Serum glucose agar, a transport medium for Streptococcus pyogenes.

Abstract

OxfordWhen planning an epidemiological study of streptococcal infection, we were recommended to try serum glucose agar which had proved a useful transport medium in another study. ' We compared serum glucose agar with Stuart's transport medium, which was our laboratory's usual transport medium for delicate organisms and with dry serum-coated swabs. We also considered the commercially available systems but found that none was sufficiently cheap and robust for our needs. This brief report describes our assessment of serum glucose agar made in our laboratory. Material and methodsThe transport media used in this study were (i) serum glucose agar, containing 0 5% glucose, 0-7% Noble Agar (Difco) and 5% horse serum and (ii) Stuart's transport medium (Oxoid). These two semi-solid agar media were dispensed in 5 ml volumes in bijoux bottles. In the study, swabs were snapped into the bottles. The dry serum-coated swabs were obtained from Medical Wire and Equipment Co Ltd. Blood agar was poured as a bilayer, using Columbia agar base (Oxoid) and 5% horse blood. Todd-Hewitt broth (Difco) was used for initial cultures and preliminary dilutions were made in brain heart infusion broth (Oxoid).In Technical method 5 Broken into Stuart's transport medium and stored at 40C for 7 days. 6 Returned to the holder and stored dry at 4°C for 7 days. 7 Inoculated onto blood agar immediately. After storage, the swabs were inoculated onto blood agar. Inoculation was standardised by using six even strokes for each plate.The number of organisms surviving was counted by starting with serial decimal dilutions of overnight cultures. Each dilution was sampled with a set of seven swabs. The dilutions giving between 20 and 200 colonies/plate were used to estimate the number of organisms present in the initial broth culture (swab 7) and on the swabs after storage (swabs 1-6). The multiplication or death of organisms was expressed as a growth index equal to the logarithm of (number of organisms after storage divided by the initial count). Thus multiplication of organisms gave a positive growth index whilst a negative index indicated cell death.The cultures tested were 25 strains of Streptococcus pyogenes in pure culture, the same 25 strains of Strep pyogenes mixed with the normal throat flora of one person, and one strain of Strep pyogenes in 10 different cultures of normal flora.An opportunity to compare serum glucose agar with direct inoculation of blood agar arose in the investigation of a school outbreak of scarlet fever. One hundred and-sixty-three pairs of throat swabs were taken from pupils and staff. The first swab in each pair was inoculated onto blood agar immediately; the second was snapped into serum glucose agar and left overnight at room temperature before subculturing onto blood agar. The blood agars were incubated anaerobically at 370C overnight. Betahaemolytic streptococci were grouped by Lancefield's method, and the findings of Strep pyogenes were analysed. Results

Recovering Streptococci from the Throat, a Practical Alternative to Direct Plating in Remote Tropical Communities

“…Stuart's transport medium has been used, apparently successfully, for more than 20 years in Central Australia, but no formal evaluation in this setting has been published (F. Morey, unpublished data). A serum glucose agar transport system has also been suggested but never widely utilized (26). Other published methods have included transport of isolates on filter paper and the use of swab transport medium kits (21,34).…”