The genus Streptococcus contains a variety of nonpathogenic species that are considered normal flora and others that are important human pathogens. Streptococci are typically catalase negative, gram-positive cocci arranged in pairs and chains. The organisms are non-motile and faculatative anaerobes. Isolation of most Streptococcus species is achieved by using enriched blood agar media. Colonies are typically small, pinpoint, and translucent or clear. Streptococci may be classified according to the type and pattern of hemolysis, by the antigenic character of a group-specific cell wall polysaccharide, and by physiological division. In this module we will look at the most common Streptococcus species isolated from humans which are pyogenes, agalactiae, bovis, pneumoniae, and the viridans group.

Streptococcus pyogenes is a group A beta streptococci. Infections are spread by respiratory secretions and some children may carry the bacterium in the respiratory tract without illness. They are the most common cause of bacterial pharyngitis which could lead to Scarlet fever due to the erythrogenic toxin produced. They are also known as "flesh eating" bacteria and can cause life-threatening disease.

Streptococcus agalacticae is a group B beta streptococci. Infection is usually associated with neonates. They are normal flora of the GI tract of humans and about 25 percent of all females will carry the bacteria as normal vaginal flora. This bacteria can cause neonatal sepsis obtained during birth, neonatal meningitis, postpartum fever, and wound infections. It can also cause endocarditis, pneumonia, and pyelonephritis in the immunosuppressed individuals.

Streptococcus bovis is a group D Streptococcus, non-Enterococcus. It is usually gamma-hemolytic but may be alpha-hemolytic. They may cause wound infection, urinary tract infection, and abdominal abscesses.

Streptococcus pneumoniae are gram-positive, lancet shaped diplococci. They are alpha-hemolytic. They are typically normal respiratory flora but may cause lobar pneumonia in the elderly and alcoholics, otitis media in infants and children, meningitis, and community-acquired bacterial pneumonia.

Viridans streptococcus are alpha-hemolytic, normal flora of the oral, respiratory tract, and GI mucosa. They are the major cause of bacterial endocarditis in people with damaged heart valves. They may enter the blood stream after dental procedures.

We will study the streptococci based on their specific hemolytic ability after incubation overnight on blood agar medium.

• Beta-hemolytic streptococci produce an enzyme that completely lyses the red blood cells in the medium, leaving a clear zone of hemolysis around the colony. This pattern is best characterized by Streptococcus pyogenes.

• Alpha-hemolytic streptococci produce an enzyme that converts red hemoglobin to green methemoglobin leaving a greenish discoloration of the culture medium surrounding the colony. This discolored area contains unlysed red blood cells and a green-colored metabolite of hemoglobin. This pattern is seen with the viridans streptococci and Streptococcus pneumoniae.

• Gamma-hemolytic streptococci are unable to hemolyze the RBCs, and therefore we should really not use the word "hemolytic" in this situation. This pattern is seen with most Lancefield group D streptococci.

Bacitracin test—Selective inhibition of group A Streptococcus to 0.02-0.04 units of bactiracin: used in the identification of streptococci.

Bile esculin—Media used to select group D Streptococcus and Enterococcus based on their ability to grow in 40% bile and to hydrolyze esculin.

Camp test—Synergistic hemolysis observed between beta-hemolytic Staphylococcus aureus and group B streptococci.

Lancefield grouping—Method to classify streptococci based on the type-specific cell wall carbohydrate.

Normal flora—Microorganisms that are found in or on a particular body site and usually do not cause infection; for example, viridans streptococci in the oral cavity.

Optochin test—Procedure used to differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococci; Streptococcus pneumoniae is inhibited by optochin (ethylhydrocupreine hydrochloride).

Pneumolysin—Virulence factor produced by Streptococcus pneumoniae associated with cytolysis.

Pyogenic exotoxin—Extracellular toxin secreted by Streptococcus pyogenes that may be associated with fever and the development of renal failure, respiratory distress, and necrosis.

Streptokinase—Extracellular enzyme produced by streptococci that cleaves plasminogen into plasmin, resulting in the lysis of a fibrin clot.

Streptolysin S and O—Hemolysins produced by streptococci; streptolysin S is oxygen stable and nonantigenic; streptolysin O is oxygen labile and antigenic.

SXT—Trimethoprim-sulfamethoxazole susceptibility test used to differentiate beta-hemolytic streptococci; Lancefield groups A and B are resistant; C, F, and G are susceptible.

Viridans streptococcus—Group of alpha-hemolytic streptococci that are normal oropharyngeal flora.

1. Albert Einstein School of Medicine

2. ASM Microbe Library

3. Bacteriology

4. Bacteriology at UW-Madison, Bacteriology 330 Home Page

5. Centers for Disease Control and Prevention

6. Harvey, Richard A., and Champe, Pamela A. (Eds.). (2001). Lippincott's Illustrated Reviews: Microbiology. Baltimore, Maryland: Lippincott, Williams and Wilkins.

7. Introduction to Clinical Microbiology

8. Medical Microbiology

9. Newcastle University Dental School

10. The Rockerfeller University

The clinical microbiology laboratory plays an important role in the diagnosis and control of infectious diseases. The person performing the tests in the microbiology laboratory must be knowledgeable and competent. This learning module can be useful in teaching the identification strategies to both the beginning student entering a laboratory career and the experienced laboratory technologist who may seek continuing education.