Study Suggests Why Good
Bacteria Sometimes Go Bad

NEW YORK (Reuters Health) - New research is shedding light on why a common group of bacteria causes life-threatening illnesses and outwits treatment in some people, yet lives happily and harmlessly in most individuals.

In the April 6th issue of Science, investigators report that some factors that make Staphylococcus aureus (S. aureus) thrive and live peacefully in humans also make them potent infectious agents.

S. aureus can cause pneumonia and meningitis, and the bacteria are among the most common causes of serious infections that patients acquire while hospitalized. However, they often can be found growing on the skin or in the nasal passages of healthy people, causing no problems whatsoever. About 60% of people in the population carry the bacteria in their nose at one time or another, and it is usually symptomless.

In a study analyzing S. aureus taken from the noses of ill and healthy individuals, Dr. Nicolas P. J. Day and his colleagues compared 61 samples of bacteria collected from patients with serious infection with 179 samples collected from healthy individuals and 94 samples from patients who picked up an infection while in the hospital.

The researchers found that certain genetic strains of S. aureus were disproportionately responsible for disease cases. Yet those same strains were also common among healthy people. This suggests that some of the factors that make S. aureus so common also make it extremely virulent in certain cases.

While the specifics are not yet known, Day suggests that certain toxins or genes related to metabolism may help S. aureus ``beat other bugs in the nose'' in order to thrive. These factors may also ``almost coincidentally'' help S. aureus evade the body's defenses and get into the bloodstream.

The report suggests that, in evolutionary terms, these bacteria have become ``fit'' by being infectious enough to spread among humans, yet not virulent enough to kill off too many of its hosts.

``If you can double your population faster than your neighbor due to an efficient metabolism, that may also help you in terms of virulence,'' Day, a researcher at University of Oxford, UK, told Reuters Health.

Day said it is not clear why some strains of bacteria become virulent while others do not, but he suggested a theory of ``evolution of ecological fitness.''

``Out of the great and varied...bacterial population, bacteria occasionally arise which are ecologically fitter than others. This leads to their massive clonal expansion, probably worldwide,'' he said. ``It so happens that these bugs are better at causing disease than others because of whatever is making them ecologically fit.''

The study has ``exciting implications for future research,'' Dr. Marc Lipsitch, an epidemiologist from Harvard School of Public Health in Boston, Massachusetts, notes in an accompanying editorial.

``Their findings suggest an answer to the riddle of why S. aureus has evolved mechanisms for causing disease when disease is not a necessary or even a common part of its life cycle,'' Lipsitch writes.

SOURCE: Science 2001;292:114-116.

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