Comparison of preservation media for storage of stool samples

Abstract

Transportation of clinical samples and long-term recoverability of pathogens are critical to epidemiological studies, particularly when conditions do not permit immediate processing. This study confirms that Cary-Blair medium (CB) is suitable for the preservation of Salmonella and Shigella isolates for more than 2 weeks at 25, 4, or -70 degrees C. Campylobacter jejuni was not recovered after 2 days of storage in CB at 25 degrees C when an inoculum of 12 x 10(8) cells per ml was used. Lower temperatures supported the recovery of this organism for 6 days. When individual pathogens were preserved with stools in CB and incubated at 25, 4, or -70 degrees C, the Salmonella and Shigella concentrations dropped from 12 x 10(8) cells to 1 x 10(3) or 1 x 10(4) cells per ml within 2 days and then remained stable for the rest of the observation period (15 days). C. jejuni survived preservation with stools for 5 to 9 days. The addition of blood and glycerol to CB improved the recoverability of all enteropathogens, particularly C. jejuni, which was consistently detected for 7 to 9 days at the different preservation temperatures used. When trypticase soy broth-glycerol (freezing medium), with or without blood, was used, there was little or no decrease in the Salmonella and Shigella concentrations during 2 weeks of preservation with stools at -70 degrees C. C. jejuni demonstrated a relatively sustained high concentration in Trypticase soy broth-glycerol with 5% blood. The use of defibrinated, laked sheep blood as a long-term freezing medium supported the recovery of low concentrations of Salmonella and Shigella spp. (10(2) to 10(3) cells per ml) for more than 14 weeks. Recovery of C. jejuni was consistent for 7 weeks when an initial concentration of 10(6) cells per ml present in stools. Laked blood provided a simple, sterile, and inexpensive medium for the preservation of individual isolates and clinical samples.