Resistance of bacterial strains to dry conditions: use of anhydrous silica gel in a desiccation model system.

The viability of 18 bacterial strains desiccated on anhydrous silica gel and stored at a temperature of 22 degrees C for at least 3 months was determined. According to their stability in the dried state, these strains could be classified into three typical groups. Group 1, containing Gram-positive strains and Salmonella serotypes, was marked by a very slow decrease of the concentration of culturable cells from day 14 on (respectively day 21 for Salmonella thompson). The rate of decrease expressed as regression coefficient (b) ranged from -0.000389 to -0.00521 log (cfp ml-1) per d. The Group 2 strains Enterobacter cloacae and Escherichia coli did not reach a comparable slow decrease in the dry material within the indicated time period. Regression coefficients were respectively -0.04406 and -0.03412 log (cfp ml-1) per d. The reciprocal values -(1/b) were respectively 23 d per log (cfp ml-1) and 29 d per log (cfp ml-1), indicating the time periods in which a reduction of 1 log unit of culturable cells occurred. Group 3 strains Pseudomonas aeruginosa, Aeromonas hydrophila and Aer. sobria were marked by a significant susceptibility to cell damage caused during desiccation and reconstitution. A high initial decrease (ID) of the concentration of culturable organisms seems to be a characteristic property of these bacterial strains: culturable organisms could not be detected after storage for 1 d (Aer. hydrophila, Aer. sobria) or 7 d (Ps. aeruginosa). The wide range of resistance of the different bacterial strains tested indicated that the silica gel model system is a suitable tool for microbiological challenge tests to investigate the survival of micro-organisms exposed to desiccation and their stability in dry materials.