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Short answer: On a Gram stain of cerebrospinal fluid (CSF), Neisseria meningitidis appears as gram-negative (red to pink), coffee-bean shaped diplococci that may sit inside or outside polymorphonuclear (PMN) leukocytes. The smear supports a presumptive result for clinical decision-making; the isolate is confirmed by culture, an oxidase test and carbohydrate utilization.
What N. meningitidis looks like on a Gram stain
The WHO/CDC laboratory manual for meningitis diagnosis states that N. meningitidis “may occur intracellularly or extracellularly in PMN leukocytes and will appear as gram-negative, coffee-bean shaped diplococci.” Gram-positive organisms stain dark violet or purple and gram-negative organisms stain red or pink from the counterstain, read under 100X oil immersion.
The same manual describes Streptococcus pneumoniae as gram-positive, lanceolate diplococci that sometimes form short chains, so colour and shape together separate it from the meningococcus on the smear. For the other gram-negative organism in the manual’s three-pathogen set, see the H. influenzae Gram stain guide; the general reagent set is covered in the Gram stain reagents and results guide.
How the CSF smear is used in the laboratory workflow
According to the manual, a presumptive diagnosis of bacterial meningitis caused by N. meningitidis, S. pneumoniae or H. influenzae can be made after a Gram stain of the CSF sediment, or by detecting specific antigens with latex agglutination or rapid diagnostic tests, in combination with a consistent clinical picture and CSF examination. Positive results “can rapidly provide evidence of infection, even if cultures fail to grow.”
The pre-stain handling points the manual lists, which is where many mis-readings begin:
- Volume decides the path. If less than 1 ml of CSF is available it is not centrifuged; it is plated directly on blood agar (BAP) and chocolate agar (CAP) and also used for the Gram stain. Above 1 ml, the CSF is centrifuged, typically at 1000 x g for 10-15 minutes.
- Cytology first. The cell examination should precede centrifugation and heating of the CSF.
- Smear thickness. The manual calls for a monolayer of organisms dense enough for easy visualization but thin enough to reveal morphology.
- Controls run alongside. Positive and negative quality-control strains are stained with the specimen (the manual lists S. aureus for gram-positive cocci and E. coli for gram-negative rods among the reference strains that may be used).
- Decolorization is a failure point. Gram-positive bacteria can be made to appear gram-negative by over-decolorization, and gram-negative bacteria can appear gram-positive by under-decolorization.
Typical bacterial-meningitis CSF findings the manual lists include turbidity, opening pressure above 180 mm water, pleocytosis (usually PMN-predominant) with WBC above 10 cells/mm3, glucose below 45 mg/dl and protein above 45 mg/dl. The Gram stain is read together with these, not in place of them.
Smear versus culture: what each can and cannot settle
| Step | What it shows | What it does not settle |
|---|---|---|
| Gram stain of CSF sediment | Gram-negative diplococci are read as N. meningitidis in the manual’s flow chart; other morphology or staining means “not N. meningitidis“ | Species and serogroup; the manual places the stain at the “clinical decision-making” step, ahead of identification |
| Growth on BAP and CAP | Round, smooth, moist, glistening, convex colonies; grey and unpigmented on BAP, large colorless-to-grey and opaque on CAP, at 35-37 C with about 5% CO2, tested at 18-24 h growth | Identity on appearance alone |
| Kovac oxidase test | A negative result means not N. meningitidis | A positive result does not confirm it: other Neisseria and unrelated bacteria can also be oxidase positive |
| CTA sugar test | Acid from glucose and maltose but not from sucrose or lactose identifies N. meningitidis | Serogroup |
| Slide agglutination serogrouping | Serogroup, the final step of the manual’s sequence | Not a first-line test; the sequence is described as an efficient way to save costly antisera and time |
The manual also directs molecular methods (PCR) to its separate chapters for identification and characterization. CDC clinical guidance for meningococcal disease notes that culture and nucleic acid amplification testing each have advantages and disadvantages, and that serogroup information informs the public health response and helps identify outbreaks.
Where it matters for labs and clinics
- Reagents and media named by the workflow. The manual’s Gram stain uses crystal violet with ammonium oxalate, Gram iodine, 95% ethanol decolorizer, and safranin or carbol-fuchsin counterstain, with 95% methanol as fixative; culture uses blood and chocolate agar plates.
- Quality control. Staining known gram-positive and gram-negative control strains with each run is the manual’s check against the decolorization errors above. General quality-system expectations are covered in the CLIA quality control guide.
- Biosafety. The manual requires BSL-2 practices for handling isolates; see biosafety level 2 requirements.
Common misunderstandings
- “Any gram-negative diplococcus is the meningococcus.” The flow chart treats the smear as the clinical decision-making step; identification continues with culture, oxidase and sugar tests.
- “Oxidase positive means N. meningitidis.” The manual states the oxidase test aids recognition, but other Neisseria species and unrelated bacteria can also react.
- “The cells are always intracellular.” The manual says the organisms may be intracellular or extracellular in PMN leukocytes, so a smear is read across fields, not only inside PMNs.
- “The Gram reaction cannot be wrong.” The manual warns that over- or under-decolorization can flip the apparent Gram reaction, which is why control strains are stained alongside.
FAQ
What does N. meningitidis look like on a Gram stain?
Gram-negative (red or pink), coffee-bean shaped diplococci that may lie inside or outside PMN leukocytes, read under 100X oil immersion (WHO/CDC manual).
Does a gram-negative diplococcus on the CSF smear confirm N. meningitidis?
No. In the manual’s flow chart the smear supports clinical decision-making; the isolate is then identified by colony appearance, the Kovac oxidase test, CTA sugar testing and serogrouping.
Is a positive oxidase test enough to identify N. meningitidis?
No. The manual notes that other Neisseria species and unrelated bacteria can also give a positive oxidase reaction, so carbohydrate utilization testing follows.
Can the Gram stain give a wrong Gram reaction?
Yes. The manual warns that over-decolorization can make gram-positive bacteria look gram-negative and under-decolorization can make gram-negative bacteria look gram-positive, which is why control strains are run with the specimen.
What biosafety level applies to N. meningitidis isolates?
The manual requires Biosafety Level 2 (BSL-2) practices for work with isolates.
Sources
- WHO/CDC, Laboratory Methods for the Diagnosis of Meningitis Caused by Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae, 2nd edition, 2011 (CDC Stacks): https://stacks.cdc.gov/view/cdc/11632. Checked 29 October 2026.
- CDC, Meningococcal disease clinical guidance for healthcare providers: https://www.cdc.gov/meningococcal/hcp/clinical-guidance/index.html. Checked 29 October 2026.
This page is general laboratory and teaching information, not medical advice and not a bench procedure.
