PARIS -- British scientists have drawn up a DNA map of the bacterium that has given the world some of its most important medicines, a potential breakthrough in the fight against antibiotic resistance, AFP reported.

Reporting in Thursday's issue of the weekly journal **** Nature ****, they describe the genetic sequence of Streptomyces Coelicolor, a bug that occurs naturally in the soil and is industrially fermented to produce an enormous range of drugs.

More than two-thirds of antibiotics derived from natural sources come from S. Coelicolor and its relatives, including Streptomycin, tetracyclin and erythromycin. The family also produces anti-cancer agents and immunosuppressants used to prevent rejection of transplanted organs.

A team led by David Hopwood of the John Innes Center in Norwich, eastern England, has spent years piecing together the roughly 8.5 million base pairs, or DNA letters, that make up the genome of the predominant strain of the clan, called S. Coelicolor A3(2).

"The genome sequence will greatly increase our understanding of microbial life in the soil as well as aiding the generation of new drug candidates by genetic engineering," they say.

Streptomycetes, as S. Coelicolor's family is called, are crucial components in soil decay, helping to break down the tough fibers of dead trees and shrubs and the hard shiny shells of dead insects into nutrients.

A3(2) reflects this tough, competitive environment. Its genome is complex and sophisticated, comprising an estimated 7,825 genes gathered in more than 20 clusters.

The big battalion of genes gives S. Coelicolor a lot of flexibility for coping with temperature extremes, lack of nutrition or water and other challenges.

As many as one in eight of the bacteria's genes are "switching genes" that turn other genetic machinery off and on, providing backup systems in survival.

This versatility is what excites pharmaceutical engineers the most, because if the switches can be identified and inserted into drug molecules, that could help to combat diseases that are fast becoming resistant to conventional antibiotics.

Another potential weapon is offered by the similarity between Streptomycetes and several evil pathogens.

Understanding exactly how S. Coelicolor works could expose weaknesses in bacterial diseases such as tuberculosis and leprosy.

The more than 7,800 predicted genes for S. Coelicolor compares with only 4,289 genes for the intestinal bug Escherichia Coli and 6,203 genes in the yeast Saccharomyces Cerevisiae, which like the mouse and Thale Cress are other closely-studied lab tools.

The human genome has been completed, and early analysis of it suggests that Homo sapiens has around 31,000 genes.