Study finds many reusable water bottles harbor high levels of bacteria, including coliforms

A study published in 2017 and recently resurfaced online found a majority of reusable water bottles tested among college students contained levels of bacteria above commonly cited safety thresholds, and about one in five carried detectable coliform organisms.

Researchers at Purdue University collected 90 bottles regularly used by students, swabbing both interior and exterior surfaces and rinsing bottle interiors to quantify bacterial growth. The team reported that roughly seven in 10 bottles exceeded 100 colony-forming units per milliliter (CFU/ml), about two-thirds topped 200 CFU/ml and approximately three in five exceeded 500 CFU/ml. The study also detected coliform bacteria in about 20 percent of samples; the most common coliform in humans and animals is Escherichia coli (E. coli), which can cause gastrointestinal illness in some cases.

Investigators used an ATP (adenosine triphosphate) test to screen external surfaces for organic residue, categorizing bottles as "clean" at 10 relative light units (RLUs) or below, "inadequately cleaned" at 11–20 RLUs and "dirty" above 30 RLUs. The study reported that the exteriors of all bottles tested met the study's criteria for "dirty," including two unused bottles purchased as controls. The researchers noted a correlation between frequency of refilling and higher ATP readings, suggesting more touchpoints—hands, caps, faucet areas and counters—may introduce additional contamination.

Bottle composition varied: about 85 percent were reusable plastic, 11 percent metal and the remainder glass; roughly one in 10 bottles had a built-in carbon filter. The authors recommended routine cleaning with hot water, dish soap and scrubbing with a brush, and advised that all parts be dried completely to limit bacterial growth.

The study was authored by researchers including Dr. Yuriko Fukuta, who is quoted in a contemporaneous report saying people constantly touch bottles with mouths and hands, creating opportunities to transfer and then grow bacteria. She told Earth.com that contamination is more likely to make people ill if their immune systems are weakened.

The paper was published in the journal Food Protection Trends in 2017 and has received renewed attention as reusable bottles have become more common. Surveys indicate as many as 155 million Americans—about 60 percent—report owning at least one reusable bottle, and the U.S. market for reusable water bottles reached roughly $2 billion in the most recent year cited in reporting about the study.

The study authors cited what they described as safe bacterial limits used in related guidance: 100 to 500 CFU/ml as a threshold range for concern. They also noted that while many detected organisms could be benign skin bacteria, other organisms such as staphylococci or coliforms may pose health risks if they enter the body or if a person is susceptible. E. coli, when pathogenic strains are present, can cause symptoms ranging from diarrhea, vomiting and muscle aches to rare complications such as hemolytic-uremic syndrome.

Limitations of the analysis include the sample size and population: the bottles came from a single university and from students who self-reported use and cleaning habits. The study did not report full species-level identification for all coliform-positive samples, so it is not possible from this report alone to determine how many bottles contained pathogenic E. coli versus nonpathogenic strains. The ATP test used for external surfaces detects general organic residue rather than identifying specific microbes.

Public-health guidance reflected in the study's recommendations emphasizes frequent and thorough cleaning of reusable bottles, using hot water and soap, scrubbing internal surfaces and drying components fully after washing. Those with weakened immune systems are advised to take extra precautions with bottle hygiene. The resurfaced findings underscore that while reusable bottles reduce reliance on single-use plastics and potential chemical exposures, they also require regular maintenance to minimize microbial risks.