TBC (Total Bacterial Count) and Legionella testing are two different types of microbiological water tests, each serving a different purpose.
1. TBC (Total Bacterial Count)
Also called:
- Total Viable Count (TVC)
- Heterotrophic Plate Count (HPC)
Purpose:
Measures the total number of viable bacteria present in a water sample. It does not identify specific bacteria or indicate whether they are harmful.
Common uses:
- Drinking water quality monitoring
- Process water testing
- Cooling towers
- Hospital water systems
- Food and beverage industries
Typical method:
- Plate Count Agar (PCA) or similar media
- Incubation at specified temperatures (commonly 22°C and/or 36–37°C) for defined periods
- Results reported as CFU/mL (Colony Forming Units per milliliter)
Interpretation:
- Low counts generally indicate good microbiological quality.
- High counts may suggest poor sanitation, biofilm formation, or stagnation, but they do not necessarily indicate the presence of pathogens.
2. Legionella Testing
Purpose:
Detects and quantifies bacteria of the genus Legionella, particularly Legionella pneumophila, which can cause Legionnaires’ disease.
Common sources tested:
- Cooling towers
- Hot and cold water systems
- Hospitals
- Hotels
- Spa pools
- Decorative fountains
Testing methods:
| Method | Detects | Time to Result |
|---|---|---|
| Culture (ISO 11731) | Live Legionella | 7–10 days |
| qPCR | Legionella DNA | Few hours to 1 day |
| Rapid antigen tests | Specific targets | Minutes to hours (limited use) |
Results:
- Reported as CFU/L for culture methods.
- qPCR results are typically reported as genome copies per liter or similar molecular units, depending on the laboratory.
Comparison
| Feature | TBC | Legionella |
|---|---|---|
| Measures | Total viable bacteria | Specific Legionella bacteria |
| Specificity | Non-specific | Highly specific |
| Indicates pathogens? | No | Yes |
| Typical unit | CFU/mL | CFU/L |
| Main purpose | General water hygiene | Public health risk assessment |
Why both tests are often performed
A water system can have:
- Low TBC but detectable Legionella, or
- High TBC but no Legionella.
Because of this, TBC cannot be used to confirm or rule out the presence of Legionella. For systems with a risk of aerosol generation (such as cooling towers, healthcare facilities, hotels, and large building water systems), Legionella testing should be performed separately as part of a water safety management program.
If you’re working with a particular standard (such as ISO, WHO guidance, ASHRAE, or local regulations), the testing frequency and action limits may differ. I can also explain the relevant requirements for those standards.
Interested in Learning more about water testing, Visit our dedicated water testing services page: WATER TESTING
