The bottles and cups you drink from may be breeding grounds for bacteria. Have you been cleaning them properly?

In order to protect the environment and save money, many people opt for bringing their own water bottles or cups. The water from taps in Western countries is generally safe for drinking, but not completely free of microorganisms; every time water is consumed from a bottle or cup, bacteria from the mouth can also get into the container. Throughout the day, the quantity of bacteria grows at a rapid pace, especially when the bottle is left at room temperature for an extended period, allowing for more bacterial growth.

According to a report from the BBC, Primrose Freestone, an associate professor of clinical microbiology at the University of Leicester in the UK, stated that bacteria that can cause human infection multiply significantly at around 37 degrees Celsius (98 degrees Fahrenheit), but they can also thrive at room temperature (about 20 degrees Celsius or 68 degrees Fahrenheit).

A study in Singapore on boiling tap water (which should kill most bacteria) found that bacteria can rapidly multiply inside water bottles with continuous use throughout the day. On average, the number of bacteria in bottled water used by adults increased from approximately 75,000 bacteria per milliliter in the morning to 1-2 million bacteria per milliliter after 24 hours.

While some bacteria in water bottles come from the water itself, most contaminants are actually introduced by the drinkers. Whether taking water bottles to work, to the gym, or simply leaving them at home, the exterior of the bottle carries numerous microorganisms. Freestone explains that these microorganisms can easily transfer into the bottle, as bacteria from the mouth also get transferred with each sip of water.

Freestone points out that individuals who do not wash their hands regularly before using water bottles may find bacteria like E. coli in their bottles.

Sharing water bottles with others can lead to the transmission or contraction of viruses. Diseases like Norovirus can easily spread through this mode of transmission.

It’s worth noting that there are usually 500 to 600 different types of bacteria living in people’s mouths. “Bacteria that may not be pathogenic for you may not always be the same for others. You could be carrying a virus without knowing it because our immune system does a good job at protecting us,” she adds.

Another way to promote bacterial growth in bottles is by introducing anything other than water into them. Freestone explains that nutrient-rich beverages can also nourish microorganisms – for example, any sugary drinks stimulate the growth of bacteria or molds present in the bottle.

“Anything other than water is a heaven for bacteria and fungi, especially protein shakes,” she says.

Those who clean their bottles using brushes or dishwashers often have lower levels of bacteria in their bottles. Using a dishwasher with a disinfection cycle may be the most effective method.

Freestone emphasizes that rinsing the bottle with cold water alone is not sufficient, as it does not remove the biofilm, a sticky bacterial layer that can accumulate on the inside surface of the bottle, providing an ideal environment for bacterial growth.

While some studies suggest that the bacterial content in plastic bottles may be higher than in stainless steel bottles, Freestone says that the most hygienic bottles are those that are easy to clean. She further notes that ensuring every part of the bottle is thoroughly cleaned, including the exterior, lid, and straw (if applicable), is crucial.

However, choosing metal or glass over plastic has another rationale.

Amit Abraham, an assistant professor of clinical population health sciences at Weill Cornell Medicine, explains that “plastic often contains chemical additives, giving them advantages like flexibility, durability, heat resistance, and lightweight.”

“These additives physically bond with the plastic, meaning they can leach into the water,” he states.

Some studies indicate that these additives (such as BPA) can disrupt our hormonal function and may be associated with an increased risk of heart disease, stroke, diabetes, and other chronic illnesses.

Abraham mentions that whether bottles are made of disposable or reusable plastic, it seems that chemicals can seep into the water. Additionally, plastic materials themselves degrade, thereby increasing the concentration of microplastics in bottled water. Choosing bottles made of glass or stainless steel is a safer option.