Science Progress (2003), 86 (1/2)

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Science Progress (2003), 86 (1/2), 9–75

Bacterial cold shock responses

MICHAEL H.W. WEBER AND MOHAMED A. MARAHIEL

As a measure for molecular motion, temperature is one of the most important

environmental factors for life as it directly influences structural and

hence functional properties of cellular components. After a sudden increase

in ambient temperature, which is termed heat shock, bacteria respond by

expressing a specific set of genes whose protein products are designed to

mainly cope with heat-induced alterations of protein conformation. This

heat shock response comprises the expression of protein chaperones and

proteases, and is under central control of an alternative sigma factor (_32)

which acts as a master regulator that specifically directs RNA polymerase

to transcribe from the heat shock promotors. In a similar manner, bacteria

express a well-defined set of proteins after a rapid decrease in temperature,

which is termed cold shock. This protein set, however, is different from that

expressed under heat shock conditions and predominantly comprises proteins

such as helicases, nucleases, and ribosome-associated components

that directly or indirectly interact with the biological information molecules

DNA and RNA. Interestingly, in contrast to the heat shock response, to date

no cold-specific sigma factor has been identified. Rather, it appears that the

cold shock response is organized as a complex stimulon in which post-transcriptional

events play an important role. In this review, we present a

summary of research results that have been acquired in recent years by

examinations of bacterial cold shock responses. Important processes such

as cold signal perception, membrane adaptation, and the modification of

the translation apparatus are discussed together with many other coldrelevant

aspects of bacterial physiology and first attempts are made to dissect

the cold shock stimulon into less complex regulatory subunits. Special

emphasis is placed on findings concerning the nucleic acid-binding cold

shock proteins which play a fundamental role not only during cold shock

adaptation but also under optimal growth conditions.

Keywords: cold shock proteins, low temperature stress adaptation,

membrane, pathogens, regulation, ribosome, temperature-dependent

gene expression

Science Progress (2003), 86 (1/2), 77–101

Water or ice? – the challenge for

invertebrate cold survival

WILLIAM BLOCK

The ecophysiology of cold tolerance in many terrestrial invertebrate animals

is based on water and its activity at low temperatures, affecting cell, tissue

and whole organism functions. The normal body water content of invertebrates

varies from 40 to 90% of their live weight, which is influenced by

water in their immediate environment, especially in species with a water

vapour permeable cuticle. Water gain from, or loss to, the surrounding

atmosphere may affect animal survival, but under sub-zero conditions body

water status becomes more critical for overwinter survival in many species.

Water content influences the supercooling capacity of many insects and

other arthropods. Trehalose is known to maintain membrane integrity

during desiccation stress in several taxa. Dehydration affects potential ice

nucleators by reducing or masking their activity and a desiccation protection

strategy has been detected in some species. When water crystallises to

ice in an animal it greatly influences the physiology of nearby cells, even if

the cells remain unfrozen. A proportion of body water remains unfrozen in

many cold hardened invertebrates when they are frozen, which allows

basal metabolism to continue at a low level and aids recovery to normal

function when thawing occurs. About 22% of total body water remains

unfrozen from calculations using differential scanning calorimetry (compared

with ca 19% in food materials). The ratio of unfrozen to frozen water

components in insects is 1:4 (1:6 for foods). Such unfrozen water may aid

recovery of freezing tolerant species after a freezing exposure. Rapid

changes in cold hardiness of some arthropods may be brought about by

subtle shifts in body water management. It is recognised that cold tolerance

strategies of many invertebrates are related to desiccation resistance, and

possibly to mechanisms inherent in insect diapause, but the role of water is

fundamental to them all. Detailed experimental studies are needed to provide

information which will allow a more complete and coherent understanding

of the behaviour of water in biological systems and aid the cryopreservation

of a wide range of biological material.

Keywords: cold tolerance, invertebrate survival

Science Progress (2003), 86 (1/2), 103–113

Morphological changes to

Escherichia coli O157:H7,

commensal E. coli and Salmonella

spp in response to marginal growth

conditions, with special reference

to mildly stressing temperatures

KAREN L. MATTICK, ROBIN J. ROWBURY AND

TOM J. HUMPHREY

Certain rod-shaped bacteria have been reported to form elongated filamentous

cells when exposed to marginal growth conditions, including refrigeration temperatures.

