Bacteriological Examination of Water: Ensuring Safety in Food Processing

Water is used at nearly every stage of food processing – from washing raw ingredients and mixing formulations to cleaning equipment and packaging. If that water carries harmful microorganisms, every product it touches becomes a potential health hazard. Bacteriological examination of water is the scientific process that stands between safe food and a potential outbreak. It uses laboratory methods to detect and count bacteria in water samples, helping food processors confirm that their water supply meets safety standards before it ever contacts food.

Why bacteriological testing of water matters in food processing 🔗

Contaminated water in a food facility can lead to widespread foodborne illness. According to research published in PubMed, the use of contaminated water during food preparation and serving is directly linked to gastrointestinal infections. The study found that 55% of water samples tested positive for total coliforms, and E. coli was detected in nearly 30% of samples – a clear indication that water quality cannot be taken for granted.

Unlike chemical contaminants, microbial contamination is invisible. Water can look perfectly clear and still harbour dangerous levels of bacteria like Salmonella, E. coli, or Shigella. That is why regulatory bodies such as the World Health Organization (WHO), the US FDA, and the Bureau of Indian Standards (BIS) mandate routine bacteriological examination of water used in food production. The WHO guidelines specify that E. coli or thermotolerant coliform bacteria must not be detectable in any 100 mL sample of drinking water.

The concept of indicator organisms 🔗

Testing water for every possible pathogen is impractical – it would be time-consuming, expensive, and technically demanding. Instead, bacteriological examination relies on indicator organisms. These are specific groups of bacteria that are commonly found in the intestines of humans and warm-blooded animals. Their presence in a water sample suggests that the water may have been contaminated with fecal matter, and therefore, disease-causing pathogens could also be present.

As the US Environmental Protection Agency (EPA) explains, coliforms and fecal streptococci are the two primary bacterial groups used as indicators of possible sewage contamination. Although these indicator bacteria are generally not harmful on their own, they signal the potential presence of pathogenic bacteria, viruses, and protozoans that share the same habitat – the digestive systems of humans and animals.

The logic is straightforward: even when a person is infected with a dangerous pathogen, they excrete millions of times more indicator organisms than actual pathogens. So if indicator organism levels in water are low, pathogen levels are very likely even lower or absent altogether.

The three key bacteriological tests for water 🔗

Bacteriological examination of water in food processing relies on three primary tests. Each one targets a different group of organisms and provides distinct information about water quality and contamination sources.

1. Plate count (heterotrophic plate count) 🔗

The plate count, also known as the heterotrophic plate count (HPC), is a method used to estimate the total number of live, culturable bacteria in a water sample. According to the Standard Methods for the Examination of Water and Wastewater, this procedure has been in use for nearly 150 years, dating back to Robert Koch’s early microbiological techniques.

The HPC does not identify specific types of bacteria. Instead, it provides a general picture of the overall microbial load in water. A high plate count suggests poor sanitary conditions, ineffective water treatment, or contamination in the distribution system. When tracked over time, HPC data helps food facilities spot trends and identify problems before they become critical.

How it works: The water sample is serially diluted (1:10, 1:100, 1:1000, and so on) in sterile water. Each dilution is then placed on a nutrient agar medium – typically plate count agar – inside a petri dish, which is sealed and incubated. One common approach uses two sets of plates: one incubated at 22°C and another at 37°C, each for 24 hours. After incubation, the visible colonies that have grown on each plate are counted. Each colony represents a colony forming unit (CFU).

For the count to be statistically reliable, the plate should ideally contain between 30 and 300 colonies. Fewer than 30 makes the data unreliable, while more than 300 often leads to overlapping colonies and inaccurate counting. Results are reported as CFU per millilitre (CFU/mL). Three common inoculation techniques are used: the pour plate method, the spread plate method, and the membrane filtration method.

2. Coliform count 🔗

The coliform count is one of the most widely used tests in water quality assessment. Coliforms are gram-negative, non-spore forming, facultatively anaerobic rod-shaped bacteria that ferment lactose to produce acid and gas within 48 hours. This group includes genera such as Escherichia, Klebsiella, Enterobacter, and Citrobacter.

