A comparison of the responses to environmental stress of the gram-positive bacterium Staphylococcus xylosus and the gram-negative bacterium Halomonas halo.

Abdulrahman Al-Humiany (1999) A Comparison of the Responses to
Enviromental Stress of the Gram-Positive Bacterium Staphylococcus xylosus
and the Gram-Negative Bacterium Halomonas Halo. PhD Thesis, Department of
Molecular Biology and Biotechnology, University of Sheffield.
Salt tolerance of the Gram-negative bacterium, Halomonas Halo, was compared with
the salt tolerance of a newly isolated Gram-positive coccus Staphylococcus xylosus.
Both organisms grew over a range of salinities from 0.1 - 3.0 M NaCI in both rich
medium containing yeast extract and in minimal medium. In the absence of yeast
extract, growth of S. xylosus was very slow at 3.0 M NaCl and its optimum salinity
for growth was 0.1 M NaCl, whereas Halomonas Halo showed optimum growth at
0.5 M NaCl. Growth experiments replacing NaCI with KC1 and the effect of Na+ on
the rate of respiration showed that Halomonas Halo had a greater requirement for
Na+ for growth than S. xylosus. When betaine was added to the minimal medium, it
greatly increased the growth rate of both organisms at 3M NaCl. The precursor of
betaine, choline, was also effective in increasing the growth rate of Halomonas Halo,
but was much less effective for S. xylosus. Both organisms transported betaine into
the cells by an energy dependent transport system; transport rates were broadly
similar, but it appeared that the halotolerant S. xylosus took up betaine more
efficiently than Halomonas Halo.
Halomonas Halo and S. xylosus were shown to grow across a pH range from 5.5 -
8.5, but S. xylosus showed optimum growth across the full range whereas Halomonas
Halo showed a distinct optimum at pH 7.0. The proton motive force (Ap) was found
to be low in both organisms and at pH 8.5, it fell below the theoretical minimum
(150 mV) which is required for ATP synthesis. Ap was significantly reduced by the
inhibitor carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and to a much lesser
extent by monensin. Both inhibitors completely stopped the growth of both
organisms at pH 7.0. The possibility that compatible solutes may protect enzymes
from thermal denaturation was examined, but the results were inconclusive.