Heat Resistance of a Halotolerant Brevibacterium sp. JCM6894 in the Presence of NaCl
Haruo MIMURA*a, Shinichi NAGATAa, and Akio ISHIDAb
a Department of Maritime Sciences, Kobe University of Mercantile Marine, 5-1-1, Fukaeminami-cho, Higashinada-ku, Kobe 658, Japan
b Department of Environmental Science, Faculty of Science, Kumamoto University, 2-40-1 Kurokami, Kumamoto 860, Japan
Microorganisms in the environment have frequently been exposed to some stresses such as osmotic stress, heat stress, oxidative stress, pH change, UV radiation, starvation etc. These stresses enhance the resistance against another stressful conditions after the cells adapted to the sublethal stress. As for a halotolerant Brevibacterium sp. JCM6894, it grew in the wide range of NaCl concentrations, 0-2 M (1), and showed the tolerance to the transient hyper-salt stress without the pre- adaptation period (2). Therefore, it seems of interest to study the effect of salt stress on the viability, osmoregulation, and cation transport of this strain. In this study, the contribution of NaCl externally added was examined for the heat resistance of the resting cells to evaluate whether or not the salt stress affects the resistance to the heat stress without pre-adaptation period.
By the heat treatment for 30 min at 47℃ in the absence of NaCl, about 5-log cycle reduction of the total viable cell numbers (1 x 108 cfu / ml) was observed. When the cells were heated in the buffer containing 2 M NaCl, the viability was maintained within less than 1-log cycle reduction after 30 min incubation at 56℃. During the heat treatment for 30 min at 47℃ in the presence of 2 M NaCl, Na+ and K+ ions in the cells increased and decreased by 13 and 26 μg ions per rng cell protein, respectively. Under this condition, the amount of free amino acids in the cells was little changed except for glutamate and hydroxyproline, those of which were reduced by 72 and 43 nmol per mg cell protein, respectively. The supplement effect of sodium glutamate as well as non-electrolytes were also examined. The viable cell numbers in the presence of 0.5 M glutamate was 7.7-log cycle for 30 min at 47℃, and 2 μg of internal Na+ ions per mg cell protein increased. When the cells were suspended in the presence of 1 M glycerol, glucose, and maltose for 30 min at 47℃, the viability was 4.4-, 4.2-, and 3.6-log cycle, respectively. These results indicate that the salt stress itself and Na+ ion existed in the cytoplasm are important factors rather than in vivo protein synthesis to prevent the thermal death of the resting cells of this strain. The effect of compatible solutes such as ectoine, glycine betaine, etc. will be reported together.
1. Nagata, S. (1988) Arch. Microbiol., 150, 302-308.
2. Mimura, H., Nagata, S., and Matsumoto, T. (1994) Biosci. Biotech. Biochem., 58, 1873-1874.
