“I closed the cap tight” A once-sipped bottled water was a bacterial incubator - 경향신문

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It is common to drink directly from a store-bought bottled water, close the cap tightly, and then drink from it again. Especially at home or in the office, people often keep a PET bottle for a day or two and use it like a water bottle instead of discarding it. On the surface, it still looks clear and clean. However, once a mouth has touched the PET bottle, the situation changes. More than the water itself, microorganisms left on the bottle mouth, cap, and inner surfaces are the problem.

There is research that examined how quickly bacteria increase in opened bottled water. A team led by Shaun Raj at the University of Texas MD Anderson Cancer Center published a study in 2005 in the international journal Water Environment Research titled How Safe Is Bottled Water?

The researchers observed bacterial growth over time and at different temperatures while storing water after a single drink. They cultured the water and measured colony-forming units (CFU). When stored at 37 degrees, an initial level of less than 1 CFU per mL rose to about 2 CFU after 2 hours, about 3000 CFU after 8 hours, about 14,000 CFU after 24 hours, and about 38,000 CFU after 48 hours.

Refrigeration, however, greatly slowed the growth rate. In the same experiment, the bacterial count was about 50% lower than at room temperature after 24 hours, and about 84% lower after 48 hours.

More recent work has identified microorganisms remaining on the bottles themselves. A research team at Seibeta University Medical College and Hospital in India published a study in 2024 in the Journal of Pharmacy and Bioallied Sciences comparing microbial contamination in PET water bottles and stainless-steel water bottles.

The researchers examined 15 PET water bottles and 15 stainless-steel bottles that were in actual use. At the initial measurement, the microbial load of PET bottles averaged 68.8 CFU/mL, higher than the 35.4 CFU/mL of stainless-steel bottles. Notably, after 3 hours of use, the microbial load of PET bottles increased by 70% compared with the initial level.

The habit of putting one’s mouth directly on the bottle also made a difference. In the same study, bottles drunk from by mouth had an average microbial load of 234 CFU/mL, higher than the 132 CFU/mL of bottles not touched by the mouth. The researchers suggested that repeatedly refilling bottles without washing them can contribute to microbial buildup. In fact, 90% of the participants did not wash their bottles each time they refilled them.

Why does merely touching the mouth to the bottle make such a difference? Because the oral cavity harbors numerous microorganisms. When lips contact the bottle rim while drinking, saliva and oral microbes can transfer to the rim and into the bottle. Microbes from the skin and surrounding environment can also be deposited when handling the cap or bottle neck. Once the bottle is closed with water still inside, the interior becomes a space where microbes can persist.

A particular concern is when microbes adhere to the bottle surface. Microorganisms can not only float in the water but also attach to surfaces and form biofilms. A biofilm is a kind of shield created when microbes adhere to a surface and produce protective substances. Simply pouring out the water once or giving a light rinse may not adequately remove microbes attached to surfaces.

There are experimental results showing that microbes can adhere to and survive on PET itself. A French environmental microbiology team observed the adhesion and survival of Mycobacterium avium on PET bottled-water surfaces, finding that over 3 months of storage at 20 degrees, the growth and adhesion strength of cells attached to the bottle surface increased.

One should also be cautious about assuming that a single rinse is sufficient. A quick rinse with water can help remove visible residues, but it is not the same as adequately removing microbes that are attached to surfaces. The U.S. Centers for Disease Control and Prevention (CDC) explains that in typical household settings, cleaning and scrubbing with soap and water alone can remove most bacteria and viruses. The key is to clean, not merely rinse once with water.

In the PET bottle study, microbial load dropped markedly after washing. Comparing before and after washing, the average microbial load fell to 11.2 CFU/mL. In other words, more important than the act of repeatedly reusing a bottle is the problem of continuing to use it without washing.

Storage temperature also matters. In the Raj study above, refrigeration greatly slowed bacterial growth but did not eliminate it. Therefore, refrigerating a PET bottle that contains water already drunk from cannot be assumed to be safe. Refrigeration is a way to slow growth, not a substitute for cleaning.

Extra caution is warranted when storing beverages that contain nutrients, such as barley tea or fruit drinks, in bottled-water containers for long periods. In opened beverages, microbial growth patterns can vary depending on temperature and composition. A 2020 study by Tokai University and Kyushu Sangyo University in the Journal of Food Hygiene and Safety also found that microbial growth in opened PET bottled beverages varied by beverage type and temperature.

The researchers inoculated microbes into 500 mL PET bottles containing tomato juice, a sports drink, bottled water, green tea, blended tea, orange juice, and coffee with milk, then observed changes for up to one week at 4·25·35℃. In tomato juice, Lactobacillus fermentum, in the sports drink, Candida albicans, and in bottled water, Klebsiella pneumoniae showed growth patterns similar to previous research.

By contrast, in green tea, blended tea, orange juice, and coffee with milk, the growth of standard strains was generally weaker. The researchers analyzed that components contained in the beverages may have inhibited microbial growth. In other words, one cannot simply claim that nutrient-containing beverages always accelerate bacterial growth; the growth rate can vary greatly depending on beverage composition, microbial species, and storage temperature.

Ultimately, the point is not that PET bottles are unconditionally dangerous. Even hygienically managed bottled water before opening is no longer a sealed container in its original state once the cap is opened and the mouth touches it. Microorganisms can enter from the mouth and hands, and the remaining water and interior moisture can create an environment for them to persist.