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Methods for Evaluating Bacteria in Drinking Water

The opportunistic bacterial pathogens present in drinking water are a concern for many health-care providers and residents. The authors of this study, Rusin, Rose, Haas, and Gerba, describe several methods of evaluating the bacteria in water. These methods include the Colilert test and measurements of heterotrophic plate count. The authors also discuss the effects of nitrite and ammonia concentrations.
  • Shikha
  • 09 Jun, 2022 6:22 am
  • Drinking Water, Bacteria, Heathcare, Legionella
  • 15

Methods of Evaluating Bacteria in Drinking Water

Colilert test

The Colilert test for bacteria in drinking waters is one of the most reliable ways to determine the quality of your drinking water. It uses a defined substrate medium and a specific reagent to detect E. coli and coliform bacteria. Results of this test are available in less than 18 hours. For faster results, use the IDEXX Quanti-Tray(r) system. The result is shown in a series of wells and is accurate in detecting coliform bacteria.

The Colilert-18 test uses two different enzyme substrates, chromogen and fluorogen. When one substrate reacts with the other, the resulting product turns yellow and fluoresces under long-wave ultraviolet light. This means that the sample contains E. coli and total coliform bacteria. However, other bacteria can produce the same enzymes and result in false-negative results. Therefore, the test is recommended only for water that has a low concentration of E. coli.

Unlike other tests, Colilert-18 has a defined bacterial enzyme detection technology that allows for simultaneous detection of E. coli and total coliform bacteria. In addition, Colilert-18 uses the enzymes ONPG, MUG, and ss-galactosidase to detect a single viable E. coli per sample. The test is available in 10mL Most Probable Number (MPN) tubes.

Measuring heterotrophic plate count

Colony-forming units (CFUs) per mL are descriptive terms. Counts should be reported as CFU/mL for spread-plate methods using 35degC for 48 hours. Colonies should be counted on at least seven consecutive squares across a plate to obtain an average colony count. If duplicate samples are used, the average colony count can be taken and divided by the sample volume.

The standard HPC method is a culture-based technique used to measure the population of heterotrophic bacteria present in drinking water. This method provides a general estimate of the total number of heterotrophs, but does not provide detailed information about the species detected. Depending on the method used, inter-specific competition, and the chosen incubation temperature, viable bacterial populations will differ. In addition, some heterotrophs are nonculturable, and the HPC media do not meet the complex nutritional requirements of all heterotrophs.

The EPA's Action Levels for Colony Forming Units (CFUs) are a more appropriate level to monitor for bacterial growth. The EPA action limit for total coliform bacteria is 500 CFU/mL, which is the standard used to monitor bacterial growth in drinking water. However, heterotrophic bacteria can interfere with some methods of total coliform recovery.

PCR techniques

Multiplex PCR methods are used for detecting bacteria in drinking water. They are more sensitive than standard culture methods and can detect some sentinel microorganisms that are associated with faecal contamination of water sources. Moreover, they do not contradict other reports. However, the detection limits of target genes are dependent on the primers used. These techniques are useful in the initial screening of drinking water.

DNA of bacteria was isolated from the samples. Different volumes of water were filtered through 0.22 um membranes. The samples were stored at -20 degC to preserve DNA. Newcastle University performed the molecular microbiology test. DNA was extracted from prokaryotic biomass using a PowerWater(r) DNA Isolation Kit manufactured by QIAGEN. Using the sample obtained from different sources, the researchers determined the amount of faecal E. coli and other coliform bacteria in water samples.

Two conventional methods of detecting bacteria in drinking water are still widely used. Traditional approaches involve culture of a water sample and diagnosing the presence of b-galactosidase in the samples. However, both methods are expensive and cumbersome. Using multiplex PCR techniques is a safer and faster alternative. The proposed method uses several sets of primers and templates to detect bacteria. For example, a single sample from location 6 may reveal the presence of Shigella, Clostridium, and Escherichia coli.

Ammonia and nitrite concentrations

A study was carried out to measure the nitrite and ammonia concentrations in water. The nitrogen concentrations were measured in a pipe loop and the apparent formation rates were calculated for a single time interval for 0.1 m3 h-1. The results were statistically significant when the nitrogen concentrations were less than the mean concentration. The nitrogen balance error was calculated by multiplying the concentrations by the volume of water.

Nitrogen is necessary for all living things and is found in various forms in the environment. It occurs in varying amounts in the environment and changes its form as it progresses through the nitrogen cycle. However, excessive concentrations of nitrite-nitrogen may cause health problems. It is vital to understand the effects of this substance on the human body. Therefore, it is vital that drinking water is filtered regularly.

The study used a biofilter obtained from an aquarium as a source of nitrites. The biofilter was used to culture NOB and AOB, which remove toxins like NH3 and NO2. Using a culture-dependent approach, a variety of nitrifying bacteria were selected. Furthermore, nitrifying activities were examined with and without synergistic interactions between different nitrifying bacteria. In addition, metagenomic sequencing of the cultures of both NOB and AOB showed that they were the same.

Pseudomonas aeruginosa

One of the most common causes of urinary tract infections is contamination of water supplies with microbial pathogens. In one study, the bacterium Pseudomonas aeruginosa was found in 25% of source water, 32.3% of carbonated water, and 4.6% of finished water. Researchers used DNA-based fingerprinting to identify and characterize virulent strains of P. aeruginosa and identified three genes responsible for microbial virulence: the ExoU gene, the ExoS gene, and the phzM gene.

Biofilms containing Pseudomonas aceticum can survive for weeks after contamination. These biofilms are formed when tap water is continuously passed through a reactor. These biofilms contain Pseudomonas aeruginosa, and this makes them much more difficult to eradicate than planktonic bacteria. They persist for at least seven days and up to five weeks under continuous flow.

The study also identified more than 30 strains of P. aeruginosa from drinking water. Of those, thirty strains were resistant to at least one antibiotic. These results differ from previous studies, but the difference may be due to the difference in source of the water samples. While most of the samples were taken from groundwater, many surface water samples were collected from sparsely-populated rural areas where bacteria are not exposed to outside influences and are rarely contaminated.

In China, P. aeruginosa was found in drinking water samples collected from various sources. Twenty-four percent of samples tested positive for P. aeruginosa were spring water. The contamination rate was 6.9% in the final product. Researchers recommend that manufacturers clean and disinfect activated carbon filters regularly. These findings are important for the prevention of contamination in drinking water.

Enteric viruses

Human feces contain many different types of viruses. Some are harmless while others can be highly harmful. Enteric viruses are primarily responsible for diarrhea and are often found in contaminated water. Human enteric viruses include the Norovirus, Rotavirus, Hepevirus, and Mamastrovirus. Human enteric viruses are easily spread through contact between infected individuals and contaminated food. Infected people can also transmit the virus to others through direct contact and contaminated water.

While there have been many studies on enteric viruses in water supplies, none of them have been conducted on the territory of a country like Kenya. Enteric virus concentrations in the water were assessed using the PCR method for adenovirus and reverse transcription PCR for adenovirus. The results showed that both HEV and AdV were detected in 106/109 samples. The results indicated that seasonality was a factor in the virus distribution and that this could affect virus levels.

The two most common types of enteric viruses are rotavirus and norovirus. Among infants, rotavirus causes severe diarrhea and causes more than half a million deaths a year. While rotavirus removal from drinking water is not an easy task, new vaccines are on the horizon. Moreover, better vaccination coverage is likely to reduce the risks of infection. In the future, it will be easier to keep viruses out of drinking water.