J Clin Pathol Qual Control.  1997 Jun;19(1):185-190.

Evaluation of Streptococcus Selective Agar (SSA) for Recovery of Group A Streptococci from Throat Cultures

• Kim SJ

Abstract

BACKGROUND: Group A streptococci (GAS) is the most common pathogen in bacterial pharyngitis. Many efforts have been made to enhance the isolation of GAS from the throat by reducing throat normal flora. Streptococcus selective agar (ssA, BBL, USA) was evaluated in terms of the recovery of GAS and other groups of beta-hemolytic streptococci from throat cultures.
METHODS
Triple throat swabs were taken from 199 elementary school children and inoculated into sheep BAP, human BAP and ssA. A bacitracin disc (0.04 U) was placed directly onto the inoculation site. Plates were incubated at 37degrees C overnight and at room temperature for an additional one day. Beta-hemolytic streptococci were identified with the bacitracin disc and latex agglutination method.
RESULTS
The isolation rates of GAS between sheep BAP, human BAP and ssA were 29.2%, 28.3% and 27.2% respectively. Group C or G streptococci grown on sheep BAP or human BAP were inhibited on ssA. The ratio of plates that showed more colonies when compared to sheep BAP (100%) was 131% on ssA, and 64% on human BAP. Four strains of GAS grew on the second day, and a further five developed so slowly that an extra day of incubation was needed in ssA. One strain was detected on the second day in sheep BAP.
CONCLUSIONS
ssA effectively suppressed oropharyngeal flora, and facilitated plate reading. A bacitracin disc directly placed on ssA was very helpful in identifying GAS presumptively in a day. However the colonies on ssA were sometimes either very small or late in developing, and therefore it is necessary to further incubate the ssA plates for 2 days. Human BAP is not recommended due to an ambiguous hemolysis pattern which is very difficult to interpret and time-consuming to confirm.

MeSH Terms

Agar*
Agglutination
Bacitracin
Child
Hemolysis
Humans
Latex
Pharyngitis
Pharynx*
Sheep
Streptococcus*
Agar
Bacitracin
Latex