Taxonomy and origin
The genus Neisseria belongs to the family Neisseriaceae. In addition to Neisseria itself, this family includes other medically significant genera such as Kingella and Eikenella.
The genus was named after the prominent German physician and microbiologist Albert Neisser. Today, the genus includes over twenty different species of bacteria. Based on their hazard level to humans, they can be divided into two large groups:
- Strict pathogens: N. gonorrhoeae (gonococcus) and N. meningitidis (meningococcus).
- Opportunistic species: N. lactamica, N. flavescens, N. sicca, and others. These microorganisms are natural inhabitants of mucous membranes and are part of the normal microflora of the upper respiratory tract in healthy humans.
Morphology and staining properties
The microscopic appearance of Neisseria is highly specific, which aids in rapid laboratory diagnosis.
- Gram stain reaction: 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.
- Mutual arrangement: In a pair, the cocci face each other with their concave surfaces, making them look like two coffee beans.
- Motility: Neisseria lack flagella, so all members of the genus are completely non-motile.
Physiology and cultural properties
Cultivating Neisseria in laboratory conditions requires special attention, as these bacteria are quite fastidious.
- Respiration: Neisseria are strict aerobes (they strictly require oxygen).
- Biochemistry: All species are oxidase-positive. Within the genus, bacteria are differentiated based on specific biochemical and antigenic features.
- Culture media: The bacteria are extremely demanding regarding cultivation conditions. Sources of iron are essential for their active proliferation.
- Growth characteristics: On solid nutrient media, they form small, non-pigmented, and transparent colonies. When inoculated on blood agar, Neisseria do not produce zones of hemolysis (they do not lyse erythrocytes).
Pathogenicity and survival factors
Although gonococci and meningococci cause clinically completely different diseases (gonorrhea and meningitis, respectively), they share an enormous genetic relationship. The degree of their DNA homology reaches 70%.
The main factor ensuring the survival of pathogenic Neisseria in the human body is their ability to acquire iron. The bacteria engage in fierce competition with host cells for this trace element. Using specific surface receptors, they can bind to the human protein transferrin and "steal" iron molecules necessary for their own metabolism and reproduction.
Briefly: answers to topical questions
What selective media are used for the cultivation of pathogenic Neisseria?
Nutrient media enriched with human or animal proteins, amino acids, and iron sources are used for the cultivation of pathogenic Neisseria.
- Blood agar — a solid enriched medium containing 5% blood.
- Serum agar — solid and semisolid (0.1%) media.
- Serum broth — a liquid enriched medium.
- Thioglycollate medium — used as part of a set of enriched media.
- Amino acid-supplemented media — used for the bacteriological inoculation of gonococci.
What virulence factors are characteristic of Neisseria meningitidis?
Neisseria meningitidis is characterized by a complex of structural components and enzymes that act as pathogenicity factors.
- Lipooligosaccharide (LOS) — the main pathogenicity factor, acting as an endotoxin with high toxicity.
- Polysaccharide capsule — protects the bacterium from phagocytosis and complement-mediated bacteriolysis.
- Type IV pili — provide adhesion to epithelial and endothelial cells.
- Outer membrane proteins — participate in attachment, invasion, and iron metabolism.
- Aggressive enzymes — include hyaluronidase, neuraminidase, fibrinolysin, as well as IgA protease, which cleaves secretory immunoglobulin A.
It is easy to remember the shape: "Two beans without flagella sit face to face and steal iron." This reflects the paired arrangement (diplococci), bean-like shape, concave surfaces, non-motility, and competition for transferrin.
Class and exam questions
Which species of Neisseria are strictly pathogenic to humans?
Only two species are strict pathogens: Neisseria gonorrhoeae (the causative agent of gonorrhea) and Neisseria meningitidis (the causative agent of meningococcal infection).
How do Neisseria appear in a microscopic smear?
They appear as Gram-negative (failing to retain the purple Gram stain) bean-shaped diplococci turned toward each other with their concave sides.
Are Neisseria capable of destroying red blood cells on blood agar?
No, when growing on blood agar, Neisseria do not form zones of hemolysis.
How do pathogenic Neisseria acquire iron in the host organism?
They use special surface receptors to compete with human cells for the transferrin protein, extracting iron from it.
What else to study on this topic
- Detailed antigenic structure of meningococci and gonococci
- Biochemical differentiation of species within the genus Neisseria
- Structure and functions of surface transferrin receptors
- Role of opportunistic species (N. lactamica, N. sicca) in microflora
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