Resurfaced Purdue study finds high bacterial contamination in reusable water bottles

A study published in 2017 and recently resurfaced has found that many reusable water bottles used by college students harbor bacterial levels that exceed accepted safety limits, including traces of coliform bacteria.

Researchers who collected 90 bottles from Purdue University students reported that about seven in 10 bottles had bacterial counts above what the researchers considered safe for drinking water, and roughly 60% exceeded the study’s maximum safety threshold. About 20% of the sampled bottles contained detectable levels of coliform bacteria, a group that includes Escherichia coli, which can cause gastrointestinal illness.

The team swabbed the inside and outside surfaces of the bottles and used an ATP (adenosine triphosphate) test on exterior surfaces to detect organic residue. The researchers reported that all exterior surfaces tested were classified as "dirty" by ATP standards, including two new bottles purchased as controls. Bottles that were refilled more frequently tended to show higher ATP readings, which investigators said could reflect additional touchpoints such as hands, caps and faucet areas.

To measure internal contamination, the researchers rinsed bottle interiors and counted colony-forming units (CFU) of bacteria per milliliter. The study noted that the CDC-referenced guidance used by the authors sets a safe bacterial limit at about 100 to 500 CFU per milliliter. The Purdue samples indicated that about 70% of bottles exceeded 100 CFU/ml, approximately two-thirds exceeded 200 CFU/ml and about three in five exceeded 500 CFU/ml.

"We're constantly touching our water bottles with our mouths and hands, so it's easy to transmit bacteria to them, and then it just grows," said Dr. Yuriko Fukuta, who led the research and is now an associate professor at Baylor College of Medicine. "In some cases this can make you sick, especially if you have a weaker immune system."

Most of the bottles in the study were made of reusable plastic (about 85%), 11% were metal and the remainder were glass; roughly 10% had a built-in carbon filter. The researchers cautioned that a range of bacteria can colonize bottle surfaces. Coliforms, present in about one-fifth of bottles, live in the intestines of humans and animals, and some strains of E. coli can cause symptoms including bloody diarrhea, vomiting and muscle aches. The study also noted staphylococci—common skin bacteria—that are typically harmless on intact skin but can cause infection if introduced into a wound.

Public health context cited in the study noted that E. coli illnesses sicken about 265,000 Americans annually. Severe complications from certain E. coli strains, such as hemolytic-uremic syndrome, are rare but can be life-threatening.

The authors recommended regular, thorough cleaning to reduce contamination. The study advised washing bottles with hot water and dish soap, scrubbing interior walls with a brush and allowing all parts to dry completely, since moisture can promote bacterial growth.

The report arrives amid growing public interest in reusable containers: survey and market data cited by news outlets indicate that as many as 155 million Americans, or roughly 60%, own a reusable bottle and that the U.S. market for such bottles reached about $2 billion last year as consumers seek to reduce exposure to single-use plastics and associated chemicals.

The Purdue study is limited by its sample size and the fact that all bottles were collected from college students at a single university, factors that the researchers acknowledged when describing the need for broader studies to assess contamination across age groups, regions and bottle types. Nevertheless, the findings underscore the importance of routine cleaning and drying of reusable bottles to limit microbial buildup and reduce potential health risks.