Science: Small but perfectly informed
The latest insight into bacterial behaviour reveals an organism sensitively tuned to its environment, writes Bernard Dixon
Tuesday 14 February 1995
Not so. Most microbes are single cells, each capable of diverse functions for which we, like other "advanced" creatures, require specialised tissues and organs. Bacteria, for example, do not have multi-layered, multi-cellular skin, with hairs, sweat cells and freckles, as a barrier to protect them from the outside world. Instead, each individual cell is shielded from the environment by a sturdy wall, with a delicate membrane beneath.
Likewise, bacteria move around by using whip-like parts of the cell called flagellae, rather than legs and feet that require shoes and socks. They break down food materials perfectly well without the benefit of a digestive tract. And they enjoy an active sex life without weird-looking organs for that purpose.
It is precisely because microbes achieve so much with so little that scientists have long used them to study the fundamental processes of life. In recent years, researchers have increasingly recognised the paramount significance of one particular microbial skill - that of sensing chemical and physical changes outside the cell. This can be crucially important when a microbe detects a new source of food in its environment, for example, or when, suddenly deprived of water, it must form a protective spore to survive until more favourable conditions return.
The other reason why scientists are interested in microbial senses is that their mechanisms can throw light on their counterparts in human cells. Although much detail remains to be clarified, it is becoming clear that both types of cell have exquisitely sensitive processes through which they respond to changes in the environment. They do so by means of sequences of signals propagated from the cell surface to the working machinery determined by the genes in the nucleus. Several diseases are associated with failures in this system. The prime example is the loss of responsiveness that characterises cancer cells and which leads to their disorderly and damaging proliferation.
The latest insight into the way microbes recognise changes in their environment comes from research carried out at the Centre for Cellular and Molecular Biology in Hyderabad, India. MK Ray and his colleagues have been studying the capacity of bacteria growing in Antarctica to react when conditions become warmer or cooler. Their work indicates that certain proteins in the cell membrane change chemically when temperature rises or falls. This change serves as a signal which in turn triggers appropriate alterations in the cell's internal genetic machinery.
The membrane around a bacterial cell is much more than simply a bag to retain the contents. Surrounded by a cell wall that is rigid but permeable, and which prevents the delicate membrane bursting, it is an active rather than passive structure. For example, it regulates the rates at which food is imported into the cell and waste substances are excreted.
The new evidence from Hyderabad has emerged from research into a bacterium which Ray and his collaborators isolated from soil in the Antarctic, where the temperature fluctuates over a surprisingly wide range. It is a strain of Pseudomonas syringae, which thrives especially well at 22C, though it can grow (more slowly) at temperatures down to 0C and (more quickly) at temperatures up to 30C.
But how does the entire, complex process of growth (including assembly of new cellular materials and provision of requisite energy) respond to warming or cooling? The Indian researchers believe the initial signal comes from two membrane proteins, altering their chemistry with changes in temperature. Like many other proteins, they can be phosphorylated (combined with phosphate), and this alters their behaviour. In this case, one of the proteins is phosphorylated only at temperatures up to 15C, while the other is phosphorylated more at higher than lower temperatures.
We do not yet understand how such changes initiate the signals affecting the expression of genes which cause a bacterium to grow more quickly or slowly. However, these are precisely the sort of changes implicated in other types of response to the environment. From preliminary tests in other bacteria, Mr Ray and his colleagues believe a similar mechanism operates in some, but not all, other Antarctic microbes.
Forty years ago this month, the Dutchmen Jan Kluyver and CB van Niel were completing their book The Microbe's Contribution to Biology, a classical text that highlighted the enormous debt science owed to unseen micro-organisms as research tools. Even then, it was clear that bacteria and other microbes, despite their minute dimensions and apparent simplicity, had in many ways helped to lay the foundations of biochemistry and genetics. Kluyver and van Niel paid tribute accordingly. Yet even they would be astonished to learn that Pseudomonas syringae is now illuminating questions of senses, signals and responses to the environment. In 1955, such matters would not have merited a moment's discussion. Microbes simply did not have senses.
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