The danger lurking in your reusable water bottle

As Australia swelters through one of its hottest summers to date, many are staying hydrated thanks to their reusable water bottles.

But drinking from your trusty flask could be doing more harm than good after recent laboratory tests revealed the containers are a breeding ground for bacteria, US television station KDVR reported.

Scientists took swabs from a selection of water bottles and compared findings to a sample taken from a dog bowl with astonishing results.

The test was looking for colony forming units (CFU), a unit used to estimate the number of viable bacteria or fungal cells in a sample.

An acceptable level for a swab is five CFU. The dog bowl expectedly produced more with 700 CFU per swab.

But one of the three water bottles tested returned over a shocking 100,000 CFU per swab after claiming to wash his bottle once a month in the dishwasher.

"It's pretty nasty... We couldn't even count that it was so full of bacteria," the microbiologist responsible for testing revealed.

One woman's bottle, which she said she rinsed daily, produced 6,000 CFU per swab.

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Doctors say stomach bacteria will kill most of the bacteria the body takes in but not washing your water bottle increases the risk falling ill.

Bottle users are advised to wash their containers daily with hot, soapy water or in the dishwasher.