The Streptococcus genus consists of Gram-positive bacteria which appear as chains under microscopic observation. Members of Streptococcus can be aerobic, anaerobic, or microaerophilic. The organisms in this genus are characterized by a coccus appearance, a thick cell wall, and aerobic action on glucose. Four different classification systems exist:

(under the microscope)


The first group in the Lancefield classification system includes only one species of Streptococcus, S. pyogenes. This particular opportunistic pathogen is responsible for about 90% of all cases of pharyngitis. A common form of pharyngitis is "Strep throat" which is characterized by inflamation and swelling of the throat, as well as development of pus-filled regions on the tonsils. Penicillin is usually administered to patients as soon as possible to quell the possibility of the infection spreading from the upper respiratory system into the lungs. Once in the lungs, the infection could give rise to pneumonia. Some cases also develop into rheumatic fever if left untreated. Other diseases linked to S. pyogenes are skin infections such as impetigo, cellulitis, and erysipelas.


The B classification of Lancefield also includes only one bacterium, S. agalactiae. For years this bacterium has been the causative agent in mastitis in cows. Currently, it has been found to be a cause of sexually transmitted urogenital infections in females. Although infection is easily treated with penicillin, proper diagnosis is necessary for women nearing labor because the infection can easily spread to the child via the birth canal.


Type D Streptococcus is the next clinically important bacterium because of the multitude of diseases it is known to cause. Although many are harmless, the pathogenic strains cause complications of the human digestive tract. This group has recently been reclassified into two divisions: Enterococcus and non-Enterococcus. The Enterococci include E. faecalis, a cause of urinary tract infections, and E. faecium, a bacterium resistant to many common antibiotics. Diseases such as septicemia, endocarditis, and appendicitis have also been attributed to group D Strep. Steptococcus is part of he normal human fecal flora . Once identified, Group D Strep can be treated with ampicillin alone or in combination with gentamicin.


Streptococcus pneumoniae

(under the microscope)
Because its surface carbohydrate antigens do not correspond to a specific Lancefield group, S. pneumoniae is discussed separately. Although not given a letter designation, S. pneumoniae can be considered a Pyogenic (puss-producing) strain of Strep. It can be distinguished from other Pyogenic bacteria by its high sensitivity to Optochin (no growth zone of inhibition). This bacterium causes pneumonia, meningitis, and otitis media. It also demonstrates alpha-hemolytic growth on blood agar.

Viridans Group

The Viridans Streptococci, consisting of S. mutans and S. mitis, are alpha-hemolytic bacteria. These bacteria inhabit the mouth. In fact, a large percentage of tooth decay can be attributed to S. mutans.

Copyright © 1995 University of Texas - Houston Medical School, DPALM MEDIC, All rights reserved.