Streptococcus Flashcards
Characteristics
Strepto= necklace. They are cocci in chains. They are Gram positive,
non-motile, and do not form spores.
They are catalase and coagulase negative. They grow poorly on simple media but grow well on media containing blood, serum or fermentable sugar. Longer chains are formed in liquid media. Virulent strains are capsulated with hyaluronic acid in the capsule.
The pathogenic species live in the naso-pharynx and GI tract of humans and other animals.
Classification
3 methods of classification are recognized, viz:
1. Bergey’s.
2. Brown’s.
3. Lancefield.
The Lancefield is most popular. It is based on the presence of polysaccharide hapteins in the cell wall which can be demonstrated by precipitin tests. Using this method, 18 groups of Streptococci are recognized; viz: A,B,C,D,E,F,G,H,K,L,M,N,O,P Q,R,S &T. They are known as Lancefield groups!
•NB: S. pneumoniae and Viridans streptococci do not possess the Lancefield antigen.
Haemolysua streptococci
a). Alpha haemolysis (partial lysis of blood around the colonies).
•b). Beta haemolysis (complete lysis of blood around the colonies)
.c). Gamma haemolysis (no lysis at all around the colonies).
•d). Alpha prime haemolysis (partial followed by complete haemolysis around the colonies).
•Haemolysis is influenced by type of media, production of haemolysin O and S, type of blood, aerobic and anaerobic incubation etc
Important group (species)
- Lancefield group A (Strep. Pyogenes).
•.It possesses capsule made up of hyaluronic acid (detectable only in young cultures, 1-2 hours old)
•TOXINS PRODUCED:
•1. Streptolysin O (Oxygen labile). It is highly antigenic.
•2. Streptolysin S. (Oxygen stable). It is non-antigenic. It is also called Leucocidin. These two toxins produce the beta haemolysis.
- Erythrogenic (Dick) toxin. It is responsible for the rash of scarlet fever. It is produced by a few strains which are lysogenized.
Beta haemolysus
It may be increased by:
•i. incubation under increased carbon dioxide.
•Ii. Anaerobic incubation
•Iii. Prolonged aerobic incubation followed by overnight refrigeration.
Infections caused by S. pyogenes.
The main site of streptococcal infection is the throat.
•1. Purulent tonsilitis. (in older chidren and adults.)
•2. Diffuse pharyngitis –in younger children .
•3. Scarlet fever- due to erythrogenic toxin.
•4. Superficial and wound infections.
•5. Erysipelas-diffuse strep skin infection involving superficial lymphatics.
- Puerperal sepsis.
•7. Rheumatic fever. Features include:
•A). Fever.
•B). Myocarditis with mitral & aortic valve damage).
•C). Arthritis- fleeting joint swellings.
•D). Chorea-uncontrolled movements of extremities.
E). Subcutaneous nodules-rubbery nodules under the skin.
•F). Rash (Erythema marginatum- ie with defined red margin.
Complications of strep throat infection.
- Peritonsilar & retropharyngeal abscesses.
•2. Diffuse cellulitis of the floor of the mouth known as ‘Ludwig angina’
•3. Spread to middle ear, chest and brain.
•4. Acute glomerulonephritis –by serotype 2.
•5. Rheumatism.
•6 .Erythema nodosum.
Other Streptococcal species.
- Enterococci.-cause pyelitis & SBE.
•2. Viridans streptococci-cause SBE.
•3. Anaerobic streptococci-(Peptostreptococci).
•They cause puerperal sepsis and wound infections. - Pneumococci-genus Diplococcus-is the only specie. It is Gram positive diplococci, catalase negative. They produce mucoid colonies, alpha haemolysis, bile soluble, ferment inulin,
Pneumococci
They are susceptible to optochin (EthylhydrocupreineHcl); Neufeld (quellung) test positive.
•Habitat: Upper respiratory tract-hence their involvement in 80-90% pneumonia in aduts in form of bronchopneumonia/lobar pneumonia.
•Acute pyogenic meningitis, Otitis media esp. in children.
•Rx: Penicillin & sulphonamide.
•Prophylaxis: Capsular polysaccharide vaccine.
