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Bacteria as causes of disease (Definitions (Pathogen - organism that…
Bacteria as causes of disease
Types of bacteria
Gram +ve
Cocci
Staphylococci
Streptococci
Entereococci
Bacilli
Bacillus
Corynebacteria
Listeria monocytogenes
Anaerobic bacilli
Gram -ve
Cocci
Neisseria
Moraxella
Anaerobic cocci
Bacilli
E coli
Campylobacter
Pseudomonas
Salmonella
Shigella
Proteus
Bacteria growing as single cells
Cocci
Gram Negative
Gram Positive
Anaerobic
Peptostreptococcus
Aerobic
Staphylococcus
Streptococcus
Beta Haemolytic
Non-Haemolytic
Alpha Haemolytic
Spirochaetes
Rods
Stains
Gram Stain
Apply a primary stain such as crystal violet (purple)
Add iodine which binds to crystal violet and helps fix it to the cell wall
Decolourise with ethanol or acetone
Counterstain with safranin (pink)
Catalase test
Flavoproteins reduce oxygen using superoxide dismutase producing hydrogen peroxide
Superoxide is converted in the following reaction by catalase: 2 hydrogen peroxide to 2 water & 1 oxygen
Staphylococci = catalase positive, streptococci = catalase negative
Coagulase Test
Distinguishes S.aureus from other staphylococci
Coagulase activates prothrombin to convert fibrinogen to fibrin
Free coagulase is released and identified by tube coagulase test
Bound coagulase (clumping factor) is identified by a slide coagulase test
S.aureus = coagulase positive, other staphylococci = coagulase negative
Haemolysis
Alpha-haemolysis - agar appears green
Beta-haemolysis - clearing of agar
Definitions
Pathogen - organism that causes or is capable of causing disease
Commensal - organism which colonises the host but causes no disease in normal circumstances(may turn opportunistic)
Opportunist pathogen - microbe that only causes disease if host defences are compromised
Virulence/pathogenicity - the degree to which a given organism is pathogenic
Asymptomatic carriage - when a pathogen is carried harmlessly at a tissue site where it causes no disease
Bacterial Cell envelope
Gram +ve - single phospholipid membrane , large peptidoglycan (cell wall)
Gram -ve - 2 membrane, inner cytoplasmic and outer membrane
Bacterial Toxin
Exotoxin - secreted proteins of gram positive and negative bacteria. interfere with nervous system, deregulation of G proteins, stop macrophages
Endotoxin - component of the outer membrane of bacteria e.g. lipopolysaccharide in gram negative bacteria