On Jun. 8, 1987, Advanced Genetic Sciences announced that its Frostban (Ice-minus) bacteria successfully protected strawberries from below-freezing temperatures and that the genetically altered bacteria did not escape from the test plot during the six-week trial. Frostban was sprayed on a small plot of strawberries April 24 in what was the nation’s first open-air testing of a genetically engineered microorganism.

Ice-minus bacteria is a variant of the bacterium Pseudomonas syringae (P. syringae) that lacks the ability to produce a certain surface protein. The “ice-plus” nucleation-active protein (INA protein) found on the outer bacterial cell wall acts as the nucleating centers for ice crystals. The crystals facilitate ice formation, hence the designation “ice-plus”. The ice-minus variant of P. syringae is a mutant, lacking the gene responsible for ice-nucleating surface protein production, and protects the plants from below freezing temperatures.