29th September 2016 Microbiology II Gram-Positive Cocci - Staphylococci The beginning They have been around for centries: - 1st description in 1881 by Sir William Ogsten (surgeon). Dissecting abscess he found grape-like clusters of Staphylococci. - One of his collegues then discovered two different types of Staph: S.aureus (gold colonies) and S.epidermidis (white colonies) S.aureus is the Staph that can cause infection. S.epidermidis is the Staph that lives as our skin and gut flora. Gram Positive Cocci Heterogeneous Spherical shape Gram-+ reaction No endospores Classification of Staphylococci Kingdom: Bacteria Family: Staphylococcaceae Gram-positive cocci: 0.5-1.0 um diameter Facultative anaerobes: 18-40? (prefers to grow with no oxygen, but can grow in the presence of oxygen) Catalase positive - breaks down hydrogen peroxide into water and oxygen(survival mechanism) Approximately 49 species can be found on the skin and mucous membranes. The major differentiating factor is whether the species is coagulase positive or negative. Coagulase production gives the microbe the ability to form a fibrin clot from fibrin (citrated plasma). This surrounds the microbe and acts a defence barrier against
29th September 2016 Microbiology II surround toxins e.g. antibiotics. When the colony of S. aureus suspended in plasma, coagulase binds to a serum factor and complex converts fibrinogen into insoluble fibrin (clot). Coagulase-positive Staphylococci: S.aureus Coagulase-negative Staphylococci: 30+ species, S. epidermidis and S. saprophyticus. S.aureus -> anterior nares S.captis > sebaceous glands S.haemolyticus and S.hominis -> apocrine glands Disease causing (pathogenic): - S. aureus - S. epidermidis - S. haemolyticus - S. lugdunensis - S. saprophyticus - MRSA Staphylococcus aureus It is an example of a true pathogen. A true pathogen must: 1. Attach to the host and begin colonising the host. 2. Evade the host defences e.g. immune system; having the ability to take over. 3. Produce its evasins and exoellular toxins. A possible fourth factor in which a true pathogen must have is resistance to antibiotics. Microbes are able to obtain resistance genes due passing them on. Capsule and slim layer: - Polysaccharide capsule - Eleven capsular serotypes: serotypes 5 and 8 are associated with infections - Capsule inhibits phagocytosis - Slim layer (biofilm) allows adhesion to tissues and foreign bodies e.g. catheters and prosthetics Peptidoglycan and associated enzymes: - Layers of glycan chains built with 10 to 12 alternating subunits of NAM and NAG - PBP are targets for penicillin and other beta-lactams. · Endotoxin-like activity: Stimulates endogenous pyrogens production Activation of complement Production of interleukin (IL) Aggregation of WBCs (Neutrophils) - forms an abscess Surface adhesion proteins - MSCRAMM: - Bound to cell wall peptidoglycan covalently - Protein A - binds Fc regions of IgG1, 3 and 4 - Fibronectin-bindng protein
29th September 2016 Microbiology II - Clumping factor protein (coagulase) Defenses against innate immunity: - Capsule protects it from phagocytosis - Protein A binds IgG upside down and prevents antibody-mediated immune clearance Virulence factors of S.aureus Adhesion proteins (Adhesins) are cell bound proteins that can bind to: - Fibrin in healthy skin - Fibronectin in slightly damaged skin -