Streptococci

They cause important human pathology like streptococcal pharyngitis and rheumatic fever and endocarditis , necrotising fasciitis etc among others. Beta hemolytic streptococci, pneumococci and Streptococcus viridans are the important ones.

Morphology

Streptococci are gram positive cocci arranged in chains or pairs. Streptococcus pneumoniae (pneumococci) are arranged as lancet (flame) shaped diplococci in pairs.

S.pyogenes (beta hemolytic strep) has hyaluronic acid capsule, S.pneumoniae and GBS (group B streptococci like S.agalactiae) have polysaccharide capsule, viridans strep do not have a capsule but they produce glycocalyx.

Classification:

On the basis of the type of hemolysis produced on blood agar Streptococci may be classified as alpha hemolytic which form a green zone of hemolysis due to incomplete breakdown of RBCs e.g. S.viridans, S.pneumoniae; beta hemolytic which produce a clear zone due to complete lysis of RBCs e.g. S.pyogenes, S.agalactiae and gamma hemolytic (non hemolytic) which do not lyse RBCs e.g. many strains of S.bovis

1. Lancefield Groups: Beta hemolytic streptococci can be divided into Lancefield Groups A to U on the basis of C carbohydrate which is located on the cell wall. S.pyogenes is included in Group A.
2. M protein: Group A beta hemolytic streptococci can be further typed depending on antigenic variations in M protein which is located on the outer surface of the cell into 80 serotypes. M protein is a virulence factor and some types show a predisposition to cause rheumatic fever while others predispose to acute glomerulonephritis.
3. Pneumococci: They can be typed on the basis of an antigen in the polysaccharide capsule into 85 serotypes. The capsule swells when it reacts with type specific antisera called ‘Quellung reaction’.

Human Pathology

Group A streptococci are the most common cause of bacterial pharyngitis (strep throat),erysipelas (infection of dermal lymphatics presenting as a demarcated, raised, inflamed area usually on the cheeks), impetigo (honey colored crusted lesions), necrotizing fasciitis (exotoxin B effect, “flesh eating bacteria”), streptococcal toxic shock syndrome, endometritis, sinusitis and mastoiditis etc.

This image depicts an intraoral view of a patient, who had presented to a clinical setting exhibiting redness and edema of the oropharynx, and petechiae, or small red spots, on the soft palate. A diagnosis of strep throat had been made, caused by group A Streptococcus bacteria. These bacteria are spread through direct contact with mucus from the nose or throat, of persons who are infected, or through contact with infected wounds, or sores on the skin.

Some people with strep throat can get a red, sandpaper like rash which blanches under pressure, starting at the face and spreading all over the body accompanied by fever and sore throat called Scarlet Fever.

Scarlet fever rash on the volar surface of the forearm, due to group A Streptococcus bacteria.

S.pyogenes can also cause nonsuppurative human diseases namely acute rheumatic fever and acute post streptococcal glomerulonephritis.

Acute Rheumatic Fever: Typically follows streptococcal pharyngitis by 1 to 5 weeks. It is due to immune cross reactivity between M proteins on streptococci and human tissues, especially heart valves, joints and central nervous system. Clinical diagnosis is made with the help of revised Jones criteria. Presence of 2 major criteria or 1 major plus 2 minor criteria are required for the diagnosis of acute rheumatic fever. Evidence of a prior streptococcal infection should be present as well, such as elevated or rising ASO titre , positive throat culture or positive rapid group A streptococcal carbohydrate antigen test. Anti DNAse B, Antistreptokinase and Antihyaluronidase are the other useful antibody tests.

**Acute Glomerulonephritis: **Typically follows S.pyogenes skin infection (impetigo) or less commonly pharyngitis/scarlet fever by 2-3 weeks. These strains are called nephritogenic strains. It is due to deposition of immune complexes on the glomerular basement membrane. On biopsy subepithelial humps are seen. Presents characteristically as periorbital edema, hypertension and ‘smoky’ urine from hematuria.

Pneumococci cause lobar pneumonia with reddish brown or rusty sputum, otitis media, sinusitis, meningitis and sepsis.

Group B streptococci causes neonatal sepsis and meningitis. Incidence can be reduced by maternal screening for GBS in the perinatal period.

Viridans streptococci (S.mutans, S.sanguis, S.mitis) are the most common cause of infective endocarditis. S.mutans cause dental plaque and caries. S.bovis is associated with endocarditis in old patients with colon cancer.

Laboratory diagnosis of streptococcal infections

1. Streptococcus pyogenes: gram positive cocci in chains, capsule will not be stained by gram stain so will look like a halo around the bacteria. Throat swabs can be taken for gram stain, rapid antigen testing and culture. Gram stain of throat swab is indeterminate as S.pyogenes looks morphologically similar to other throat commensals. Blood agar culture will show beta hemolytic colonies which are inhibited by Bacitracin disc (sensitive to Bacitracin).

   In streptococcal toxic shock syndrome blood cultures are usually positive (unlike in staphylococcal TSS).

   In a patient presenting with acute rheumatic fever, throat culture for streptococcus is usually negative (as it happened a few weeks ago).

2. Group B streptococci: Rectal and vaginal samples can be taken for gram stain and culture. Rapid testing for DNA can give results in an hour. They form beta hemolytic colonies on blood agar, hydrolyze hippurate, Bacitracin resistant and give a positive CAMP test.

3. Group D streptococci (EXCEPT Enterococci): They are alpha or non hemolytic on blood agar, characteristically hydrolyse esculin in the presence of bile which produces a black pigment on bile esculin agar, do not grow in 6.5% NaCl.

4. Viridans streptococci: They are alpha hemolytic on blood agar, resistant to Optochin and resistant to bile lysis/ not bile soluble.

5. Pneumococci: They appear on gram stain as gram positive lancet shaped diplococci (cocci in pairs) with surrounding halo because of the capsule. They are alpha hemolytic on blood agar. They are characteristically sensitive to Optochin and are bile soluble. Quellung reaction can be used to detect Pneumococci. Blood cultures will be positive in about 20% of patients with pneumococcal pneumonia. A latex agglutination test on csf can be done for rapid diagnosis of pneumococcal meningitis. Bacteremia and pneumonia can be indirectly detected by the presence of pneumococcal capsular antigen in urine.

Numerous, Gram-positive, Streptococcus pneumoniae