Evaluation of Sponge Impregnated with Preservative for Protection of Urinary Tract Pathogens During Transport

Background: Delays in transport of urine specimens may result in overgrowth of some species or die-off of others even when preservatives are used. The objective of this study was to evaluate a sponge (patent pending) impregnated with different concentrations and combinations of preservatives to measure viability of bacteria without inhibition or overgrowth over extended periods of time.
Methods: Seven ATCC strains of urinary pathogens were tested: Staphylococcus aureus 6538, Staphylococcus saprophyticus 15305, Pseudomonas aeruginosa 27853, Escherichia coli 25922, Enterococcus faecalis 29212, Klebsiella pneumoniae 700603 and Proteus mirabilis 7002. A 0.5 McFarland standard of each organism was prepared and diluted to contain 103 to 105 CFU/mL. Standard length sponges were impregnated with various concentrations and combinations of boric acid (BA) and sodium formate (SF), dried and sterilized by gamma irradiation. 2 mL of filter-sterilized (0.22 µM) urine was inoculated with each species and added to tubes containing the sponges. The sponge was squeezed several times, and excess urine was poured off. Each sponge nominally took up 1 mL of urine. All tubes were held at room temperature and sampled at 0, 1, 2 and 3 days. At each test time 1 µL of urine was plated in triplicate to appropriate media, incubated overnight at 35oC and colony counts were performed. As a control the same procedure was followed using the BD Vacutainer® Urine Preservative tube containing BA and SF.
Results: Colony counts for all species tested remained stable for up to 3 days in the impregnated sponges when the concentrations of BA and SF were adjusted to the appropriate concentration of sodium borate and sodium formate in a 2:1 ratio. By comparison, P. aeruginosa colony counts decreased significantly (by 2 logs) and E. faecalis increased significantly (by 1-2 logs) in the BD system held for the same period of time. Higher concentrations of BA in the sponges caused inhibition of both P. aeruginosa and E. coli, and lower concentrations resulted in overgrowth, particularly by E. faecalis and P. mirabilis. Conclusions: Colony counts of urinary tract pathogens can be maintained static on a sponge impregnated with specific concentrations of BA and SF for protracted periods of time.

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