The survival of Escherichia coli on drying and rehydration.

By direct observation of individual organisms in a micro-culture chamber and by viable plate counts, a study has been made of the function of sugar/colloid mixtures in promoting the survival of Gram-negative bacteria on drying. The high death rate after drying in sugar alone was due mainly to cell-wall damage caused during rehydration by the temporary osmotic pressure set up by the sugar within the cell, leading to the formation of spheroplasts not capable of division. Spheroplast formation was largely prevented and survival greatly enhanced by controlled rehydration, showing that the sugar component was the primary protective agent. The complementary role of the protein or ‘protective colloid’ appears to lie in its ability to compress the cell wall against the contracted plasma membrane in plasmolysed cells, thus decreasing the volume of the interspace between the two membranes and so limiting the sugar trapped therein to a safe amount. These observations were corroborated by cell volume measurements in the ultracentrifuge.