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 g el and stored at a temperature of 22 degrees C for at least 3 months w as determined. According to their stability in the dried state, these strains could be classified into three typical groups. Group 1, contai ning Gram-positive strains and Salmonella serotypes, was marked by a v ery slow decrease of the concentration of culturable cells from day 14 on (respectively day 21 for Salmonella thompson). The rate of decreas e expressed as regression coefficient (b) ranged from -0000389 to -0.0 0521 log (cfp ml(-1)) per d. The Group 2 strains Enterobacter cloacae and Escherichia coli did not reach a comparable slow decrease in the d ry material within the indicated time period. Regression coefficients were respectively -0.04406 and -0.03412 log (cfp ml(-1)) per d. The re ciprocal 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 red uction of 1 log unit of culturable cells occurred. Group 3 strains Pse udomonas aeruginosa, Aeromonas hydrophila and Aer. sobria were marked by a significant susceptibility to cell damage caused during desiccati on and reconstitution. A high initial decrease (ID) of the concentrati on of culturable organisms seems to be a characteristic property of th ese bacterial strains: culturable organisms could not be detected afte r storage for 1 d (Aer. hydrophila, Aer. sobria) or 7 d (Ps. aeruginos a). The wide range of resistance of the different bacterial strains te sted 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.