Bottled water is an industry built on the perception of purity, often leading consumers to believe the product is completely sterile. This belief is a misconception, as bottled water is not medically sterile and naturally contains a low level of microorganisms. The presence of some bacteria is normal and expected, even in water sourced from protected springs or purified through advanced treatment systems. The real question for consumers is not simply whether bacteria exist, but whether the levels and types of microorganisms present pose a risk to health.
The Microbial Reality of Bottled Water
Water, regardless of its source, contains naturally occurring bacteria often measured by the heterotrophic plate count (HPC). Even after modern purification and treatment, a small population of these microorganisms remains at the time of bottling. Processing methods, such as filtration and disinfection with ozone or ultraviolet (UV) light, effectively reduce microbial load but do not achieve complete sterilization. Ozone, commonly used as a final disinfectant, dissipates quickly, leaving no residual agent to prevent later microbial growth.
The bacteria that survive this process are generally non-pathogenic, often belonging to genera like Pseudomonas, Flavobacterium, and Moraxella-Acinetobacter. Since the water lacks the complex nutrients found in other beverages, these organisms multiply slowly, utilizing trace elements or dead bacterial cells as a food source during storage. The U.S. Food and Drug Administration (FDA) does not set a specific maximum limit for HPC because these organisms are not considered a direct health threat to the average healthy person. Their presence confirms that bottled water is a natural product and not a bacteria-free environment when it leaves the plant.
Sources of Contamination After Sealing
The most significant increase in bacterial levels occurs after the factory seal is broken and the bottle enters the consumer environment. Every time a person drinks directly from the bottle, oral flora, including harmless Streptococcus species, are introduced. These bacteria find a nutrient source in tiny amounts of transferred saliva or organic matter, allowing them to multiply rapidly. Studies show that bacterial counts in opened bottles can increase dramatically, sometimes reaching tens of thousands of colonies per milliliter, especially when stored at warmer temperatures.
Improper storage conditions can quickly encourage microbial proliferation. Exposing a sealed or opened bottle to elevated temperatures or direct sunlight creates an ideal incubation environment. Reusing single-use plastic bottles without thorough cleaning introduces bacteria from hands and the environment. Repeated use can also cause microscopic cracks and scratches in the plastic, creating protected niches where bacteria and biofilm can accumulate and resist simple rinsing.
Regulatory Safety and Health Risk Assessment
The safety of bottled water is primarily ensured by regulatory standards that focus on preventing pathogenic, or disease-causing, bacteria. In the United States, the FDA regulates bottled water as a packaged food product, setting quality standards that must be met before sale. These standards mandate frequent testing for indicator organisms, which signal potential contamination from sources like sewage or feces.
Specifically, the finished product must not contain any detectable Escherichia coli (E. coli), which indicates possible fecal contamination and renders the water unsafe for consumption. For total coliforms, a broader group of bacteria, the standard requires that no more than one organism be present per 100 milliliters of water. The microorganisms that grow naturally in sealed bottles, such as the HPC bacteria, are fundamentally different from these regulated pathogens and pose a minimal health risk for the average consumer.
Minimizing Bacterial Growth in Stored Water
Consumers can take simple steps to significantly reduce bacterial growth in their bottled water. The most effective action is to store both sealed and opened bottles in a cool, dark, and dry location, away from direct sunlight or high heat. High temperatures, such as those found in a car or garage, accelerate the metabolic activity of any bacteria present, leading to rapid multiplication.
For bottles consumed over time, minimizing the introduction of new bacteria is essential. Avoid touching the bottle rim or cap threads with unwashed hands to prevent the transfer of skin-borne organisms. If a single-use plastic bottle is reused, it should be thoroughly washed daily with hot, soapy water and a brush. Allowing the bottle to air-dry completely after washing is a final step, as trapped moisture can otherwise promote a new round of bacterial or mold growth.