Understanding what water quality tests detect, what they mean, how the results are calculated, and what regulatory limits apply is practical knowledge for environmental microbiologists, public health scientists, food industry microbiologists, pharmaceutical quality control staff, and infection prevention and control teams dealing with waterborne HAI risks (including Legionella).
This page covers the core indicator organism concept, the standard methods for testing drinking water and recreational water, Legionella risk assessment and testing, pharmaceutical water quality requirements, and the calculation of most probable number (MPN) from membrane filtration colony counts.
The Indicator Organism Concept
Testing water directly for every possible pathogen (Salmonella, Vibrio cholerae, Cryptosporidium, norovirus, Hepatitis A virus, and dozens of others) is technically complex, time-consuming, and expensive. The practical solution is the indicator organism concept: test for organisms whose presence correlates with faecal contamination and therefore with the likely presence of faecal pathogens.
Total coliforms are gram-negative, non-spore-forming, facultative anaerobic rods that ferment lactose at 35 degrees Celsius within 48 hours, producing acid and gas. They are found in the intestinal tract of warm-blooded animals and extensively in soil and vegetation. Total coliforms indicate the general microbiological status of water and potential contamination during treatment or distribution, but their presence does not necessarily indicate recent faecal contamination.
Thermotolerant (faecal) coliforms are a subset of total coliforms that ferment lactose at 44.5 degrees Celsius. Escherichia coli is the primary thermotolerant coliform and the most specific indicator of recent faecal contamination. The distinction between "E. coli" (specific faecal indicator) and "total coliforms" (general indicator including environmental organisms) is critical for interpreting results.
WHO Guidelines for Drinking-water Quality specify that E. coli or thermotolerant coliforms must not be detectable in any 100 mL sample of treated drinking water. Total coliforms must also not be detectable in any 100 mL sample. These are the standard limits for water entering distribution.
Enterococcus: faecal streptococci (Enterococcus faecalis, E. faecium) are also indicators of faecal contamination and are used alongside E. coli for monitoring bathing water and for verifying water treatment. Enterococci are more resistant to chlorination than coliforms and are considered a more conservative indicator of recent faecal pollution.
Membrane Filtration: The Standard Method
The membrane filtration method is the standard quantitative approach for testing drinking water and treated wastewater for E. coli and total coliforms. It is the basis for the WHO and most national regulatory testing protocols.
Procedure: a measured volume of water sample (100 mL is standard for drinking water) is drawn through a cellulose acetate or polycarbonate membrane filter (0.45 micrometer pore size) by vacuum or pressure. All bacteria present in the sample are retained on the membrane surface. The membrane is then placed on a selective and differential culture medium (for total coliforms and E. coli: lactose-containing chromogenic agar, or the traditional tryptose sulphate cycloserine (TSC) medium or lauryl sulphate broth) and incubated for 18 to 24 hours.
Colonies appearing on the membrane are counted and the presence of E. coli-specific chromogenic colonies (typically blue-green or blue on chromogenic media) confirms E. coli presence. Results are expressed as colony-forming units (CFU) per 100 mL. A result of 0 CFU/100 mL (for E. coli) is required for drinking water compliance.
Most Probable Number (MPN): The Statistical Alternative
For turbid water samples that cannot be filtered (because suspended particulates block the membrane), the most probable number (MPN) method is used. MPN is a statistical estimate of bacterial density derived from a series of dilution tubes inoculated with the water sample.
The standard MPN method uses a 3-tube or 5-tube series at each of 3 dilutions (for example, 10 mL, 1 mL, and 0.1 mL of sample inoculated into tubes of lauryl sulphate broth). After incubation, gas production (for coliforms) or the colour change/fluorescence (for E. coli on selective media) indicates a positive tube at each dilution level. The combination of positive and negative tubes across the dilution series is looked up in an MPN table (or calculated using the statistical formula) to give the MPN per 100 mL.
Example: in a 3-tube series at 10/1/0.1 mL dilutions, if 3/3/1 tubes are positive (all three 10 mL, all three 1 mL, one of three 0.1 mL), the MPN table gives approximately 430 MPN per 100 mL.
Legionella in Water Systems: The Healthcare Risk
Legionella pneumophila (serogroup 1 in particular) causes Legionnaires' disease: a severe pneumonia with case fatality rates of 10 to 15 per cent in hospitalised patients. The organism thrives in warm water systems (optimal growth at 30 to 45 degrees Celsius) in biofilms within water distribution pipework, cooling towers, showerheads, taps, and any water system with stagnation or scale and sediment build-up.
Infection occurs through inhalation of aerosol droplets containing Legionella, generated by showers, cooling towers, air conditioning systems, hydrotherapy pools, and decorative fountains. Person-to-person transmission does not occur.
Legionella risk is controlled through: maintaining hot water above 60 degrees Celsius at the calorifier and delivering above 50 degrees Celsius at outlets, maintaining cold water below 20 degrees Celsius, flushing little-used outlets weekly, descaling and disinfecting showerheads and taps regularly, and thermal or biocide disinfection of cooling towers.
