ASK A MICROBIOLOGIST: HOW OFTEN SHOULD I WASH MY WATER BOTTLE?
Reusable water bottles – whether they’re made of plastic, aluminium, glass or stainless steel – are one of the environmentally conscious items that went from unheard of to ubiquitous in the past decade. And while many of us now rely on them daily, most do not wash them with every use – despite doing this with all other cutlery, glassware, and crockery that touches our mouths.
One reason is that we might assume that tap water is sterile. But according to Primrose Freestone, associate professor in clinical microbiology at the University of Leicester, that’s not really the case.
“What comes out of the tap is not sterile – it’s monitored for microbial safety [an absence of harmful microorganisms], not sterility [an absence of all microorganisms]. Even bottled water, which is a lot more clean than it used to be, isn’t completely sterile [but is microbially safe].”
In a study using tap water that had been boiled, which be pretty sterile, found that on average the water in adult water bottles went from 75,000 bacteria per millilitre to more 1-2 million per millilitre over the course of 24 hours.
The majority of the microbes don’t come from the water but from us, says Aiden Taylor, lecturer in microbiology at the University of Reading.
“Every time you take a sip from a bottle, even if it’s a new bottle that you’ve never used before, the bacteria and saliva from your mouth is going back into the bottle,” he says. “And then there’s the bacteria that’s on your hands that transfer when you’re handling the bottle. Those are all routes to get bacteria into that water bottle.”
Bacteria love moist environments and if you are adding anything into your water bottle (whether that’s a slice of lemon or a protein shake), these nutrients mean that number of bacteria will expand even more rapidly. While many of these bacteria will be ones that are already in your body and not causing you harm, that doesn’t mean they don’t have the potential to cause harm.
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“We will carry different types of bacteria in our mouths depending on what we’ve eaten and other factors,” says Aiden. “Some are good and some are bad, but most of them are probably not going to be harmful.”
That said, some people may well have an underlying infection in their mouth like gingivitis and not even know it, which is why it’s best to avoid sharing a water bottle.
However the real hazard would be from bacteria associated with our intestinal tract, such as E.coli, which is easily spread by lazy hand hygiene.
The expert conclusion then is that you definitely do need to wash your reusable bottles regularly with hot water and a dish detergent.
“If you’ve got a very robust immune system, you can probably get away with it for a while,” Primrose says, “but eventually, the water is going to start smelling funny, tasting funny, and texturally be on the slimy side. That is not going to do you any good.”
Aiden adds that while we need bacteria and don’t need things to be sterile at all times, it’s a good idea to wash a water bottle regularly. The longer the bacteria is left to propagate, the more they will build up until they form a biofilm – a structure on the walls of the bottle. And that can’t be cleaned off with just water, even hot water.
So how frequently is ‘regularly’? Primrose errs on the side of caution, recommending you wash the bottle with every other use, or at least three times a week. She personally doesn’t use a reusable water bottle for this reason: “You’re going to end up with something growing there if you don’t empty out the contents and properly wash the bottles after every one or two uses.”
However, Aiden is a bit more lax. “I’m as guilty as most people: I’m looking at my water bottle now that I use every day, and I probably haven’t cleaned that in a month. Most people, if you just use it for water, will just refill it until it starts to taste a bit dodgy.
“It doesn’t have to be every day, but ideally you should be properly washing it once a week.” He points out that washing your hands properly is far more important when it comes to preventing infection.
To keep your water bottle clean and hygienic, the experts offer three golden rules:
- Only put water in it. If you add anything else, you’re going to promote the growth of bacteria and fungi, which are going to colonise and then become harder to remove.
- In terms of washing, you need agitation to remove the biofilms: either a bottle brush or dishwasher is best. If that’s not possible, the combination of hot and soapy water can help detach any bacteria that may be growing on the interior surface. The water should be at least 60 degrees, as that’s the temperature that kills pathogenic bacteria. Shake the bottle, leave for at least 10 minutes, then rinse very well and leave to dry overnight. (Drying is antimicrobial too).
- Use untreated stainless steel where possible: there’s not as much for the bacteria to grab onto and form a biofilm, while plastic is far more sticky. But be sure to check that the bottle isn’t coated with plastic on the inside as sometimes happens with aluminium bottles.