Imagine taking an ocean-side vacation, with the sun, sand, and water lulling you to relaxed bliss. After day at the beach, you experience an intense bout of stomach cramps and – more delicately put – GI distress. A rare day off is ruined because of a bug you picked up. Next, imagine a situation where a task as innocuous as drinking tap water sends you running to the nearest bathroom. Even worse, imagine a situation where an entire town experiences simultaneous diarrhea from contaminated tap water.
Clean water is one of the more obvious necessities in life. The above situations are both examples of what can happen if microbial pathogens contaminate recreational or consumable water sources – and both are real-life examples of contamination accidents. Type “boil water advisory” into your favorite internet search engine, and the results demonstrate that ongoing adulterated water issues can affect any community in any country. These examples underscore the importance of surveillance for public health – last discussed on this blog regarding food contaminants, it is no less important to track where potential water contamination has taken place.
As seen in other types of surveillance, diagnostic, and typing methods, there are technological advances that allow faster, wider screening of waterways to help stop contamination from spreading. Surface water has traditionally been monitored by counting contaminating fecal indicator bacteria, which act as a sentinel for other fecal-oral pathogens. New research published last week in Applied and Environmental Microbiology proposes using molecular detection, via a specialized microarray, to screen for the presence of pathogenic microbes.
Of course, any microbial contamination is not like a pure culture grown in rich medium; the cells will at very dilute concentrations due to competing microbial neighbors and the suspending water. So the assay, developed by a team of scientists headed by Jennifer Weidhaas at West Virginia University, uses a time-tested method to concentrate the microbial milieu: ultrafiltration. The filtration method used, dead-end ultrafilturation (DEUF) used a pore size small enough to trap even the smallest microorganism (usually), viruses.
One advantage of using a microarray screening technique is that the device can be used to monitor many different types of microbes at once – viruses, bacteria, fungi, or protozoans that may lurk in the water can be identified this way. This takes advantage of the hundreds of sequences associated with potential pathogens. The array, first described in Environmental Science and Technology ($), contains 411 distinct controls and probes, including for antimicrobial resistance genes. To increase target sequences, total DNA amplification was performed prior to annealing cDNA target.
Researchers tested sewage-spiked marine and freshwater, as well as sewage, freshwater, and marine samples. The array was able to differentiate sources of fecal pathogens in surface water samples, as samples spiked with different sewage sources were sorted into clusters (see figure, right). This indicates the microarray can be a good predictor for human pathogens, which are most likely to be associated with human sewage contamination.
The array was compared to another molecular method, qPCR, which identified the same pathogens the array found, such as polyomavirus and adenovirus, and some pathogens the array missed, such as norovirus. Discriminating between contamination sources is not as useful as identifying contaminant microbes, so the microarray sensitivity needs refinement before replacing current technology. While qPCR may be more sensitive, the ability to scale up microarrays gives the technology an advantage for screening many sample types and timepoints.
Despite the scalability advantage, some technical glitches may hold this microarray method back from widespread use for the time being. DNA amplification from environmental samples fails to differentiate between living and dead organisms, which could lead to false positives. More importantly, the WGA step amplified targets unevenly, making it difficult to measure the proportion of contaminant DNA from all microbes. A false-positive can be followed up with a more sensitive step for verification; a false-negative can release unsafe water for human consumption.
Development of this promising technology will lead to a faster, cheaper alternative to waterway surveillance. As the recent revelation of Legionnella pneumophila cases in Flint, Michigan continues to demonstrate, the better we can screen water for contaminants, the safer the public will be.
-- Julie Wolf