A lot of people are afraid of E. coli but most strains are harmless to our gastrointestinal tract. In fact, the majority of E. coli strains live in the intestines of both humans and animals. However, not all strains are harmless. Some strains of E. coli, like the O157 H7 strain, possess toxins that could cause harm to humans. Since these bacteria reside in the intestines of both infected humans and animals, both of them are able to shed it off through their feces. When the feces of these infected humans and animals would mix with the drinking water, both animals and humans who would drink this contaminated water may become infected as well. Furthermore, even if the E.coli that is present in the water is not E. coli O157 H7 but just the usual strain. This would still make the drinking water unfit for drinking. The usual strain of E. coli may not cause illness but this is an indication that the water has been contaminated with feces and that other diseases that are transmitted through the fecal-oral route may be present.3
Why do we test our drinking water for E. coli?
The main reason as to why we test our drinking water for E. coli is to find out whether or not the water has been contaminated with fecal material.3
Waste from humans and animals may end up in our drinking water through a variety of ways. Using manure as fertilizers in rainy seasons is one way. The rain may wash the manure towards the streams and end up in the drinking water reservoir. Improperly built septic tanks may also contaminate our drinking water reservoir. During rainy seasons they may fill-up and spill its contents which could wind up in the streams.3
Fecal contamination of water is not exclusive to agricultural and urban areas. Even in remote rural areas, this still happens. If a lot of birds are present near a body of water then their feces may contaminate this body of water.3
Although some strains of E. coli could potentially cause harm to humans. That is not the main reason why we’re trying to look for it in water. E. coli is a bacteria which is referred to as an ‘Indicator organism’. It is called as such because the presence of E. coli indicates fecal contamination.2
The characteristics of E. coli is well documented. It is a Gram-negative rod-shaped facultative bacteria and is a member of the Enterobacteriaceae family. It is capable of fermenting lactose and mannitol and produces acids, gas, and indole from tryptophan as metabolic-by-products. In the microbiology laboratory, there are biochemical tests that would aid in the identification of the bacterial isolate. One growth medium, the MacCongkey agar, contains lactose. If the unknown bacteria is able to ferment lactose, the acids that they would produce through the fermentation process would turn the pH indicator red. This growth medium is used in the process of identifying E. coli. If an isolate is able to ferment lactose then the bacteria may be E. coli. Further biochemical tests would narrow down the identity of the bacteria until a definite identity is found.2
Bacteriological analysis of Drinking Water
Water isn’t really safe for drinking not unless they are tested for fecal contamination through the detection of E. coli using bacteriological methods.
Where should samples be taken?
Bacteriological analysis of drinking water is supposed to be conducted both at the reservoir and to where the water is accessed by the consumer. This is important because investigators can track down where the contamination happened in the event that it does occur.2
Depending on the nature of the provider, water samples may be collected in their reservoir, treatment plant, storage facilities, distribution network, and points at which water is delivered to the customers.2
How often do we need to collect samples?
Frequent sample collection and analysis is ideal because water quality may change quickly. Furthermore, the constant analysis may allow those who are responsible for monitoring water quality to catch the contamination in its early stages. However, this is not the case most of the time.2
How do we store samples for microbiological analysis?
