Neisseria

The genus Neisseria consists of Gram-negative, aerobic, nonsporing, nonmotile cocci, typically arranged in pairs (diplococci) with adjacent sides flattened together. The bacteria belonging to this genus are oxidase positive and mostly catalase positive. They ferment sugars with production of acid but no gas. The genus Neisseria consists of 10 species. Neisseria gonorrhoeae and Neisseria meningitidis are the two important species that cause human infections. These two species are strictly pathogens for humans, whereas the other Neisseria species are commensals of the mouth and upper respiratory tract, and hence cause opportunistic infections.

Genus Neisseria bercirikan: Gram-negatif, aerobik, nonspora, coccus nonmotil, biasanya berpasangan (diplococci) dengan sisi berdekatan diratakan bersama. Bakteri genus ini termasuk oksidase positif dan sebagian besar katalase positif. Mereka memfermentasi gula dengan produksi asam tapi tidak ada gas. Genus Neisseria terdiri dari 10 spesies. Neisseria gonorrhoeae dan Neisseria meningitidis adalah dua spesies penting yang menyebabkan infeksi pada manusia. Kedua spesies menjadi patogen kuat bagi manusia, sedangkan spesies Neisseria lainnya adalah commensalism pada mulut dan saluran pernapasan bagian atas, dan karenanya menyebabkan infeksi oportunistik (infeksi yang disebabkan oleh organisme yang biasanya tidak menyebabkan penyakit pada orang dengan sistem kekebalan tubuh yang normal, tetapi dapat menyerang orang dengan sistem kekebalan tubuh yang buruk). 

 

Neisseria gonorrhoeae

N. gonorrhoeae is a strict human pathogen. It is the causative agent of gonorrhea, one of the most common sexually transmitted disease worldwide. Gonococci when transmitted nonsexually from the mother’s genital tract to the newborn during birth cause ophthalmia neonatorum.                 

N. gonorrhoeae adalah patogen pada manusia. Ini adalah agen penyebab gonore, salah satu penyakit seksual yang paling umum ditularkan di seluruh dunia. Gonococci ketika ditransmisikan secara nonseksual dari saluran kelamin ibu pada bayi yang baru lahir selama kelahiran penyebab ophthalmia neonatorum (bentuk konjungtivitis dan jenis penyakit infeksi neonatal pada bayi yang baru lahir selama persalinan, mata bayi yang terkontaminasi selama proses melalui jalan lahir dari ibu yang terinfeksi Neisseria gonorrhoeae).

Morphology

N. gonorrhoeae shows following features:

1.      N. gonorrhoeae are Gram-negative and aerobic diplococci. They are mostly intracellular found within the polymorphonuclear (PMN) leukocytes and some cells contain as many as hundred cocci. Smears from the pus sample show the intracellular kidney-shaped cocci, typically arranged in pairs with concave sides facing each other.

2.      Freshly isolated bacteria may be capsulated. They do not form endospores.

3.      They are nonmotile.

Culture

N. gonorrhoeae is a fastidious coccus. It requires complex media for growth. The cocci grow on enriched media, such as blood or chocolate agar. These cannot grow on ordinary media, such as nutrient agar or Mueller–Hinton agar. They are aerobes but can also grow anaerobically. They grow optimally at a temperature range of 35–36°C. They fail to grow at temperature less than 25°C or greater than 37°C. The growth of bacteria is enhanced by incubation in humid atmosphere supplemented with 5–10% CO₂.

1.     Blood agar: On blood agar at 24 hours, N. gonorrhoeae produces convex small colonies measuring 0.6 x1.4  µm in diameter. These colonies are translucent with entireedges and finely granular surface. They are soft and easily emulsifiable. Gonococci are inhibited by fatty acids and trace metals present in the digested products of peptone found in the blood agar. Addition of soluble starch to the media neutralizes the toxic effects of the fatty acids.

2.      Selective media: Thayer Martin medium (chocolate agar medium containing antibiotics, such as colistin, nystatin, and vancomycin) and modified New York City medium (a translucent medium containing vancomycin, colistin, trimethoprim, and either nystatin or amphotericin B) are selective media used for isolation of gonococci from the clinical specimens containing mixed microbial flora. In these media, the growth of contaminating bacteria is suppressed including that of commensal Neisseria. On these media, N. gonorrhoeae produces small, translucent, and convex colonies, which are soft and easily friable.

3. Transport medium: Stuart’s transport medium is used for the collection and transport of clinical specimens to the laboratory for isolation and demonstration of N. gonorrhoeae.

