The Streptococci and Enterococci

 

The Streptococci and Enterococci are gram positive cocci arranged in chains. The chain-like morphology is particularly evident in smears made from cultures grown in broth; the chain morphology is less evident when smears are made from solid media cultures. These organisms are all catalase negative, therefore the catalase test is useful in their identification.

Different species exhibit characteristic types of hemolysis on blood agar which is differential for them. Thus it is important to note if the colonies are alpha, beta, or gamma hemolytic.

These organisms are also classified based on their reaction with specific antibodies (or antisera) directed against unique carbohydrate antigens in the bacterial cell wall. This type of serologic typing is called 'Lancefield Typing' and the organisms are designated as belonging to specific Lancefield 'groups' which are designated with capital letters.

1. Group A Streptococcus - Streptococcus pyogenes

This microaerophilic, beta-hemolytic organism is also bacitracin sensitive. It is a serious pathogen and it is the cause of strep throat, rheumatic fever, puerperal fever, glomerulonephritis, scarlet fever, impetigo, erysipelas, toxic shock syndrome and necrotising fasciitis. It produces a number of exotoxins and enzymes that contribute to its disease causing potential.

2. Group B Streptococcus - Streptococcus agalactae

This beta-hemolytic organism is an important cause of septicemia, pneumonia and menigitis in newborn children who acquire the organism from their carrier mothers at birth.

3. Group D Streptococci:

Group D Enterococcus - Enterococcus faecalis

This facultatively anaerobic organism, which can be gamma, alpha or beta hemolytic, is a normal inhabitant of the large intestine. It is also a frequent cause of bladdder infections. Enterococcal infections are particularly common in hospital patients with urinary or intravascular catheters and in patients that have been hospitalized for long periods of time.

Group D Non-Enterococcus -- Streptococcus bovis

This organism is not generally considered to be a pathogen but may sometimes cause endocarditis and bacteremia. This organism is often classified as a viridans streptococcus (see below).

 

4. Streptococcus pneumoniae

This organism, which is technically classified as a viridans streptococcus, is a normal inhabitant of the repiratory tract. This alpha-hemolytic, encapsulated diplococcus is a common cause of otitis media and meningitis and it is a frequent cause of pneumonia in people who have been compromised by other illness.

 

5. The viridans streptococci

This is a heterogeneous group of alpha and gamma hemolytic streptococci which commonly colonize the oropharynx, the gastrointestinal tract and the genitourinary tract. With the exception of Streptococcus pneumoniae, these organisms are not particularly pathogenic, but when the are, they are most commonly associated with dental caries and subacute bacterial endocarditis.