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Short answer
Salmonella is a Gram-negative rod, so on a Gram stain it appears pink to red rather than purple. The stain only places an organism in a broad group. It does not name the genus, let alone one of the 2,500-plus serovars, and for diagnosis CDC calls culture the gold standard.
What is a Gram stain, and what is its purpose?
The Gram stain is a differential staining test: it separates bacteria into two groups according to how their cell walls handle a sequence of dyes. OpenStax Microbiology describes it as a method developed by the Danish microbiologist Hans Christian Gram in 1884 that is still among the most frequently used staining techniques. Its purpose is to sort bacteria by cell-wall type and show their shape under the microscope, which narrows what a laboratory looks for next. See the differential staining guide for the wider family of stains.
How does a Gram stain work?
A heat-fixed smear is treated with crystal violet (the primary stain), then Gram’s iodine (a mordant that locks the dye into the cell wall), then a decolorizer such as ethanol, and finally a counterstain, usually safranin. Cells with thick peptidoglycan layers resist the decolorizer and stay purple. Cells with thinner peptidoglycan lose the crystal violet, and the counterstain then colors them pink (OpenStax Microbiology, section 2.4). The reagent sequence is the concept; the bench protocol, timings and reagent preparation are covered in the Gram stain procedure, reagents and results guide.
What color does Gram-negative stain?
Gram-negative cells appear pink to red after the counterstain, while Gram-positive cells stay purple. OpenStax describes the Gram-negative cells as the red, safranin-dyed ones. WHO’s meningitis manual names safranin or carbol-fuchsin as the counterstain in its Hucker-modification method.
Why Salmonella reads as Gram-negative rods
The World Health Organization fact sheet on non-typhoidal Salmonella states that Salmonella is a genus of Gram-negative rods belonging to the Enterobacteriaceae family, with more than 2,500 serotypes identified to date. A correctly stained smear of Salmonella therefore shows pink to red rod-shaped cells. This is the answer to the common phrasings “salmonella on gram stain” and “gram stain of salmonella”: Gram-negative, rod-shaped, pink to red.
What a Gram stain cannot tell you about Salmonella
The stain reports cell-wall type and shape, and both are shared across many organisms. WHO’s laboratory manual for meningitis shows the same principle: Neisseria meningitidis, Haemophilus influenzae and Streptococcus pneumoniae are separated by shape and by Gram reaction together, not by color alone.
| Organism | Gram reaction and shape | Source |
|---|---|---|
| Salmonella | Gram-negative rods | WHO, Salmonella (non-typhoidal) fact sheet |
| N. meningitidis | Gram-negative, coffee-bean-shaped diplococci | WHO meningitis laboratory manual |
| H. influenzae | Small, pleomorphic Gram-negative rods or coccobacilli | WHO meningitis laboratory manual |
| S. pneumoniae | Lanceolate, Gram-positive diplococci | WHO meningitis laboratory manual |
Two different Gram-negative rods look alike on a stained slide, so a pink rod is a lead, not an identification. CDC’s clinical overview of salmonellosis recommends laboratory testing, particularly culture, as the basis for diagnosis, notes that when a culture-independent test such as a PCR multiplex panel identifies Salmonella a follow-up (reflex) culture is recommended, and states that serologic testing is unreliable and not advised. Confirmed cases are reportable in the United States.
Gram stain versus culture: where each fits
The stain is fast and shows morphology; culture isolates the organism so it can be identified. In the WHO manual Basic laboratory procedures in clinical bacteriology, Salmonella and Shigella are told apart on selective and differential media by colony appearance, which no stain can show:
- MacConkey agar: lactose-nonfermenting bacteria such as Salmonella and Shigella give small colourless colonies, while lactose fermenters such as E. coli and Enterobacter/Klebsiella produce pink to red colonies. See the MacConkey agar guide.
- XLD agar: Shigella and Salmonella produce small red colonies, and most Salmonella strains have a black centre. See the XLD agar guide.
- Kligler’s iron agar: one of three differential media WHO recommends for initial screening of Salmonella and Shigella isolates; the related tube reactions are covered in the triple sugar iron agar results guide.
Where it matters for labs
For a teaching or clinical laboratory, the practical point is the order of evidence. The Gram reaction and cell shape are the first observation, selective and differential plates are the next, and culture-based identification confirms the genus, in line with CDC’s description of culture as the gold standard. The same order applies to other organisms; the Haemophilus influenzae Gram stain guide and the Bacillus subtilis Gram stain guide show how the stain pairs with other findings for those organisms.
Common misunderstandings
- “Pink means Salmonella.” Pink means Gram-negative. Many bacteria share that result.
- “A stain result is a diagnosis.” CDC points to culture, with reflex culture after a culture-independent test, as the route to diagnosis.
- “Every smear is reliable.” OpenStax notes that older cells with damaged walls can look Gram-negative even when the species is Gram-positive, and that leaving the decolorizer on too long can change the result, so fresh cultures and careful technique matter.
Frequently asked questions
What is a Gram stain?
A differential staining technique that separates bacteria into Gram-positive (purple) and Gram-negative (pink to red) groups based on cell-wall structure.
How to Gram stain bacteria?
The method applies crystal violet, Gram’s iodine, a decolorizer and a counterstain such as safranin to a heat-fixed smear. Reagent details and timing are in the procedure guide.
What is the purpose of a Gram stain?
To classify bacteria by cell-wall type and show their shape under the microscope, so that the laboratory can choose the next test.
What color does Gram-negative stain?
Pink to red, the color of the safranin counterstain. Gram-positive cells stay purple.
What is a Gram stain test?
A microscopy test on a stained smear of a specimen or culture. It reports Gram reaction and morphology, not a species name.
Is Salmonella Gram-positive or Gram-negative?
Gram-negative. WHO describes Salmonella as a genus of Gram-negative rods in the Enterobacteriaceae.
Sources
- World Health Organization. Salmonella (non-typhoidal) fact sheet. Read 2026-10-29.
- Centers for Disease Control and Prevention. Clinical Overview of Salmonellosis. Read 2026-10-29.
- OpenStax. Microbiology, 2.4 Staining Microscopic Specimens. Read 2026-10-29.
- WHO. Laboratory Methods for the Diagnosis of Meningitis Caused by Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae (WHO/CDS/CSR/EDC/99.7). Read 2026-10-29.
- WHO. Basic laboratory procedures in clinical bacteriology (Vandepitte et al., 2nd ed., 2003), WHO IRIS handle 10665/42696. Read 2026-10-29.
