Neisseria meningitidis, commonly known as meningococcus, is a bacterium that can cause meningitis and septicemia. This gram-negative bacterium is not only a significant human pathogen but also a challenging microorganism to identify using standard laboratory methods. One of the most common questions asked by researchers and students of microbiology is whether Neisseria meningitidis is gram-positive or gram-negative. In this article, we’ll unravel the mystery and explore the gram staining characteristics of this bacterium.
What is the Gram Stain?
The Gram stain is a differential staining technique that is widely used to differentiate bacterial species into two major groups: gram-positive and gram-negative. It is a crucial test that helps microbiologists to identify and classify bacteria based on their cell wall structure. This staining technique is based on the ability of the cell wall to retain crystal violet dye, which is followed by counterstaining with safranin.
Principle of Gram Stain
The ability of a bacterium to retain the crystal violet dye during the staining process is primarily dependent on the composition of its cell wall. Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet stain, while gram-negative bacteria have a thin peptidoglycan layer and an outer membrane that makes them lose the crystal violet stain but take up the safranin stain.
Characteristics of Neisseria meningitidis
Neisseria meningitidis is a gram-negative, aerobic, non-motile, oxidase-positive bacterium that typically appears as a diplococcus. It is a part of the normal human pharyngeal flora and can cause invasive disease in susceptible individuals.
Cell Wall Structure of Neisseria meningitidis
The cell wall of Neisseria meningitidis is composed of an outer membrane, containing lipopolysaccharides, porins, and proteins, which is separated from the thin peptidoglycan layer by the periplasmic space. The membrane contains several virulence factors, including pili, which are responsible for the bacterium’s attachment to host cells.
Gram Stain Characteristics of Neisseria meningitidis
As mentioned earlier, Neisseria meningitidis is a gram-negative bacterium, which means it loses the crystal violet stain but takes up the safranin stain during the Gram staining process. The cells appear red or pink when viewed under a microscope, indicating that they are gram-negative.
Identification of Neisseria meningitidis
The Gram stain result is just the first step in identifying Neisseria meningitidis in the laboratory. Several other tests are needed to confirm the identification of this bacterium, including:
- Oxidase test
- Catalase test
- API-NH system
- Serology
- PCR
Oxidase Test
Neisseria meningitidis is oxidase-positive, which means it produces cytochrome oxidase, an enzyme that can reduce artificial electron acceptors like N,N,N’,N’-tetramethyl-p-phenylenediamine dihydrochloride to form a purple color.
Catalase Test
Neisseria meningitidis is catalase-positive, which means it produces catalase enzyme that can break down hydrogen peroxide into water and oxygen gas.
API-NH System
This is a biochemical identification system that is used to identify Neisseria and related species. The API-NH system is based on the ability of bacteria to metabolize various substrates, such as sugars, amino acids, and other compounds, to produce acid or gas. The results are then analyzed using a database to identify the bacterium to the species level.
Serology
Serology involves testing for the presence of antigens or antibodies in the blood of the patient. The meningococcal polysaccharide capsule is the primary virulence factor in Neisseria meningitidis, and it can be detected using serology tests.
PCR
PCR is a molecular technique that can amplify specific genes from the DNA of bacteria, allowing for rapid and accurate identification of Neisseria meningitidis.
Clinical Significance of Neisseria meningitidis
Neisseria meningitidis is a significant human pathogen that can cause meningitis and septicemia, which are potentially life-threatening conditions if not diagnosed and treated timely. The bacterium is highly infectious and can cause outbreaks, especially in crowded settings, such as college dormitories, military barracks, etc.
Meningitis
Meningitis is an inflammation of the meninges, which are the protective membranes covering the brain and spinal cord. Symptoms of meningitis include fever, headaches, stiff neck, vomiting, and photophobia. Meningitis caused by Neisseria meningitidis is known as meningococcal meningitis and is one of the most severe forms of bacterial meningitis.
Septicemia
Septicemia, also known as blood poisoning, is a systemic infection caused by the spread of bacteria, viruses, or toxins in the bloodstream. Symptoms of septicemia include fever, chills, low blood pressure, rapid heartbeat, and a rash that can progress to purpura or gangrene. Septicemia caused by Neisseria meningitidis is known as meningococcal septicemia and can be fatal within a few hours.
Conclusion
In conclusion, Neisseria meningitidis is a gram-negative bacterium that loses the crystal violet stain but takes up the safranin stain during the gram staining process. Its identification requires several tests, including oxidase, catalase, serology, API-NH, and PCR. The bacterium is a significant human pathogen that can cause meningitis and septicemia, and it is essential to diagnose and treat infections timely to prevent severe complications.
FAQs
- Is Neisseria meningitidis gram-positive or negative?
- What is the primary virulence factor in Neisseria meningitidis?
- What are the symptoms of meningitis caused by Neisseria meningitidis?
- What is septicemia?
- How is Neisseria meningitidis identified in the laboratory?
Neisseria meningitidis is a gram-negative bacterium.
The meningococcal polysaccharide capsule is the primary virulence factor in Neisseria meningitidis.
The symptoms of meningitis caused by Neisseria meningitidis include fever, headaches, stiff neck, vomiting, and photophobia.
Septicemia, also known as blood poisoning, is a systemic infection caused by the spread of bacteria, viruses, or toxins in the bloodstream.
Several tests are needed to confirm the identification of Neisseria meningitidis, including oxidase, catalase, API-NH, serology, and PCR.
References
- Cartwright, K., & Stuart, J. (2010). Meningococcal disease. Bmj, 340.
- Green, B., & Robertson, L. (2014). Meningococcal infections. Bmj, 349.
- Hamilton, H. L., & Dillard, J. P. (2006). Natural transformation of Neisseria gonorrhoeae: from DNA donation to homologous recombination. Molecular microbiology, 59(2), 376-385.
- Sanders, C. C. (2002). Antibiotic resistance in Neisseria meningitidis. Clinical infectious diseases, 35(Supplement_1), S58-S63.
- Tzeng, Y. L., & Stephens, D. S. (2000). Epidemiology and pathogenesis of Neisseria meningitidis. Microbes and infection, 2(6), 687-700.