Citrate Utilization (Simmons Citrate)
An original Simmons Citrate slant guide for reading alkalinization in Enterobacterales and other Gram-negative rod profiles.
Bench card visual
Simmons Citrate slants A-B
Read first
Organisms capable of utilizing sodium citrate grow and shift the pH indicator to deep royal blue.
A
PositiveEnterobacter aerogenesGrowth with a deep royal blue color shift throughout the medium.
B
NegativeEscherichia coliNo growth; medium remains its original forest green color.
PositiveEnterobacter aerogenes
Growth with a deep royal blue color shift throughout the medium.
NegativeEscherichia coli
No growth; medium remains its original forest green color.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Royal Blue Medium | Alkaline products shift bromothymol blue indicator at alkaline pH (>7.6) | Shorthand: Positive (+) |
| Visible Growth | Survival and replication on citrate slant | Can be positive even with trace green |
| Forest Green Medium | Medium remains unshifted (neutral pH) | Shorthand: Negative (-) |
Citrate study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Positive | Growth with royal blue color shift | Presumptive Klebsiella, Enterobacter, Serratia, Citrobacter, or Salmonella species. |
| Negative | No growth and unchanged forest green medium | Presumptive Escherichia coli, Shigella, Morganella, or Yersinia species. |
| Trace Positive | Growth on slant without intense blue color | Interpret as positive if clear growth is visible; check controls. |
Key takeaways
- Key component of the classic IMViC profile (Indole, Methyl Red, Voges-Proskauer, Citrate).
- E. coli is classically Citrate negative; Klebsiella pneumoniae is Citrate positive.
- Differentiates closely related Gram-negative rods in fecal workups.
Remember
E. coli is IMViC (++--); Klebsiella pneumoniae is IMViC (--++).