To expand upon these observations, the filamentation of commensal

Escherichia coli, E. coli O157:H7 and Salmonella spp was investigated, following

exposure to certain, mildly stressing, levels of temperature, pH or water

activity (aw), with levels of cellular protein being monitored during cell elongation,

in some experiments. Our studies indicated that cellular filamentation

could be demonstrated in all 15 strains of the above organisms tested, following

exposure to marginal conditions achieved by incubation at high or low temperatures,

high or low pH values and low aw. The level of environmental stress

causing filamentation tended to be specific to the particular organisms. For

example, Salmonella spp formed filamentous cells at 44°C, whereas E. coli

strains, including O157, grew by binary fission at that temperature, but formed

filamentous cells at 46°C. In addition, plate count techniques to enumerate bacteria

during filamentation, failed to reflect the increase in cell biomass that was

occurring, whereas measurements of protein concentration demonstrated the

increase quite strikingly. These findings have important implications for our

understanding of the ability of food-borne pathogens to cause disease, since the

infectious dose of a microorganism implicated in an outbreak of such disease is

typically determined by a viable count method, which could underestimate the

number of potential infectious units present in a food that had been stored in

such a way as to provide marginal growth conditions.

Keywords: Escherichia coli, Salmonella, cell elongation, mildly stressing

temperatures

Science Progress (2003), 86 (1/2), 115–137

Lethal effects of heat on bacterial

physiology and structure

A.D.RUSSELL

High temperatures have profound effects on the structural and physiological

properties of sporulating and non-sporulating bacteria, with membranes,

RNA, DNA, ribosomes, protein and enzymes all affected. Nevertheless, it is

apparent that no one single event is responsible for cell death. The induction

of intracellular heat-shock proteins and the activation of extracellular alarmones

in vegetative cells exposed to mildly lethal temperatures are important

cell responses. In bacterial spores, several factors contribute to the

overall resistance to moist (wet) and dry heat; the latter, but not the former,

induces mutations. Heat resistance develops during sporulation, when

spore-specific heat-shock proteins are also produced. Heat sensitivity is

regained during germination of spores.

Keywords: lethal temperature, bacterial physiology and structure

Science Progress (2003), 86 (1/2), 139–156

Extracellular proteins as

enterobacterial thermometers

ROBIN J. ROWBURY

Biological thermometers are cellular components or structures which

sense increasing temperatures, interaction of the thermometer and the

thermal stress bringing about the switching-on of inducible responses, with

gradually enhanced levels of response induction following gradually

increasing temperatures. In enterobacteria, for studies of such thermometers,

generally induction of heat shock protein (HSP) synthesis has been

examined, with experimental studies aiming to establish (often indirectly)

how the temperature changes which initiate HSP synthesis are sensed;

numerous other processes and responses show graded induction as temperature

is increased, and how the temperature changes which induce these

are sensed is also of interest. Several classes of intracellular component

and structure have been proposed as enterobacterial thermometers, with

the ribosome and the DnaK chaperone being the most favoured, although

for many of the proposed intracellular thermometers, most of the evidence

for their functioning in this way is indirect. In contrast to the above, the

studies reviewed here firmly establish that for four distinct stress responses,

which are switched-on gradually as temperature increases, temperature

changes are sensed by extracellular components (extracellular sensing

components, ESCs) i.e. there is firm and direct evidence for the occurrence

of extracellular thermometers. All four thermometers described here are

proteins, which appear to be distinct and different from each other, and on

sensing thermal stress are activated by it to four distinct extracellular

induction components (EICs), which interact with receptors on the surface

of organisms to induce the appropriate responses. It is predicted that many

other temperature-induced processes, including the synthesis of HSPs, will

be switched-on following the activation of similar extracellular thermometers

by thermal stimuli.

Keywords: biological thermometer, extracellular protein, enterobacterial

thermometer