Microbiological monitoring: routine quantitative Legionella culture is performed on water samples from cooling towers, hot water systems, and other high-risk outlets. Results above 100 CFU/L require investigation and remediation. Results above 1,000 CFU/L represent a significant risk and require immediate remedial action and enhanced disinfection.
Pharmaceutical Water Quality
Pharmaceutical manufacturing requires water of defined microbiological quality for different applications. USP, EP, and other pharmacopoeias specify limits:
Purified Water (PW): total aerobic microbial count (TAMC) alert limit 10 CFU/mL. Used for non-sterile pharmaceutical manufacturing.
Water for Injections (WFI): must be produced by distillation or reverse osmosis combined with other purification steps. TAMC should be not more than 10 CFU/100 mL. Endotoxin limit: not more than 0.25 EU/mL (endotoxin is the primary clinical risk from microbially contaminated WFI used in injections).
Highly purified water (HPW): intermediate between PW and WFI, used in some specific applications.
Endotoxin testing for pharmaceutical water uses the Limulus Amoebocyte Lysate (LAL) test or the recombinant Factor C (rFC) assay, which quantify bacterial endotoxin (LPS) at picogram levels. Endotoxin, not the live bacteria themselves, causes the pyrogenic reaction in patients receiving contaminated injectable products.
Frequently Asked Questions
What is an indicator organism in water testing?
An indicator organism is a microorganism whose presence in water indicates the likely presence of faecal contamination and therefore potential faecal pathogens. The ideal indicator is consistently present in faecal matter, easy to detect by standard methods, absent from uncontaminated water, and not pathogenic itself. E. coli is the gold-standard indicator of recent faecal contamination.
What is the regulatory limit for E. coli in drinking water?
The WHO Guideline and most national drinking water standards require E. coli (or thermotolerant coliforms) to be undetectable (0 CFU/100 mL) in any 100 mL sample of treated drinking water entering the distribution system. In the EU, Drinking Water Directive 2020/2184 sets the same standard.
What is membrane filtration?
Membrane filtration is the standard method for testing water for E. coli and total coliforms. A measured water volume (typically 100 mL) is filtered through a 0.45 micrometer membrane, retaining all bacteria on the surface. The membrane is incubated on selective chromogenic agar. E. coli colonies produce a specific colour (typically blue or blue-green) and are counted directly.
What is MPN?
Most Probable Number (MPN) is a statistical estimate of bacterial density in water, derived from the pattern of positive and negative results across a series of dilution tubes inoculated with the water sample. It is used for turbid samples that cannot be filtered and produces a probabilistic estimate rather than a direct count.
What Legionella level triggers action in a water system?
ACOP L8 (UK Approved Code of Practice) and HSE guidance: Legionella counts above 100 CFU/L trigger investigation and re-sampling. Counts above 1,000 CFU/L represent a significant risk requiring immediate remedial action, enhanced disinfection (thermal flush or biocide treatment), and increased monitoring. Counts above 10,000 CFU/L require immediate precautionary closure of the implicated system pending disinfection.
What temperature controls Legionella in hot water systems?
Legionella pneumophila multiplies at 20 to 45 degrees Celsius (optimal around 37 degrees Celsius) and is killed at 60 degrees Celsius within 2 minutes, at 55 degrees Celsius within 5 to 6 hours. Hot water calorifiers must store water at a minimum of 60 degrees Celsius, and outlets must deliver water at above 50 degrees Celsius within 1 minute of opening. Cold water must be kept below 20 degrees Celsius.
What is Water for Injections (WFI)?
WFI is the highest purity pharmaceutical grade water, used for preparing injectable pharmaceutical products, IV infusions, and for dissolving or diluting injectable drugs. It is produced by distillation or (in the EU since 2017) membrane-based purification methods. USP and EP require a TAMC of not more than 10 CFU/100 mL and endotoxin not more than 0.25 EU/mL.
What is the LAL test?
The Limulus Amoebocyte Lysate (LAL) test detects bacterial endotoxin (LPS) using a coagulation cascade from the blood cells (amoebocytes) of the horseshoe crab (Limulus polyphemus). LPS from gram-negative bacteria activates the cascade, causing turbidity or gel formation proportional to LPS concentration. The test detects endotoxin at picogram-per-mL levels. It is mandatory for testing injectable pharmaceuticals and medical devices for pyrogen safety.
What is heterotrophic plate count (HPC)?
HPC measures the total number of aerobic, heterotrophic bacteria in water by plating on non-selective media and counting all colonies. HPC is not a safety indicator (HPC organisms are mostly environmental, non-pathogenic bacteria) but is used to monitor water treatment efficiency and distribution system integrity. An elevated HPC in a treated water system indicates inadequate disinfection, stagnation, or contamination during distribution.
What is recreational water quality testing?
Recreational water (bathing beaches, inland waters, swimming pools) is monitored for E. coli and intestinal enterococci to assess faecal contamination risk. EU Bathing Water Directive 2006/7/EC classifies bathing water quality as Excellent, Good, Sufficient, or Poor based on E. coli and enterococcal counts measured over four consecutive seasons. Poor quality bathing water should trigger investigation of pollution sources and management measures.