Egg-Based Media for Delayed Processing of Nasopharyngeal Swabs in Colonization Studies of Streptococcus pneumoniae

Abstract.

This report describes the development and evaluation of a selective egg-based medium for ambient-temperature storage and transport of nasopharyngeal (NP) swabs in colonization studies of Streptococcus pneumoniae. Egg-thioglycolate-antibiotic (ETA) medium is based on Dorset's egg medium but made with thioglycolate broth and the addition of gentamicin (3 mg/l), nalidixic acid (15 mg/l), and amphotericin B (2.5 mg/l). Laboratory studies were conducted using mock swabs from 34 NP samples with known colony counts of pneumococci, which had previously been frozen in STGG (skim milk, tryptone, glucose, glycerol) broth. ETA facilitated better recovery of pneumococci than did either Stuart's or Amies' transport media. In a field study of 117 children, NP swabs were placed in ETA, after being vortexed in STGG and direct-plated for colony counts. Of 52 swabs that were culture positive for pneumococci, all 52 grew pneumococci after 4 days of storage in ETA, and 49 isolates were recovered after 7 days. Transport media such as Stuart's and Amies' require the processing of swabs within about 24 h, and storage in STGG broth requires freezing at –70°C. ETA should be a useful addition to the storage media available for use in epidemiological studies of pneumococcal colonization, especially in situations where prompt processing, rapid transport, or low-temperature storage are not possible.