Survival of dessicated root-nodule bacteria

It has been demonstrated that the fast-growing root-nodule bacteria are more susceptible to desiccation than the slow-growing species. This reaction to drying has been related to the amount of water retained by the two bacteria] groups at any vapour pressure. Invariably, the fast-growing bacteria retained more water than the slow-growing species. This may be a reflection of the higher surface energies involved in 'wetting the fast-growing species at any vapour pressure, and the larger surface area available for adsorption of water by these bacteria relative to the slow-growing rhizobia. The different responses of the two' bacterial groups to desiccation is probably related to the different amounts of water retained rather than to differences in the rate of movement of water between the bacteria and the environment on desiccation. Very little effect of desiccation um lysozyme sensitivity could be detected for either fast-or slow-growing rhizobia. Consequently, it is Unlikely that sub-lethal. damage involves alterations of the integrity of the lipopolysacchnride layer of the bacteria surviving desiccation. However, desiccation did cause drastic changes to the surface features of rhizobia as determined by fluorescence of bacteria in the presence of l-anilino-8-naphthalene sulphonate. This could reflect damage to the cytoplasmic membrane and leakage of intracellular material from killed bacteria. Montmorillonite to was found to protect the fast-growing but pot the slow-growing rhizobia from desiccation. Protection of the former group of bacteria is dependent upon the precise clay/bacterium association as not all combinations resulted in increased ability of these bacteria to survive drying . It has been postulated that the mechanism of action or montmorillonite is to decrease the intracellular water content of desiccated fast -growing rhizobia below an undefined critical level . As the water content of desiccated slow-growing rhizobia is normally below this value, further removal intracelluular water by clay has to effect upon survival . Of all the other additives tested, only maltose, sucrose, glucose and polyvinylpyrolidone (PVT) protected both the fast -and slow -growing rhizohia from dryng. The effect of polyethylene glycol (MW 6000) was similar t o the effect of montmorillionite as the polymer protected the fast- growing bacteria but not the slow-growing species. All other additives tested were either detrimental or had no effect upon survival of desiccated bacteria. The mechanism of protection of sugars and PVP is unknown but it has been suggested that the site of action may involve the cytoplasmic membrane.