Effect of swab composition and use of swabs versus swab-containing skim milk-tryptone-glucose-glycerol (STGG) on culture- or PCR-based detection of Streptococcus pneumoniae in simulated and clinical respiratory specimens in STGG transport medium

Abstract

To detect Streptococcus pneumoniae colonization, the nasopharynx is sampled using a swab placed in skim milk-tryptone-glucose-glycerol (STGG) transport medium, and then the swab specimen or STGG medium is cultured or subjected to PCR. We evaluated the effect of swab composition and compared the sensitivities of detection of culture and PCR using swabs and swab-containing medium. Calcium alginate, Dacron polyester, or rayon-tipped swabs were inoculated with pneumococci or were immersed in nasal wash specimens from children and then placed in STGG medium. Swabs and medium inoculated with pneumococci were cultured. Swabs grew significantly more colonies than medium. The number of colonies cultured from rayon swabs or medium was significantly higher than the number cultured from the calcium alginate swab or medium. The number of colonies from both the Dacron polyester swabs and medium were significantly lower than with either calcium alginate or rayon swabs. When DNA was separately extracted from the calcium alginate swab and medium and subjected to PCR for pneumococcal detection from either S. pneumoniae-inoculated swabs or clinical specimens that grew S. pneumoniae, the sensitivity was at least 10 times higher using the swab. With Dacron polyester or rayon-tipped swabs, there was no consistent difference between the sensitivity of PCR using swabs and that of PCR using medium. Thus, calcium alginate swabs may be superior to STGG medium for the culture and PCR-based detection of S. pneumoniae. For culture, rayon swabs are superior and Dacron polyester swabs are inferior. The sensitivity of the swab and swab-containing medium for culture or PCR detection of S. pneumoniae varies with swab composition.