Neisseria

For medical students2 min readUpdated 2026-10-10

Neisseria is a large genus of aerobic Gram-negative bacteria that typically appear in pairs and have a characteristic bean-like shape. While most species are completely harmless and reside in our respiratory tracts, two species are strict human pathogens responsible for severe infections.

MorphologyGram-negative diplococci resembling coffee beans or kidney beans.

GeneticsGonococci and meningococci are genetically remarkably close, sharing approximately 70% DNA homology.

EnzymesAll members of the genus are strict aerobes and oxidase-positive.

SurvivalSuccessfully compete with human cells for iron by acquiring it from transferrin.

Taxonomy and Origin

The genus Neisseria belongs to the family Neisseriaceae. In addition to Neisseria, this family includes other medically important genera such as Kingella and Eikenella.

The genus is named after Albert Neisser, the prominent German physician and microbiologist. Currently, the genus includes over twenty different bacterial species. Based on human pathogenicity, they can be divided into two main groups:

Strict Pathogens: N. gonorrhoeae (gonococcus) and N. meningitidis (meningococcus).
Commensal/Opportunistic Species: N. lactamica, N. flavescens, N. sicca, and others. These microorganisms are natural inhabitants of mucous membranes and form part of the normal flora of the upper respiratory tract in healthy individuals.

Morphology and Staining Characteristics

The microscopic appearance of Neisseria is highly specific, aiding in rapid laboratory diagnosis.

Gram Stain: They are Gram-negative bacteria.
Arrangement: They are typical diplococci, meaning they always occur in pairs.
Cell Shape: The bacteria are bean-shaped or kidney-shaped.
Relative Alignment: Within each pair, the cocci face each other with their concave surfaces, making them resemble two coffee beans.
Motility: Neisseria lack flagella and are strictly non-motile.

Physiology and Cultural Characteristics

Culturing Neisseria in a laboratory requires special care because these bacteria are quite fastidious.

Respiration: Neisseria are strict aerobes (oxygen is essential for their survival).
Biochemistry: All species are oxidase-positive. Within the genus, species are differentiated based on specific biochemical and antigenic profiles.
Culture Media: The bacteria are highly demanding regarding growth conditions. Iron sources are mandatory for active replication.
Colony Morphology: On solid media, they form small, unpigmented, transparent colonies. When plated on blood agar, Neisseria do not produce zones of hemolysis (they do not lyse erythrocytes).

Pathogenicity and Survival Factors

Although gonococci and meningococci cause completely distinct clinical syndromes (gonorrhea and meningitis, respectively), they share an exceptionally high genetic relationship, with a DNA homology reaching 70%.

The primary survival factor for pathogenic Neisseria in the human body is their ability to scavenge iron. These bacteria engage in fierce competition with host cells for this micronutrient. Utilizing specific surface receptors, they bind to human transferrin and extract iron molecules required for their own metabolism and proliferation.

Mnemonic

Form mnemonic: "Two bean-shaped coffee beans without flagella sit face-to-face and steal iron." This reflects their pairing (diplococci), bean-like shape, concave surfaces, non-motility, and competition for transferrin.

Frequently asked questions

What enriched media are used for culturing pathogenic Neisseria?

Culturing pathogenic Neisseria requires media enriched with human or animal proteins, amino acids, and iron sources.

Blood agar — a solid enriched medium containing 5% blood.
Serum agar — solid and semi-solid (0.1%) media.
Serum broth — a liquid enriched medium.
Thioglycolate medium — used as part of enriched media sets.
Amino acid-supplemented media — used for the isolation of gonococci.

What virulence factors are characteristic of Neisseria meningitidis?

Neisseria meningitidis possesses a complex array of structural components and enzymes that act as virulence factors.

Lipooligosaccharide (LOS) — the main pathogenicity factor functioning as a highly toxic endotoxin.
Polysaccharide capsule — protects the bacterium from phagocytosis and complement-mediated bacteriolysis.
Type IV pili — mediate adhesion to epithelial and endothelial cells.
Outer membrane proteins — involved in attachment, invasion, and iron metabolism.
Aggressive enzymes — include hyaluronidase, neuraminidase, fibrinolysin, and IgA protease, which cleaves secretory immunoglobulin A.

Which Neisseria species are obligate human pathogens?

Only two species are strict pathogens: Neisseria gonorrhoeae (the causative agent of gonorrhea) and Neisseria meningitidis (the causative agent of meningococcal disease).

What do Neisseria look like under a microscope in a clinical smear?

They appear as Gram-negative, bean-shaped diplococci with their concave sides facing each other.

Do Neisseria cause hemolysis on blood agar?

No, when grown on blood agar, Neisseria do not form zones of hemolysis.

How do pathogenic Neisseria acquire iron within the host?

They utilize specialized surface receptors to compete with human cells for transferrin, extracting iron from it.

Go deeper

Detailed antigenic structure of meningococci and gonococci
Biochemical differentiation of species within the genus Neisseria
Structure and function of surface transferrin receptors
Role of commensal species (N. lactamica, N. sicca) in the microbiome