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Printable Version
Translations available in Spanish.
Growth Inhibition of Streptococcus pyogenes by Bacitracin
Resource Type: Visual: Image
Publication Date: Prior to 1/1/2002
Figure 1

Streptococcus pyogenes (Labeled view)
Authors
Neal Chamberlain
Department of Microbiology
Kirksville College of Osteopathic Medicine
Kirksville, Missouri 63501
USA
Email: nchamberlain@kcom.edu
A cotton swab was used to obtain a sample from the peritonsilar region of a 5-year-old girl with a possible streptococcal throat infection. The swab was rolled on the first sector of a blood agar plate. The organisms were then separated from each other using a sterile loop and streaking for isolation. A bacitracin containing disc (Taxo A disc) was placed on the first sector and the plate was incubated at 37 degrees Celsuis with 5% carbon dioxide for 16 hours.

One way to differentiate beta-hemolytic group A Streptococcus from other beta-hemolytic streptococci is by determination of their sensitivity to bacitracin. Streptococcus pyogenes (group A beta-hemolytic) is sensitive to bacitracin and will not grow around the antibiotic-containing disc. The other beta-hemolytic streptococci are not sensitive to bacitracin and will grow next to the antibiotic-containing disc.

The reason Streptococcus pyogenes must be identified is because patients with this type of pharyngitis (sore throat) are more likely to suffer from nonsuppurative complications if not treated with antibiotics. Whereas, the other beta-hemolytic streptococci do not usually cause these nonsuppurative complications and are considered normal flora in the oropharynx.

This blood agar plate demonstrates that the beta-hemolytic colonies were sensitive to bacitracin and did not grow around the antibiotic containing disc. It also demonstrates that other colony types are present in this sample that are not hemolytic.