyogenic infections: e.g endocarditis, impetigo, cellulitis, septic arthritis and osteomyelitis; food poisoning; severe pneumonia (often secondary to influenza); organ abscesses following from bacteraemia, scalded skin syndrome and TSS. PATHOGENESIS is through both causing pyogenic inflammation and through producing toxins.

• Typical lesion is an ABSCESS where the bacteria become inaccessible to phagocytes and opsonins and general immune surveillance through a wall of fibrin and cells. These undergo central necrosis and usually drain to the outside; however there is the risk that the organisms can DISSEMINATE into the bloodstream and seed organs- much more serious. DRAINAGE is the cornerstone of abscess treatment.

• Important predisposing factors are INDWELLING FOREIGN BODIES e.g. tampons (these can cause TSS) and OPEN WOUNDS. Also REDUCED HUMORAL IMMUNITY especially predisposes to infection due to the pyogenic nature of this bacterium. This might include low levels of antibodies, complement, or neutrophils. CHRONIC GRANULOMATOUS DISEASE PATIENTS are also especially prone.

• VIRULENCE FACTORS: - Polysaccharide capsule: 11 serotypes, is anti-opsonic. However high levels of specific anticapsular antibodies are protective in that they promote opsonisation; indivduals who have already been infected however are usually not protected from subsequent infections. Teichoic acids have LPS-like immunogenic properties, and through binding to fibronectin they enable adherence to mucosal cells. Catalase: breaks down H2O2 which is important part of the neutrophil’s oxidative burst killing arsenal. Distinguished from other types of staphylococci by coagulase production, which catalyses the formation of fibrin and contributes to abscess formation. Protein A normally binds to the Fc receptor of IgG, preventing the activation of complement and other humoral effector mechanisms. Enterotoxin is heat resistant and causes food poisoning characterised by watery non-bloody diarrhoea- acts as a SUPERANTIGEN within the G.I. TSST also acts as an antigen and causes toxic shock. Exfoliatin causes scalded skin syndrome- acts as a protease that cleaves the desmosomes between keratinocytes in the epidermis, separating it from the granular cell layer. Membrane damaging toxins: e.g leukocidins (many types); kill leucocytes, haemolysin (causes the beta-haemolysis). Hyaluronidase “spreading factor”. Beta-lactamase production is common; thus most strains resistant to penicillin. Chemotaxis inhibitory protein of staphylocci (CHIPS) is secreted by 60% of strains- blocks neutrophil chemotaxis by binding to the formyl peptide receptor and the c5a receptor to block cognate chemotactic agonists from binding. Dlt proteins cause alanine substitutions of teichoic acid that neutralise the cell surface –ve charge that normally attracts cationic antimicrobial peptides secreted into the phagosome.

• The bacteria EXPLOITS ANTIBODIES targeted against its fibronectin-binding proteins (e.g. teichoic acids) as the bound IgG engages a receptor on the nearby platelet surface, causing a clustering of receptors and subsequent platelet aggregation. Then grows in thrombi, escaping the attention of neutrophils. Important facet of ENDOVASCULAR INFECTIONS.

• Mice were infection with 15 Staph aureus strains, each with a differing degree of CATALASE ACTIVITY- a correlation between catalase activity and lethality was observed. Addition of EXOGENOUS CATALASE led to a further increase in lethality.

• Staph aureus can degrade NEUTROPHIL EXTRACELLULAR TRAPS by converting them to deoxyadenosine through the action of nuclease and adenosine synthase, triggering the caspase-3 mediated death of macrophages and their exclusion from abscesses. Pathogenesis has evolved to anticipate host defences and repurpose them for the destruction of the immune system.

• DLT MUTANTS are more susceptible to killing by cationic antimicrobial peptides and neutrophils in vitro.

• TRANSCRIPTIONAL MICROARRAY STUDIES of mRNA from different strains of Staph.aureus following ingestion by neutrophils indicates a large number of differentially activated genes such as those coding for superoxide dismutases and carotenoid pigment biosynthesis. Similar study with STREP PYOGENES (see below)...

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