Samples are best stored in a light-proof icebox with ice packs included inside. These samples are fit for analysis for up to 6 hours. Although it is not recommended, samples below 24 hours are still acceptable. In situations where-in there are no ice packs present. The samples must be analyzed within 2 hours.2
Multiple Tube Method (or Most Probable Number - MPN)
This technique for the bacteriological analysis of water is often used for assessing the safety of drinking water. It is a technique that approximates the number of bacteria present in a water sample by using statistical tables as a reference. This procedure is time-consuming because it involves the process of culturing the bacteria and encouraging it to grow through incubation. Aside from this, the Multiple Tube Method is labour-intensive because it requires a lot of glassware and consumables. Although there are other methods for the bacteriological analysis of water, this one is preferred because it is relatively cheaper and relatively more accurate.2
In a nutshell, the multiple-tube method is done by preparing various dilutions of the same sample. Each diluted sample is then mixed with the culture medium, a medium that encourages bacterial growth, and incubated. After incubation, the tube that has some bacteria present would exhibit bubble formation and turbidity. The number of tubes that would exhibit a positive result would then be counted. The ‘most probable number’ of bacteria present per 100ml of the sample is then determined by consulting a statistical table.2
The culture medium that is contained in every tube is a sterile solution containing needed nutrients that would encourage the bacteria to grow. After a sample is added to this solution, it is thoroughly mixed and then a small inverted glass tube is placed inside to catch bubbles. It is incubated for 24 to 48 hours to encourage bacterial growth. After incubation, the most probable number would then be determined. At this point, the approximate number of bacteria present would be referred to as total coliform. Coliforms are Gram-negative, aerobic or facultative anaerobic non-spore-forming rods. Coliform bacteria were once used as an ‘indicator organism’ but advances in microbiological studies have shown that there are a lot of coliform bacteria that are naturally present in our surroundings so they are not specific as an indicator organism for fecal contamination.1
Isolating fecal coliforms would improve the specificity of this microbiological methodology. Tubes that are positive for bacterial growth in the previous stage, the Total Coliform, would then undergo fecal coliform analysis. A sample is taken from each total-coliform-positive tube and then transferred to a separate tube that contains a kind of solution that would discourage the growth of other bacteria but encourage the growth of fecal coliforms. Again, they are mixed thoroughly and a small tube used to catch bubbles would be placed inside. Basically, the same thing is done as with total coliform at this point. MPN is also determined.1
To improve the specificity of this microbiological methodology further. Yet another sample is taken from each fecal-coliform-positive tube and then transferred to a separate tube that contains a kind of solution that would discourage the growth of other bacteria except E. coli. The same thing is done here as with the other stages, total coliform and fecal coliform. The MPN is determined so that we could arrive at an approximation of how many E. coli is present per 100 mL of the sample. If your samples would yield a positive result at this point, then that is a reliable indication that the water being tested is contaminated with fecal matter.1
Characteristics of the Multiple Tube Method
● The multiple tube method is very slow. It would take a week before you could get the result of the microbiological test; 24 to 48 hours for the total coliform, then 24 to 48 hours for the fecal coliform, and finally 24 to 48 hours for E. coli.2
● Aside from taking too long, the multiple tube method is very labour-intensive. You’ll have to prepare several dilutions of the sample. Then transfer each of these dilutions to their dedicated bacterial medium. Not to mention that you have to wash all of these tubes at the end.2
● The multiple tube method demands a lot of culture medium. Especially if most of the tubes would exhibit a positive result from total coliform to fecal coliform, and to E. Coli.2
● Compared to other methods, the multiple tube method is very sensitive and can easily detect fecal contamination.2
● Although MPN is very sensitive, when it comes to determining the number of microorganisms present in the sample. It is not so accurate and precise because it is based on statistical approximation.2
● This thing is impossible to do in the field. For you to be able to perform this test, you need incubators, weighing scales, glassware, and many more. You’ll have to set up a minilab if you want to conduct MPN in the field; an undertaking which is quite impractical.2
● The bacteria present in the water may not always be healthy. There are instances that the bacteria in the water being tested are stressed or most of them are dead. However, this is not an issue for MPN as the total coliform step would encourage the growth of the stressed bacteria in an optimal environment.2
Advantages of real-time E. coli monitoring
The great thing about real-time E. coli monitoring is that when fecal contamination occurs you would know the exact time and date of its occurrence. This could even serve as a warning system. If E. coli levels would increase, a remedy to the issue could be applied swiftly.
Our product provides real-time E. coli monitoring through the use of labelled antibodies. This is how our product works, a label that is detectable by our equipment is attached to one end of an antibody. The other end is designed to attach to E. coli specifically. All these properties have made it possible for our product to detect the presence of E. coli and quantify it in real-time.