Mutants: enhanced tolerance or sensitivity to WORLD OF MICROBIOLOGY AND IMMUNOLOGY404•
structure, and education have been recognized by his investi-
ture as an officer of the Order of Canada in 1998.
Murray received his early education in Britain, but
moved to Montreal in 1930 where his father was Professor of
Bacteriology and Immunology at McGill University. He
attended McGill from 1936 to 1938,then returned to England
to study at Cambridge University (B.A. in Pathology and
Bacteriology in 1941 and with a M.A. in the same discipline
in 1945). In 1943 he also received a M.D. degree from McGill.
In 1945, Murray joined the faculty of the Department of
Bacteriology and Immunology at the University of Western
Ontario in London as a Lecturer. He remained at Western for
the remainder of his career. He was appointed Professor and
Head of the department in 1949 and served as head until 1974.
Since his retirement in 1984 he has been Professor Emeritus.
Murray has served as President of the American Society
for Microbiology in 1972–1973 and was one of the founders
of the Canadian Society for Microbiologists in 1951. In 1954,
he became the founding editor of the Canadian Journal of
Microbiology, which continues to publish to this day.
His interest in taxonomy continued a family tradition
begun by his father, E.G.D. Murray, who was a trustee of the
Bergey’s Manual of determinative Bacteriology from 1936
until his death in 1964. Robert Murray succeeded his late fam-
ily on the Board of Trustees of the Manual. He chaired the
Board from 1976 to 1990.
In addition to these responsibilities, Murray has served
the microbiology community by his editorial guidance of var-
ious journals of the American Society for Microbiology and
other international societies.
During his tenure at the University of Western Ontario,
Murray and his colleagues and students conducted research
that has greatly advanced the understanding of how bacteria
are constructed and function. For example, the use of light and
electron microscopy and techniques such as x-ray diffraction
revealed the presence and some of the structural details of the
so-called regularly structured (or RS) layer that overlays some
bacteria. In another area, Murray discovered and revealed
many structural and behavior aspects of a bacterium called
Deinococcus radiodurans. This bacterium displays resistance
to levels of radiation that are typically lethal to bacteria.
Such research has been acknowledged with a number of
awards and honorary degrees. Murray’s contribution to
Canadian microbiology continues. He is a member of the
Board of Directors of the Canadian Bacterial Diseases
Network of Centres of Excellence.See alsoBacterial ultrastructure; Radiation resistant bacteriaMUTANTS: ENHANCED TOLERANCE OR
SENSITIVITY TO TEMPERATURE ANDPMutants: enhanced tolerance or sensitivity to temperature and pH rangesH RANGES
Microorganismshave optimal environmental conditions under
which they grow best. Classification of microorganisms in
terms of growth rate dependence on temperature includes thethermopiles, the mesophiles and psychrophiles. Similarly,
while most organisms grow well in neutral pHconditions,
some organisms grow well under acidic conditions, while oth-
ers can grow under alkaline conditions. The mechanism by
which such control exists is being studied in detail. This will
overcome the need to obtain mutants by a slow and unsure
process of acclimatization.
When some organisms are subjected to high tempera-
tures, they respond by synthesizing a group of proteins that
help to stabilize the internal cellular environment. These,
called heat shock proteins, are present in both prokaryotes and
eukaryotes. Heat stress specifically induces the transcription
of genes encoding these proteins. Comparisons of amino acid
sequences of these proteins from the bacteriaEscherichia coli
and the fruit fly Drosophilashow that they are 40%–50%
identical. This is remarkable considering the length of evolu-
tionary time separating the two organisms.
Fungiare able to sense extracellular pH and alter the
expression of genes. Some fungi secrete acids during growth
making their environment particularly acidic. A strain of
Asperigillus nidulans encodes a regulatory protein that acti-
vates transcription of genes during growth under alkaline con-
ditions and prevents transcription of genes expressed in acidic
conditions. A number of other genes originally found by analy-
sis of mutants have been identified as mediating pH regulation,
and some of these have been cloned. Improved understanding
of pH sensing and regulation of geneexpression will play an
important role in gene manipulation for biotechnology.
The pH of the external growth medium has been shown
to regulate gene expression in several enteric bacteria like
Vibrio cholerae.Some of the acid-shock genes in Salmonella
may turn out to assist its growth, possibly by preventing lyso-
somal acidification. Interestingly, acid also induces virulence
in the plant pathogen (harmful microorganism) Agrobacterium
tumefaciens.
Study of pH-regulated genes is slowly leading to knowl-
edge about pH homeostasis, an important capability of many
enteric bacteria by which they maintain intracellular pH.
Furthermore, it is felt that pH interacts in important ways with
other environmental and metabolic pathways involving anaer-
obiosis, sodium (Na+) and potassium (K+) levels, DNArepair,
and amino acid degradation. Two different kinds of inducible
pH homeostasis mechanisms that have been demonstrated are
acid tolerance and the sodium-proton antiporter NhaA. Both
cases are complex, involving several different stimuli and
gene loci.
Salmonella typhimurium( the bacteria responsible for
typhoid fever) that grows in moderately acid medium (pH
5.5–6.0) induces genes whose products enable cells to retain
viability (ability to live) under more extreme acid conditions
(below pH 4) where growth is not possible. Close to 100% of
acid-tolerant (or acid-adapted) cells can recover from
extreme-acid exposure and grow at neutral pH. The inducible
survival mechanism is called acid tolerance response. The
retention of viability by acid-tolerant cells correlates with
improved pH homeostasis at low external pH represents
inducible pH homeostasis.womi_M 5/7/03 7:53 AM Page 404