While total coliforms can be found in soil, vegetation, and other environmental sources (and therefore do not always indicate fecal contamination), fecal coliforms and specifically E. coli are much more reliable indicators of contamination from human or animal waste. As noted in the Biology LibreTexts microbiology manual, fecal coliforms (also called thermotolerant coliforms) can grow in the presence of bile salts and produce acid and gas at 44°C within 48 hours, distinguishing them from non-fecal coliforms.

How it works: Two primary methods are used to detect and enumerate coliforms:

Multiple tube fermentation (Most Probable Number method): Measured portions of the water sample are added to tubes containing a selective nutrient broth, often with a small inverted Durham tube inside. The tubes are incubated at a set temperature. Gas production within the Durham tube and turbidity in the broth indicate the presence of coliforms. By testing multiple dilutions and recording positive and negative tubes, analysts use statistical tables to calculate the Most Probable Number (MPN) of coliforms in the original sample.

Membrane filtration method: A known volume of water is filtered through a sterile membrane filter with a standard pore size. The filter is then placed on selective media – such as mFC agar for fecal coliforms or MacConkey agar for gram-negative bacteria – and incubated. Colonies that grow on the filter are counted directly, with the results reported as CFU per 100 mL.

For drinking water and water used in food processing, the standard is strict. The US EPA’s Revised Total Coliform Rule requires that coliforms should not be found in more than 5% of monthly samples. For E. coli, the acceptable limit in potable water is zero CFU per 100 mL.

3. Fecal streptococci test 🔗

The fecal streptococci test targets bacteria from the genus Streptococcus that are naturally present in the intestinal tract of humans and warm-blooded animals. This group includes species such as Streptococcus faecalis (now reclassified as Enterococcus faecalis), S. faecium, S. bovis, and S. equinus. Enterococci are a subgroup within the fecal streptococcus group and are considered particularly valuable indicators.

What makes fecal streptococci especially useful is their environmental resilience. According to a study published in PubMed, fecal streptococci are resistant enough to survive long after the original contamination event, making them effective indicators of past fecal contamination that other tests might miss. The same study found that when water was tested only for E. coli and coliforms, 22% of fecally contaminated samples went undetected – highlighting the importance of including the fecal streptococci test in a comprehensive monitoring program.

Research published in the journal Water, Air, & Soil Pollution also demonstrated that fecal streptococci were generally more resistant to natural water conditions and purification processes than coliforms. In some cases, their survival patterns matched those of enteric viruses better than coliform bacteria, suggesting they can be a more reliable indicator of viral contamination risk.

How it works: The test typically uses selective media containing sodium azide, which inhibits gram-negative bacteria while allowing streptococci to grow. Common methods include membrane filtration (as described in ISO 7899-2) and the multiple tube technique using azide dextrose broth as a presumptive medium, followed by confirmation on ethyl violet azide broth. Plates or tubes are incubated at 36-37°C for 48 hours, after which positive results are recorded and quantified.

Proper sampling: the foundation of accurate results 🔗

No laboratory test can compensate for a poorly collected sample. Sampling technique is the single most important factor determining the reliability of bacteriological examination results. Several key principles must be followed:

Sterile containers: All sample containers must be sterilized before use – either autoclaved at 121°C for 15 minutes or supplied as factory-sealed, pre-sterilized bags. Any contact between the sample and a non-sterile surface will invalidate the results.

Proper collection technique: Samplers must avoid touching the inside of the container or its cap. If the water is chlorinated, the collection container should contain a neutralizing agent (such as sodium thiosulphate) to prevent residual chlorine from continuing to kill bacteria after collection, which would give a falsely low count.

Time and temperature control: Samples must be kept on ice and analysed within 6 hours of collection. Extended delays allow bacterial populations to change – some organisms multiply while others die off – leading to results that do not reflect actual water conditions at the time of sampling.

Multiple sampling points: In food processing facilities, water should be sampled at several locations: the source (borewell, municipal supply), after treatment, and at the actual points of use within the facility (such as wash stations, mixing tanks, and CIP systems). Contamination can occur anywhere in the distribution system, so testing only at the source is not enough.

