SALT LAKE CITY (Reuters Health) - Just because water comes in a bottle doesn't mean it's sterile, according to an expert who spoke here Tuesday at the American Society for Microbiology's annual meeting.

However, it is not a great idea to share bottled water after it has been opened, or to leave an open bottle sitting around in warm weather for too long, Rosenberg told Reuters Health.

While there haven't been widespread outbreaks of illness linked to consumption of contaminated bottled water, Rosenberg said, bottled water can indeed contain microbes at levels capable of making a person with a weak immune system sick. Bacteria may come from the water source, or can be introduced during the bottling process.

The US does not monitor bottled water for the presence of Pseudomonas aeruginosa, which can cause illness, is often resistant to antibiotics and is a reliable indicator that contamination has occurred during bottling, according to Rosenberg.

The US Food and Drug Administration (FDA) has said such testing would be too expensive, he noted, but both Europe and Canada monitor bottled water for this bug.

The FDA does watch bottled water for contamination with coliform bacteria. But standards for municipal water--which is monitored by the Environmental Protection Agency (EPA)--are actually tougher. While the FDA allows for the presence of tiny amounts of coliform bacteria in 1 out of 10 bottles tested, the EPA has zero tolerance.

Glass bottles, the researcher said, are less hospitable to growth of bacteria than plastic ones. And the organism that causes cholera can survive in flat bottled water, but dies off in carbonated water within a day.

As bottled water is capable of harboring "medically important" microbes, Rosenberg stressed that "frequent analysis and stringent regulation are crucial to maintaining human health."

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