Studies reveal: reusable water bottles contain a high bacterial load

In recent years, reusable water bottles have become an everyday accessory in almost every bag – at work, at school, in the gym and on trips. However, behind the healthy and ecological image sometimes hides a real microbial risk.

A series of laboratory tests and academic studies published in recent decades indicate that a bottle that is not washed properly may contain an extremely high bacterial load, sometimes even higher than that found in a public toilet.

One of the most cited reports in the field was published in 2016 by the fitness site Treadmill Reviews, which asked to check how clean athletes’ water bottles really are. The tests were performed at the EmLab P&K laboratory – a certified environmental laboratory in the United States.

As part of the report, 12 reusable water bottles were tested that were used continuously for a week, without washing with soap or in the dishwasher, but only refilled. Samples were taken from the surfaces of the spout and the stopper, and the bacterial load was measured in CFU units – colony forming units – per square centimeter.

When a division was made according to the type of traffic jam, the differences were even sharper. Bottles with a removable spout were found to be the most contaminated, with an average load of 933,340 CFU per square centimeter – almost a million bacteria.

Most of them were defined as gram-negative bacteria, a group that includes strains with the potential for antibiotic resistance. Squeeze bottles contained an average of 161,971 bacteria, and screw bottles 159,060.

In contrast, straw bottles were found to be almost completely clean, with only 25 CFU. The explanation offered in the report was that the straw leads the water directly to the mouth and reduces contact of saliva and moisture with the outer plastic surfaces.

Even earlier, already in 2003, an academic study was published in the Canadian Journal of Public Health, which examined the state of water bottles among elementary school students in Canada.

The study, carried out by University of Calgary researchers, involved collecting 75 water bottles from students. The researchers tested both the water inside the bottles and the surfaces of the spout.

The results revealed that in 64.4% of the bottles bacteria levels exceeding the standard allowed for drinking water were found. A particularly worrisome finding was the presence of fecal coliforms in approximately 9% of the bottles – an indication of contamination that probably originates from poor hand hygiene after using the toilet. The researchers specifically recommended daily cleaning of the bottles in a dishwasher or boiling water, and warned against prolonged use without disinfection.

In 2018, another study was published that focused on a sports environment and concerned shaker bottles used to mix protein powders. The study, published in the Journal of Exercise Physiology Online and corresponding environmental journals, tested 30 shaker bottles of exercisers at gyms in Brazil.

Here, too, the picture was clear: in 83% of the bottles some kind of bacterial contamination was found. In 27% of them, the dangerous bacterium Staphylococcus aureus was detected, a well-known skin bacterium that can cause skin infections, pneumonia and even blood infections.

In 17% of the bottles, the E. coli bacterium was found, indicating contamination from a fecal source. The researchers linked the findings to the complex cork structure of the shakers, which includes grooves and internal networks that make complete cleaning difficult and allow bacteria to hide.

Another aspect tested in later studies is the effectiveness of cleaning methods and the relationship to the material from which the bottle is made. A review published in the International Journal of Environmental Health Research showed that regular washing with only lukewarm water reduces the amount of bacteria temporarily and only marginally.

From a medical point of view, most of the contamination found in the bottles is defined as “self-contamination” – bacteria originating from the mouth, hands and skin of the user himself, which return to him in high concentrations.

However, the main risk is turning the bottle into a vector for the transmission of diseases, especially viral diseases such as influenza, corona or EBV, as well as bacterial skin infections, due to the frequent contact of the hands and mouth with the nozzle.

The recommendations that emerge from the studies are unequivocal: a reusable water bottle should be washed daily in hot water with soap, while completely disassembling the cap and straw. Once every few days it is recommended to carry out a more thorough disinfection using boiling water, a dishwasher at a high temperature or soaking in a diluted vinegar or bleach solution – according to the manufacturer’s instructions. Bottles with complex grooves or wear marks are recommended to be replaced, especially if they are made of plastic.