Characteristics

They are catalase-negative, Gram-positive spherical or oval cocci in pairs and chains; 0.7–0.9 µm in diameter. Chain formation is best seen in liquid cultures or pus.

Culture

These cocci grow well on blood agar, although enrichment of media with glucose and serum may be necessary. Typical haemolytic reactions are produced on blood agar (Fig. 11.1):

Fig. 11.1 α- and β-haemolysis: β-haemolytic colonies (e.g. Streptococcus pyogenes) produce complete translucence of blood agar, whereas α-haemolytic colonies (e.g. Streptococcus pneumoniae) do not. Note also the sensitivity of S. pneumoniae to a disc impregnated with optochin.

Serology

The carbohydrate antigens found on the cell walls of the organisms are related to their virulence. Hence, serogrouping, termed Lancefield grouping, is useful in the identification of the more virulent β-haemolytic species. Currently, 20 Lancefield groups are recognized (A–H and K–V) but not all are equally important as human pathogens. The following are worthy of note:

Habitat and transmission

The normal habitat of this species is the human upper respiratory tract and skin; it may survive in dust for some time. Spread is by airborne droplets and by contact.

Characteristics

It is found as a commensal in the nasopharynx of a minority of healthy adults, but more commonly (about 10%) in children. It grows well on blood agar, with a characteristic halo of β-haemolysis. Some strains produce mucoid colonies as a result of having a hyaluronic acid capsule. This may contribute to virulence by offering resistance to phagocytosis.

Culture and identification

Culture on blood agar yields characteristic β-haemolytic colonies (lysis of blood due to streptolysins O and S). A Gram-stained smear may show characteristic cocci in chains (Fig. 11.2); these are more developed in liquid than in solid media. The isolate can be presumptively identified as Streptococcus pyogenes if it is sensitive to bacitracin.

Fig. 11.2 Scanning electron micrograph of a chain of streptococci.

If rheumatic fever is suspected, then testing the patient’s antistreptolysin O (ASO) antibody titre will demonstrate previous exposure to Streptococcus pyogenes.

Pathogenicity

Streptococcus pyogenes causes a number of infections; the most notable are:

Treatment and prevention

Penicillin is the drug of choice; erythromycin is suitable for patients hypersensitive to penicillin. No vaccine is available.

Habitat and transmission

Found in the human vagina; sometimes anorectal carriage occurs. Babies acquire infection from the colonized mother during delivery or during nursing.

Characteristics

Gram-positive cocci in chains.

Culture and identification

Gram-stained smear and culture yielding β-haemolytic colonies on blood agar; colonies on blood agar are generally larger than Streptococcus pyogenes. Lancefield group is determined by antiserum against cell wall polysaccharide.

Pathogenicity

No toxins or virulence factors have been identified. This species causes neonatal meningitis and septicaemia; it is also associated with septic abortion and gynaecological sepsis.

Treatment and prevention

Penicillin is the drug of choice; erythromycin is suitable for patients hypersensitive to penicillin. Prophylactic antibiotics may be given to neonates if the mother is culture-positive.