Biochemical reactions

N. gonorrhoeae shows following features:

1.      Gonococci ferment glucose with the production of acid but no gas.

2.      They do not ferment maltose, lactose, sucrose, or fructose. This is an important feature to differentiate N. gonorrhoeae from N. meningitidis. N. gonorrhoeae utilizes glucose only, whereas N. meningitidis utilizes both glucose and maltose.

3.      They do not reduce nitrates, and they do not produce hydrogen sulfide.

4.      They are oxidase and catalase positive.

Other properties

Susceptibility to physical and chemical agents: The gonococci are highly delicate bacteria. They die rapidly on drying. They are also killed by soap, and many other disinfectants, such as phenol, chlorhexidine, and hexachlorophene and antiseptics. They are killed at a temperature as low as 25°C. Freeze drying or storing in liquid nitrogen are the most effective methods for storage of gonococci for a longer period.

Cell Wall Components and Antigenic Properties (Komponen Sel Dinding dan property antigenik)

     The cell wall of N. gonorrhoeae like any other Gram-negative bacteria consists of three layers: outer cell surface, middle peptidoglycan layer, and inner cytoplasmic membrane. These contain following proteins (Fig. 26-1).

 

·         Outer membrane proteins

     The outer membrane proteins (OMP) are present in the outer membrane. They mediate the uptake of iron essential for  growth and metabolism of the cocci. They promote intake of iron by binding hemoglobin, transferrin, and lactoferrin. These proteins are of three types:

     1)      The Por proteins

     2)      The Opa proteins

     3)      The Rmp proteins

The Por proteins: The Por proteins, earlier known as protein I, are porin proteins that form pores or channels in the outer membranes. Por proteins are of two types: Por-A and Por-B, each with a variety of antigenic variations. Strains producing Por-A proteins are commonly associated with disseminated disease because these proteins prevent killing of gonococci in the serum by the serum complement components. The antigenic variations observed in Por proteins form the basis for the serotype classification of N. gonorrhoeae.

The Opa proteins: These proteins, also known as opacity protein, were formerly known as protein II. These proteins are found in the membrane and mediate adherence of the bacteria to each other, and also to the eukaryotic cells. Strains producing Opa proteins produce opaque colonies in culture.

The Rmp proteins: These proteins, also known as reduction modifiable proteins, were formerly known as protein III. These are proteins found in the outer membrane of gonococci and lead to the production of antibodies that block serum bactericidal activity against gonococci.

·         Other important gonococcal proteins

Lipo-oligosaccharide (LOS) is another major antigen present in the cell wall of the bacteria. This antigen consists of lipid A and oligosaccharide similar to that of lipopolysaccharide (LPS) of Gram-negative bacteria. However, LOS does not show antigenic variation as found in LPS. LOS possesses endotoxic

activity. IgA1 protease and beta-lactamase are the other important proteins. IgA1 protease degrades secretory IgA1, whereas beta-lactamase degrades beta-lactam rings in the penicillin.

    Virulence factors (factor tingkat patogenitas N. gonorrhoeae)

N. gonorrhoeae menyebabkan penyakit baik dengan multiplikasi dalam jaringan dan menyebabkan peradangan. Bakteri ini tidak menghasilkan racun/toksin. Namun ada beberapa bagian sel yang menyebabkan pathogen antara lain:

1)     Capsule: N. gonorrhoeae does not form a true carbohydrate capsule unlike N. meningitidis. Instead, it forms a polyphosphate capsule, which is loosely associated with its cell surface. Capsule is most evident in freshly isolated gonococci and is antiphagocytic. It prevents phagocytosis of the gonococci.

2)     Pili: Pili are hair-like structures that extend from the cytoplasmic membrane through the outer membrane. The pili are composed of the proteins known as pilins, which are repeating protein subunits. The expression of protein pilin is controlled by P gene complex. The pilins of all the strains of gonococci are antigenically different. There is a marked antigenic variation in gonococcal pili as a result of chromosomal rearrangement. More than 100 serotypes are known. The pili are important virulence factors:

   a)  They play an important role in the virulence of the bacteria. They mediate attachment of gonococci to  nonciliated epithelial cells.

   b)  They also contribute to virulence by preventing ingestion and killing of gonococci by neutrophils.

Other virulence factors: These include: (Faktor virulensi lainnya)

   a.   Por protein of outer membrane protein (OMP) confers resistance to serum killing of gonococci by                preventing fusion of phagolysosome in neutrophils.

   b. Opa proteins mediate bacterial adherence of bacteria to each other and to the eukaryotic cells.

   c.  Rmp proteins produce antibodies that block serum bactericidal activity against gonococci.

   d.  Lipooligosakarida bakteri memiliki aktivitas endotoksik.

Sumber bacaan:

Textbook of Microbiology and Immunology 2nd Edition