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.

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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.

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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

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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