Streptococcus

Gram positive cocci, that are facultative anerobes

Classification:

Haemolysis

• α-haemolytic (turn blood agar green)
• β-haemolytic (turn blood agar clear)
• non-haemolytic (don’t affect the blood agar)

Cell envelope antigens

• The β-haemolytic are further subdivided by the possession of specific carbohydrate antigens in the cell envelope.
• The important medical groups are A, B, C, F and G.

S. pyogenes

About

• Commensal of the upper respiratory tract
• Group A β-haemolytic
• M-proteins, which prevent phagocytosis, and allow binding to proteins.

Pathology

• Upper resp; pharyngitis, tonsillitis and otitis media
• Skin; cellulitis, necrotizing fasciitis, impetigo, wound infection and scarlet fever.
• Invasive: septicaemia
• RHEUMATIC FEVER! usually occurs about 2-3 weeks after the initial infection, where the antibodies to the M proteins start to attack cardiac myofibres.

Treatment

• Penicillin is the drug of choice.
• Some strains are resistant, so erythromycin is used

S. agalactiae

About

• Commensal of the perineum
• Group B β-haemolytic cocci
• Type-specific antigensproteins, work in a similar way to the M-proteins.

Pathology

• Neonates: Important cause of septicaemia and meningitis
• Adults: occasional UTIs, endocarditis, postpartum sepsis, septicaemia.

Treatment

• Penicillin is the drug of choice.

S. pneumoniae

About

• α-haemolytic cocci
• Commensal of the upper respiratory tract
• Produces IgA1 protease, which cleaves IgA1 antibody
• Produces pneumolysin, inhibit neutrophil chemotaxis, phagocytosis and Ig synthesis

Pathology

• Commonest cause of lower respiratory tract infection
• Bacterial meningitis, especially in elderly.
• People without spleens are particularly susceptible to this, as they have a reduced capacity to produce antibodies to carbohydrate antigens.

Treatment

• They are similar to S. aureus, in that they have widespread resistance. Sadly, treatment often needs vancomycin.

Viridans

About

• α-haemolytic cocci
• Commensal of the oral cavity, and nearby to it
• Optochin resistant, unlike S. pneumoniae

Pathology

• Commonest cause of lower respiratory tract infection
• Bacterial meningitis, especially in elderly.
• People without spleens are particularly susceptible to this, as they have a reduced capacity to produce antibodies to carbohydrate antigens.

Treatment

• They are similar to S. aureus, in that they have widespread resistance. Sadly, treatment often needs vancomycin.

Other Types

• Micrococcus – forms squares, facultative anaerobe, rarely causes disease except in immunocompromised.
• Peptococcus & peptostreptococcus – forms chains, anaerobic, can cause skin infections and bacteraemia