Comparative Evaluation of Selective and Nonselective Culture Techniques for Isolation of Group A Beta-Hemolytic Streptococci

Abstract

A new selective blood agar medium, Strep A Isolation Agar (SI) from Remel (Lenexa, KS), was compared with Becton Dickinson's Streptococcus Selective Agar (SA) (Becton Dickinson Microbiology Systems, Cockeysville, MD) and with a nonselective Columbia Blood Agar (CB) (Difco, Detroit, MI). Throat swabs from patients with acute pharyngitis were cultured with the use of a single swab to inoculate each of the three plates in a specific order, rotating in three-week cycles. Plates were examined (each medium by a different technologist) after 24 and 48 hours of incubation at 35 degrees C in 5% carbon dioxide, and beta-hemolytic streptococci were serogrouped with the use of coagglutination. The positivity rate was significantly greater for SI (25%) and SA (26%) than for CB (18%) (P less than 0.001). The respective rates of Group A streptococcal detection by SI, SA, and CB were 91%, 95%, and 67%, respectively. However, a feature associated with the use of SI or SA, in contrast to CB, was delayed identification of isolates by 24-48 hours because of small colony size, slower growth rate, and inability to serogroup colonies taken directly from primary culture plates. Recovery of non-Group A beta-hemolytic streptococci occurred with CB (12%) greater than SI (8%) greater than SA (6%). SI is superior to a nonselective medium, such as CB, and is equal to SA for recovery of Group A streptococci from throat cultures.

Novel, rapid optical immunoassay technique for detection of group A streptococci from pharyngeal specimens: comparison with standard culture methods

“…Routine cultures using SSA and TSA compared with the broth-enriched cultures demonstrated sensitivities of 75.4 and 76.3%, respectively. Despite these findings, the throat culture is still used as the "gold standard" for GAS testing and considered to be more sensitive than most rapid methods (12,16,17,21,26).…”

“…The sensitivity and specificity of these culture methods are dependent on a number of variables, including sample collection, time and conditions of transport to the laboratory, and choice of media and culture conditions. Sensitivities from 75 to 95% are reported with a single blood agar or selective culture plate when the results of these methods are compared with the total number of positive isolates encountered with several methods, including broth-enriched methods and dual-plate systems (5,11,15,25,26).…”

Effects of media, atmosphere, and incubation time on colonial morphology of Arcanobacterium haemolyticum

“…In spite of the significantly larger colonies and zones of hemolysis on TSA medium in CO2, it is notable that neither medium nor atmosphere had a statistically significant effect on the number of nonhemolytic cultures of A. haemolyticum; this suggests that any conventional throat culture VOL. 31,1993 on September 25, 2020 by guest http://jcm.asm.org/ Downloaded from procedure that includes a 48-h incubation should be an adequate method for recovery of most strains of this species.…”

“…The purpose of the present study was to determine which of the routine throat culture methods produce the most recognizable colonies of A. haemolyticum. We examined media, atmospheres, and incubation times that are commonly recommended for throat cultures (7,15,18,19,24,25,31) to determine the best combinations for recognition of the typical A. haemolyticum morphology. * Corresponding author.…”

Which is the best method to trace group A streptococci in sore throat patients: culture or GAS antigen test?

“…However, the latest version of the Abbott GAS antigen test makes it possible to perform a semiquantitative test where the strength of the colour reaction of a positive result gives an indication of the number of colony forming units (CFU) present. Third, the use of throat cultures as a reference standard has rarely been questioned (17,18). It has by and large been taken for granted that GAS can be identified by use of a standard media.…”