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Short answer: on a Gram stain, Listeria monocytogenes, the cause of listeriosis, is gram-positive: it stains purple and appears as small, regular rods. The CDC identification chart describes cerebrospinal fluid (CSF) smears showing polymorphonuclear leukocytes, monocytes and gram-positive rods that may be intracellular. The stain reports shape and colour only; naming the organism takes the catalase, hemolysis and motility results that follow it.
Definition and scope
A Gram stain sorts bacteria by cell-envelope structure. Gram-positive cells keep the crystal violet-iodine complex and look purple; gram-negative cells lose it at the decolorizing step and take up the red-pink safranin counterstain instead. The reagents, timings and reading of the method are covered in the CASRAI guide to Gram stain procedure, reagents and results, so they are not repeated here. This page covers one organism: what Listeria looks like on a smear and how far that finding can be pushed.
The CDC clinical overview calls Listeria a “rod-shaped, gram-positive bacterium” that can be readily isolated in standard bacterial culture of normally sterile body sites. It lists sepsis, meningitis and meningoencephalitis as the common clinical presentations, with the highest risk in older adults, people with weakened immune systems, and pregnant women and their newborns.
What does Listeria look like on a Gram stain?
Gram-positive, purple, and rod-shaped. The CDC Listeria monocytogenes identification chart describes “regular small Gram (+) rods,” which “may be short or longer.” In CSF the chart describes polymorphonuclear leukocytes (PMNs), monocytes and gram-positive rods, and notes the rods may be intracellular. The chart names CSF and blood as the only places this major pathogen is found among its specimen types.
How does a Gram stain work, and what is its purpose?
The purpose of a Gram stain is a fast first sort of a specimen or colony into gram-positive or gram-negative and by shape (rods, cocci). It works because the two cell-envelope types handle the crystal violet-iodine complex differently, as described above. A Gram stain test is therefore a screening observation, not an identification. For the bench method, including how to gram stain bacteria step by step, use the procedure guide; for why gram-positive and gram-negative walls differ, see cell wall examples.
Gram stain versus the tests that follow
The stain narrows the field to gram-positive rods. The CDC chart then separates Listeria with further tests, and each row below comes from that chart.
| Observation | What the CDC chart says | Why it matters |
|---|---|---|
| Gram stain | Regular small Gram (+) rods, short or longer | Places the isolate with gram-positive rods; does not name it |
| Catalase | Catalase (+) | The chart pairs it with the Gram stain to tell Listeria from group B streptococci |
| Colony appearance | Looks like group B streptococci | The chart says to differentiate by Gram stain and the positive catalase reaction |
| Hemolysis | Beta hemolysis that does not extend beyond the edge of the colony | If it extends beyond the colony, the chart says the organism is not Listeria |
| Motility | Tumbling motility at 26 °C; non-motile at 35 °C | Temperature-dependent motility is part of the identification pattern |
This cross-walk is the practical gain over a plain “it is gram-positive” answer: the colony resembles group B Streptococcus, and the Gram stain (rods, not cocci) plus catalase is what the chart uses to split the two.
Where it matters for labs and clinics
Specimens
The CDC chart frames Listeria as a major pathogen found in CSF and blood only, and the clinical overview says it is readily isolated by culture of normally sterile sites. It also says listeriosis cannot be detected by routine stool culture, so stool culture is not the route to this diagnosis.
Consumables the workflow touches
This is a checklist of the items the tests above imply, not a purchasing recommendation:
- Gram stain reagent set and glass slides, for the smear (see the procedure guide).
- Catalase reagent, for the catalase test.
- Blood agar, to observe the hemolysis and its extent relative to the colony edge.
- Motility test medium and incubation at both 26 °C and 35 °C, for the temperature comparison.
For Listeria surveillance on surfaces rather than patient specimens, see Listeria environmental monitoring. For another selective and differential medium used in bacteriology, see the bile esculin agar test, and for a medium that selects for gram-negative organisms, MacConkey agar.
Common misunderstandings
- “A gram-positive rod means listeriosis.” No. The stain reports morphology and colour; the CDC chart relies on catalase, hemolysis and motility to identify the organism.
- “Gram-positive means it stains pink.” Gram-positive cells are purple. Gram-negative cells stain red-pink.
- “Beta hemolysis alone settles it.” Per the chart, the zone must not extend beyond the colony edge; if it does, the organism is not Listeria.
- “Stool culture will find it.” The CDC states Listeria cannot be detected by routine stool culture.
FAQ
What is a Gram stain?
A staining method that sorts bacteria into gram-positive (purple) and gram-negative (red-pink) groups by cell-envelope structure, and shows their shape.
How to gram stain bacteria?
The sequence of crystal violet, iodine, decolorizer and counterstain is laid out in the CASRAI Gram stain procedure guide.
What is the purpose of a Gram stain?
A fast first sort by colour and shape. For Listeria, it points to gram-positive rods; the follow-on tests identify the organism.
What color does gram negative stain?
Red to pink, from the safranin counterstain. Listeria monocytogenes is gram-positive and does not stain this way.
What is a Gram stain test?
A smear-based screening observation made on a specimen or a colony. It is not a species-level identification.
How does Gram stain work?
Gram-positive cells retain the crystal violet-iodine complex after decolorizing; gram-negative cells lose it and take up the counterstain.
Sources
- CDC REACH, Listeria monocytogenes Identification Chart (job aid in the CDC course “Biochemicals and Gram-Positive Organism Identification”): https://reach.cdc.gov/sites/default/files/job-aids-resources/Listeria_monocytogenes_ID_Chart_Branded_508.pdf. Checked 2026-10-29.
- CDC, Clinical Overview of Listeria: https://www.cdc.gov/listeria/hcp/clinical-overview/index.html. Checked 2026-10-29.