Quality control measures: As recommended by the US EPA, field blanks (sterile water processed like a regular sample) and duplicate samples should be collected at about 10% of sampling sites to detect collection errors and verify analytical precision.

Interpreting results and taking action 🔗

Once laboratory results are available, they must be compared against established regulatory standards. For water used in food processing, the expectations are typically as strict as those for drinking water:

Plate count: Acceptable heterotrophic plate counts vary by standard, but a general benchmark for treated drinking water is fewer than 500 CFU/mL (at 37°C). Sudden spikes in HPC values – even if they remain below the threshold – may indicate a developing problem that warrants investigation.

Coliforms: Total coliforms should be absent from treated water. Any detection of E. coli or fecal coliforms in water used for food processing is a critical finding that requires immediate corrective action.

Fecal streptococci: Their presence indicates fecal contamination and demands immediate investigation of the water source and distribution system.

When test results exceed acceptable limits, food processing facilities must respond quickly. Typical corrective actions include increasing disinfection levels (chlorination, UV treatment, or ozonation), identifying and repairing the source of contamination, and in severe cases, halting production until safe water quality is restored. All actions and test results must be documented as part of the facility’s HACCP (Hazard Analysis and Critical Control Points) plan.

Limitations to keep in mind 🔗

Bacteriological tests are essential, but they are not without limitations. The plate count method only detects bacteria that can grow under the specific laboratory conditions provided – certain species require different nutrients, temperatures, or atmospheric conditions that standard methods do not cover. Additionally, stressed or injured bacteria (for example, those partially affected by chlorine) may fail to grow on standard media, potentially leading to underestimates of the actual microbial population.

The time required for results is another challenge. Standard plate counts and coliform tests need 24-48 hours for incubation. During this waiting period, contaminated water could continue to be used. This is why modern facilities increasingly supplement traditional methods with rapid testing technologies like ATP (adenosine triphosphate) bioluminescence assays, which can provide results in minutes, though they measure overall biological activity rather than specific bacterial counts.

Finally, as the PMC review on indicator organisms notes, fecal streptococci do not multiply in the environment, which is a useful property for indicating true contamination events, but the organisms can persist long after contamination has been resolved – sometimes causing unnecessary alarm.

Building a comprehensive water monitoring program 🔗

No single test provides a complete picture of water safety. The most reliable approach combines all three tests – plate count, coliform count, and fecal streptococci test – into a regular monitoring schedule. Each test provides different but complementary information: the plate count reveals overall microbial load, the coliform count signals potential fecal contamination, and the fecal streptococci test catches contamination events that coliform testing alone might miss.

A well-designed monitoring program includes scheduled testing at defined frequencies (weekly, monthly, or as determined by risk assessment), sampling from multiple points in the water system, trend analysis to detect gradual changes, and clear procedures for corrective action when results exceed limits. Staff training is critical – everyone involved must understand proper sampling techniques, the meaning of results, and the steps to take when something goes wrong.

What do you think? If a single coliform test can miss over 20% of fecally contaminated water samples, how should food processors decide which combination of tests to prioritise – and how often should they be running them to truly protect consumers?

References

1. https://pubmed.ncbi.nlm.nih.gov/38743188/
2. https://www.who.int/publications/i/item/9789240045064
3. https://www.eurofins.in/food-testing/blog/bacteriological-examination-of-water/
4. https://archive.epa.gov/water/archive/web/html/vms511.html
5. https://www.standardmethods.org/doi/abs/10.2105/SMWW.2882.188
6. https://en.wikipedia.org/wiki/Bacteriological_water_analysis
7. https://bio.libretexts.org/Bookshelves/Microbiology/Introductory_Bacteriology_Lab_Manual_(Turnbull)/01:_Labs/1.09:_Bacteriological_Analysis_of_Water
8. https://www.epa.gov/dwreginfo/revised-total-coliform-rule-and-total-coliform-rule
9. https://pubmed.ncbi.nlm.nih.gov/2141396/
10. https://link.springer.com/article/10.1007/BF00572392
11. https://www.iso.org/standard/14853.html
12. https://pmc.ncbi.nlm.nih.gov/articles/PMC7169830/