Nucleating characteristics of Pseudomonas aeruginosa above 0 °C and the role of l-leucine

It has been reported earlier that the bacteria Pseudomonas aeruginosa have the ability to nucleate ice over a long temperature range below 0 °C. In the present work, the same bacteria are shown to have nucleating ability even above 0 °C. It is found to reach a peak at 4 °C and then gradually falls off as the temperature is raised, vanishing altogether at 12 °C. When the bacteria are irradiated by UV-rays, the nucleating ability above 0 °C has the same variational pattern with change of temperature, though the ability is significantly less. When the bacterial medium is boiled for 20 min, the bacteria loose their nucleating ability completely.Since ice cannot exist above 0 °C, some supporting experiments are performed to establish the identity of the nucleated crystals: (a)Crystals grown around a small sample of cultured P. aeruginosa in an incubator were shown to be identical with the nucleated crystals in the Cold Room experiments.(b)X-ray diffraction (XRD) spectrum of the grown crystals is found to be consistent with the presence of amino acids like l-leucine, isoleucine and l-isoleucine, amongst which l-leucine is the most abundant.When droplets of an aqueous solution of l-leucine are seeded, their nucleation characteristics are similar to those with P. aeruginosa. From XRD, nuclear magnetic resonance (NMR) and thermal analysis, one can conclude that the crystals formed by P. aeruginosa above 0 °C are hydrate crystals with water phase caging around the seeded agent; the amino acid l-leucine present in the protein of the bacteria plays the dominant role in the nucleation.