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Spotted: acidocalcisomes in a trypanosome. Credit: COURTESY OF KILDARE MIRANDA

Bacteria have specialized compartments that were previously thought to exist only in higher organisms, according to research published earlier this month.

The discovery upsets a long-held notion about the nature of bacterial cells, researchers say. Whereas the cells of all other living things rely on a wide assortment of internal enclosures, known as organelles, to carry out their metabolic tasks, bacteria were thought to manage with a single open space devoid of partitions.

Now a team led by microbiologist Roberto Docampo at the University of Illinois in Urbana say they have found the first organelles in bacteria that have a direct counterpart in higher organisms (M. Seufferheld et al. J. Biol. Chem. doi:10.1074/jbc.M304548200; 2003). The organelles — called acidocalcisomes — are thought to store energy and help control acidity within the cell.

Docampo's bacterial organelles seem to be widespread; in fact, microscopists have been looking at them for a century without knowing it. They stain purple with certain dyes and have been called 'metachromatic' granules, but they were thought to have no membrane or enzymes — essential components of organelles.

Docampo has shown by electron microscopy and antibody techniques that metachromatic granules are surrounded by a membrane bearing proton-pumping enzymes that make its contents more acidic. For his experiments, he used Agrobacterium tumefaciens, a plant pathogen that causes crown gall. But a variety of other bacteria also have metachromatic granules, including the ulcer bug, Helicobacter pylori, and the bacterium that causes diphtheria, Corynebacterium diphtheriae.

The origin of the organelles is a mystery. In most cells, organelles fall into one of two categories. They are either components of an elaborate intracellular trafficking system, which bacteria lack, or they are derived from formerly free-living organisms that joined up with the ancient ancestors of modern cells.

Neither possibility seems particularly likely in this case, says Andrew Roger, who studies organelles at Dalhousie University in Nova Scotia and was surprised by the news. “Which of these might happen in bacteria is not clear,” he says.

As one approach to this question, Docampo's group is now searching for more organisms with acidocalcisomes for comparison, by scouring sequence databases for the gene that encodes the proton pump. In addition, as the proton pump in the membrane is present in some harmful bacteria but not in human cells, drugs that target it might be effective against these infections, Docampo suggests.