Many drinking water treatment facilities worldwide disinfect water with ultraviolet light because it’s quick and efficient, kills protozoa such as Giardia, and doesn’t introduce potentially harmful disinfection by-products. But a new study shows that UV treatment alone can push bacteria into a dormant state instead of killing them, and that in some cases, the bacteria can later revive and proliferate (Environ. Sci. Technol. 2015, DOI: 10.1021/es505211e).

In contrast with chlorination, which kills by destroying the bacterial cell membrane, UV treatment leaves the membrane intact but damages bacterial DNA to block replication. But it can also induce bacteria into a viable but nonculturable state in which the bacteria are dormant and no longer form colonies, but can later wake up and start dividing again.

Xin Yu, an environmental engineer at the Chinese Academy of Sciences’s Institute of Urban Environment, wanted to determine the risk that bacteria in this dormant state might pose to drinking water safety for facilities in which only UV treatment is used. In the U.S. and Canada, UV is used as a secondary treatment after chlorination or other disinfection methods. But in China, UV alone is used in many small-scale drinking water facilities and for point-of-use disinfection, Yu says, and it’s being proposed as an effective alternative to chlorination on a larger scale.

So he and his colleagues grew liquid cultures of two types of bacteria, Escherichia coli and Pseudomonas aeruginosa, to concentrations of 1 billion colony-forming units (CFU)/mL. Then they exposed these cells to UV light at doses from 0 to 300 mJ/cm3. A typical UV dose for water disinfection is 30 to 40 mJ/cm3.

The researchers quantified the UV-treated bacteria in two ways. First, they counted the colonies formed in a petri dish, a method called plate counting that is used by many water treatment facilities. Second, they measured the expression of bacterial genes using reverse transcription with quantitative polymerase chain reaction.

In all cases, the plate counting assays showed that UV treatment decreased the bacterial concentration to 0.0001 to 1% of their starting levels. But to Yu’s surprise, the PCR tests showed no significant change in expression of a ribosomal gene. This indicates that most of the UV-treated bacteria retained the ability to synthesize proteins and thus were not dead.

The team next wanted to determine whether these bacteria could revive and form colonies. So they diluted the UV-treated bacteria to 0.1 CFU/mL or less, which is far below the 100 CFU/mL safety threshold used by most treatment plants. They then added a nutrient-rich broth and increased the temperature to 37 °C to simulate conditions in the human body. After 24 hours, they tested these cultures for bacterial growth.

At 0.1 CFU/mL, E. coli cells came back to life for all levels of UV treatment. For P. aeruginosa at that concentration, only cells exposed to a fairly low level of UV treatment, 25 mJ/cm3, showed resuscitation.

The results, Yu says, suggest that water treatment facilities that use plate counting to assess bacterial concentration may underestimate the concentration of viable cells.

Although dormant bacteria can resuscitate under some conditions, the study shows that not all of them can, says Sébastien P. Faucher, a microbiologist at McGill University, in Montreal. “This is good news.” He estimates that dormant bacteria pose a minimal threat to drinking water safety, because bacterial concentrations of water entering the UV treatment stage in plants are significantly lower than in the samples the researchers treated with UV—at least 99.99% lower.