SUSCEPTIBILITY OF BACTERIAL STRAINS TO DESICCATION - A SIMPLE METHOD TO TEST THEIR STABILITY IN MICROBIOLOGICAL REFERENCE MATERIALS

The purpose of this work was to predict the behaviour of bacteria with in spray-dried powders used for the production of microbiological refe rence materials. The survival of eighteen bacterial strains desiccated on anhydrous silica gel and stored at 22 degrees C for at least 3 mon ths was determined. According to their percentage survival after desic cation, storage and reconstitution after certain time periods a classi fication of these bacterial strains was possible. Gram-positive strain s such as Enterococcus faecium showed a relatively high resistance to desiccation with more than 30% survival on each day of sampling. Certa in Gram-negative strains such as Aeromonas hydrophila were very suscep tible to this treatment; culturable cells could not be determined as s oon as 1 week after desiccation. Other Gram-negative organisms (e.g., Salmonella serotypes, Escherichia coli and Enterobacter cloacae) were less susceptible to drying than this group but showed a lower resistan ce than the Gram-positive bacterial strains. The use of anhydrous sili ca gel is probably a suitable basis for the development of a screening system to characterize the susceptibility of bacterial strains to des iccation. Further, the suitability of methods to improve the stability of the concentration of microorganisms in dry materials could be inve stigated with the help of this desiccation system.