Recovery of nosocomial fecal flora from frozen stool specimens and rectal swabs: comparison of preservatives for epidemiological studies

Abstract

The recovery of antibiotic-susceptible and -resistant aerobic Gram-negative bacilli from stool specimens and from mock rectal swabs after freezing (-20 degrees C) for as long as 4 weeks was studied using three preservatives: Cary-Blair (CB) transport medium, buffered glycerol saline (BGS), and Para Pak C&S solution (CS). In addition, the recovery of enterococci from rectal swabs was investigated after storage of swabs in Stuart's transport media at 4 degrees C as long as 4 weeks. The log10 decreases in bacterial counts from seeded stool suspensions frozen in BGS were 0.64 (i.e., fourfold) and 1.16 after 1 and 4 weeks, respectively, which were significantly less (p < .05) than 1 and 4 week decreases following freezing in CB (1.57 and 2.85) or in CS (1.50 and 2.45). The recovery of Gram-negative bacilli from patients' rectal swabs preserved in BGS was consistent with the results of the experiments with seeded stool suspensions. There was no detectable decrease in recovery of enterococci from rectal swabs stored at 4 degrees C. BGS performed well as a preservative for freezing stool specimens or rectal swabs for later recovery of nosocomial Gram-negative bacilli; enterococci survived well in refrigerated rectal swab specimens.