Recovering streptococci from the throat, a practical alternative to direct plating in remote tropical communities

Abstract

Throat swabs are regarded as the "gold standard" for diagnosing streptococcal pharyngitis and for surveillance research. Culturing throats in remote tropical settings is logistically difficult, and these settings are commonly burdened by high rates of streptococcal disease. The survival of streptococci on swabs may depend on whether they are of "throat" or "skin" type, as determined by emm pattern typing. The aims of this study were to compare the recovery rates of beta-hemolytic streptococci (BHS) using three different transport methods and to determine whether the recovery rates correlated with the emm pattern type. Monthly duplicate throat swabs were taken from occupants of selected households in three remote Aboriginal communities. Paired swabs were separated and handled in one of three ways: (i) direct inoculation onto culture media with cold-box transport (plated), (ii) sealed in a bag with a silica gel desiccant and cold-box transport (desiccant), and (iii) transport at ambient temperature and humidity (ambient). emm pattern typing was done by standard methods. Over 23 months, 4,842 throat swabs were taken, and 4,122 were paired. BHS were recovered on 11.5% of the 4,842 occasions (group A, 4.5%; group C, 1.7%; group G, 5.4%). Results from paired swabs showed the plated method was superior to desiccant and desiccant was better than ambient. Pooled data indicated that plated and desiccant were equivalent, and both were significantly better than ambient. There was no correlation between the emm pattern type and recovery of group A streptococci by different methods. In tropical and remote settings, cold-box transport with desiccant and subsequent inoculation of culture plates in the laboratory is a practical alternative to direct plating.

FIG. 1.

Recovery of BHS using paired throat swabs: swabs inoculated onto culture media in the community clinic (plated) were compared with swabs transported with desiccant (desiccant) and swabs transported under ambient conditions (ambient) (see Materials and Methods). (Top) Number of isolates, 258 of 2,571 paired swabs. Ambient sensitivity, 59.3% (95% confidence interval [CI], 53.2% to 65.1%), and plated sensitivity, 76.7% (95% CI, 71.2% to 81.5%); P < 0.001. (Middle) Number of isolates, 101 of 521 paired swabs. Desiccant sensitivity, 72.3% (95% CI, 62.5% to 80.7%), and plated sensitivity, 83.2% (95% CI, 74.4% to 89.9%); P = 0.04. (Bottom) Number of isolates, 69 of 515 paired swabs. Ambient sensitivity, 66.7% (95% CI, 54.3% to 77.6%), and desiccant sensitivity, 85.5% (95% CI, 75.0% to 92.8%); P = 0.009.

FIG. 2.

Recovery of GAS using paired throat swabs. (Top) Number of GAS isolates, 101. Ambient sensitivity, 52.5% (95% confidence interval [CI], 42.3% to 62.5%), and plated sensitivity, 71.3% (95% CI, 61.4% to 79.6%); P = 0.006. (Middle) Number of GAS isolates, 29. Desiccant sensitivity, 72.4% (95% CI, 52.8% to 87.3%), and plated sensitivity, 79.3% (95% CI, 60.3% to 92.0%); P = 0.54. (Bottom) Number of GAS isolates, 37. Ambient sensitivity, 67.6% (95% CI, 50.2% to 83.0%), and desiccant sensitivity, 78.4% (95% CI, 61.8% to 90.2%); P = 0.29.

FIG. 3.

Pooled data (not paired) for all swabs taken showing the recovery rates of BHS using three different transportation methods.

FIG. 4.

Comparison of emm pattern types of GAS isolates recovered; on-site inoculation of cultures plates (plated) and transport under ambient conditions (ambient). Number of GAS isolates, 101. There was no significant difference in the proportions of emm pattern types between methods.