Salt Tolerance Test (6.5% NaCl)
A bench guide to the 6.5% NaCl salt tolerance test, used to differentiate salt-tolerant Enterococci (positive) from salt-sensitive nonenterococci (negative).
Salt tolerance tubes A-B
Organisms are inoculated into brain-heart infusion broth with 6.5% NaCl. Growth (turbidity) and acid (yellow) are positive.
Heavy visible growth (turbidity) accompanied by a color shift from purple to yellow, indicating active glucose fermentation in 6.5% NaCl.
No growth (completely clear broth) and no color change (stays deep purple/magenta). The high salt concentration completely inhibited growth.
Heavy visible growth (turbidity) accompanied by a color shift from purple to yellow, indicating active glucose fermentation in 6.5% NaCl.
No growth (completely clear broth) and no color change (stays deep purple/magenta). The high salt concentration completely inhibited growth.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Visible Turbidity / Cloudiness | Growth of the bacteria in high salt concentration | Primary indicator of positivity |
| Yellow color shift | Acid produced from glucose fermentation in the BHI base | Confirms active metabolic growth |
| Clear Purple broth | No bacterial growth; high salt fully inhibited the organism | Negative reaction |
Salt Tolerance Interpretive Matrix
| Pattern | What to call it | How to think about it |
|---|---|---|
| Turbid broth (with or without yellow) | Positive | Salt tolerant. Differentiates Enterococci (positive) from Group D Streptococci. |
| Clear broth (no turbidity, stays purple) | Negative | Salt sensitive. Typical for Streptococcus bovis, Streptococcus pneumoniae. |
- The 6.5% NaCl broth is both selective (inhibiting most Gram-positive cocci) and differential (detecting glucose fermenters).
- Bromcresol purple is the pH indicator: purple is neutral (negative), yellow is acid (positive).
- Enterococci are highly resistant to high salt concentrations, making this a cornerstone test.
Salt tolerance is about growth. Clear purple = Negative. Turbid (usually yellow) = Positive (Enterococci).