Streptococci

 

 

Streptococci are gram-positive cocci which grow in chains. The initial classification is based on their ability to haemolyze RBCs  B-haemolytic streptococci are further differentiated immunologically into Lancefield groups. B-haemolytic streptococci produce a clear zone of complete haemolysis on blood agar plates. This is due to the release of streptolysin O and S. a-haemolytic streptococcus are surrounded by a narrower zone of haemolysis. Green discolouration of the haemolyzed zone frequently occurs, hence the synonym term S.viridans. g-Streptococci produces no haemolysis. S. faecalis is a typical nonhaemolytic species. This classification of streptococci based on haemolysis is not entirely reliable, because there is considerable strain variation e.g. certain members of Lancefield antigenic groups B, C, D, H, and O are non-haemolytic, despite the usual classification of these groups in the B-haemolytic category.

 

Group	Haemolysis	Usual habitat	Pathogenicity
A	+	Man	Many human diseases
B	±	Man and cattle	Neonatal menigitis
C	±	Man and  animals	Mild respiratory infections
D	±	Diary products GI tract of man and animals (enterococci)	Urinary tract and wound infections, endocarditis
E	+	Milk, swine	Unknown
F	+	Man	? found in the respiratory tract
G	+	Man, dogs	Mild respiratory infections
H	±	Man	? found in the respiratory tract
K	±	Man	? found in the respiratory tract
L	+	Dogs	Genital tract infections
M	+	Dogs	Genital tract infections
N	-	Diary products	None
O	±	Man	Found in the respiratory tract; endocarditis

 

 

B-haemolytic streptococci

 

B-haemolytic streptococci may be separated into immunologically specific groups (Lancefield groups) on the presence of group-specific C carbohydrate Ags. Group A streptococci can be further differentiated into more than 55 immunological types by their M protein. A number of extracellular products are produced:

 

1.    Erythrogenic toxin (A, B, C)

2.    Streptolysin O, and S

3.    Diphosphopyridine nucleotidase

4.    Streptokinases A, and B

5.    DNAses A, B, C, D

6.    Hyaluronidase

7.    Proteinase

8.    Amylase

9.    Esterase

 

Pathogenicity

 

Group A - S. pyogenes causes both suppurative and non-suppurative diseases. Suppurative diseases include pharyngitis and complications such as otitis media, mastoiditis, peritonsillar abscesses, meningitis, peritonitis, and pneumonia. Scarlet fever may or may not occur. Other suppurative conditions include puerperal sepsis, cellulitis, impetigo, lymphagitis, and erysipelas. The principal diseases of the non-suppurative group are acute glomerulonephritis and rheumatic fever. Group A can be transmitted to humans via food.

 

Group B - this group plays a particularly important role in neonatal infections. They are frequently responsible for outbreaks of neonatal meningitis and septicaemia.

 

Group C - may be responsible for mild cases of pharyngitis

 

Group D - non-haemolytic strains of group D (S. faecalis or enterococcus) are common inhabitants of the human G.I. tract. They occasionally cause subacute bacterial endocarditis. They may cause a clinical syndrome similar to that of staphylococcal intoxification. Diarrhoea, abdominal cramps, nausea, vomiting, fever and chills occur in 2-36 hours after ingestion. The infectious dose is probably high >10⁷ organisms. Group D can be transmitted to humans via food. As in group A, a number of foods may be affected, including milk, ice cream, eggs, steamed lobster, ham and salads. In almost all cases, the foodstuffs were allowed to stand at room temperature for several hours between preparation and consumption. Entrance into food is the result of poor food hygiene, ill food handlers, or the use of unpasteurised milk.

 

Group G - may be responsible for mild cases of pharyngitis

 

 

a-haemolytic streptococci

 

This group is often referred to collectively as the viridans group. They have a very low degree of pathogenicity compared with pneumococci, which are also a-haemolytic. Unlike pneumococci, S. viridans are neither bile-soluble nor sensitive to optochin. S. viridans is the major cause of subacute bacterial endocarditis.

 

 

Non-haemolytic streptococci

 

Many non-haemolytic species such as S. faecalis possess the same group-specific cell wall Ags as certain haemolytic streptococci. Non-haemolytic streptococci are generally of low pathogenicity for man except as an occasional cause of subacute bacterial endocarditis.

 

 

 

Laboratory diagnosis

 

Streptococci are readily cultured from blood agar plates where a or B haemolysis may be seen. The colonies may be mucoid, matt, or glossy. Pour plates should be used in order to be certain of detecting B-haemolysis. Lancefield typing may be carried out on extracted carbohydrate C Ags classically by the precipitin test with specific rabbit antisera. However, a simple method may be used to identify group A since group A strains are exquisitely sensitive to bacitracin.

 

The patient’s sera may be tested for the presence of anti-M antibodies by a variety of serological tests. However, test results may be often misleading because of cross-reacting Ags. Serological tests for Abs to extracellular products are much easier to perform and are widely used to obtain evidence of recent streptococcal infection. The antistreptolysin test measures Abs against streptolysin O. Antibodies against streptokinase, hyaluronidase, or DNAse may be also be used.

 

 

Treatment

 

B-haemolytic streptococci are among the most susceptible pathogens to antimicrobial agents. Penicillin is the drug of choice in the treatment of S. pyogenes infection. Where a history of hypersensitivity to penicillin exits, erythrocmycin may be used instead. Prophylactic penicillin is often given to patients with rheumatic fever in order to prevent a recurrence.