Viability of respiratory pathogens cultured from nasopharyngeal swabs stored for up to 12 years at − 70 °C in skim milk tryptone glucose glycerol broth

Abstract

Nasopharyngeal carriage studies are needed to monitor changes in important bacterial pathogens in response to vaccination and antibiotics. The ability to store original specimens frozen in skim milk tryptone glucose glycerol broth (STGGB) allows additional studies to be conducted without the need for further expensive field collection. Although sub-cultured isolates remain viable in this medium for many years, limited data are available to indicate viability of relatively low numbers of organisms present in nasopharyngeal specimens stored frozen over long periods of time. We conducted several studies whereby swabs stored in STGGB at − 70 °C for up to 12 years were thawed and aliquots cultured. Recovery of Streptococcus pneumoniae (72% positive from 269 swabs), Haemophilus influenzae (62% from 214) and Moraxella catarrhalis (81% from 162) was not significantly different from the original cultures: 69% (Risk Difference [RD] 3.0, 95% Confidence Interval [CI] − 4.7, 10.7), 66% (RD − 4.7, 95% CI − 13.8, 4.4) and 78% (RD 3.1, 95% CI − 5.7, 11.9) positive respectively. There was no trend in recovery from swabs stored for increasing lengths of time. We conclude that studies which rely on the viability of these respiratory pathogens can be conducted using original swabs stored at − 70 °C for at least 12 years.