Date: 22. November 2017
Introduction to Neisseria
In this lecture educator explains the Neisseria and in the first section of lecture educator presents the introduction of Neisseria. The genus Neisseria contains two important human pathogens: Neisseria meningitidis and Neisseria gonorrhoeae. N. meningitidis mainly causes meningitis and meningococcaemia. Neisseria meningitidis is also known as the meningococcus (plural, meningococci), and Neisseria gonorrhoeae is also known as the gonococcus (plural, gonococci). Then educator explains the Neisseria of Medical Importance and Important Clinical Features of Neisseria in detail.
Important Properties includes Neisseria are gram-negative cocci that resemble paired kidney beans
N. meningitidis (meningococcus) has a prominent polysaccharide capsule. Enhances virulence by its antiphagocytic action. It enables the organism to resist phagocytosis by polymorphonuclear leukocytes (PMNs). Antigen detected in the spinal fluid of patients with meningitis, Antigen that defines the 13 serological types and Antigen in the vaccine.
N. gonorrhoeae (gonococcus) has no polysaccharide capsule but has multiple serotypes based on the antigenicity of its pilus protein. Endotoxin of Neisseriae consist of lip-oligosaccharide (LOS), in contrast to the lipopolysaccharide (LPS) found in enteric gram-negative rods. Grown on chocolate agar containing blood heated to 80°C, which inactivates the inhibitors.
Neisseria Meningitidis - I
Humans are the only natural hosts for meningococci. The organisms are transmitted by airborne droplets; they colonize the membranes of the nasopharynx and become part of the transient flora of the upper respiratory tract. Carriers are usually asymptomatic. From the nasopharynx, the organism can enter the bloodstream and spread to specific sites, such as the meninges or joints, or be disseminated throughout the body (meningococcemia).
Pathogenesis: Meningococci have three important virulence factors, (1) A polysaccharide capsule that enables the organism to resist phagocytosis by polymorphonuclear leukocytes (PMNs), (2) Endotoxin, which causes fever, shock, and other pathophysiologic changes (in purified form, endotoxin can reproduce many of the clinical manifestations of meningococcaemia), (3) An immunoglobulin A (IgA) protease that helps the bacteria attach to the membranes of the upper respiratory tract by cleaving secretory IgA.
N. meningitidis ranks second to S. pneumoniae as a cause of meningitis but is the most common cause in persons between the ages of 2 and 18 years. Group A meningococci are most likely to cause epidemics of meningitis. Group B meningococci cause many cases of meningitis in developed countries because it is not present in the vaccine.
Neisseria Meningitidis - II
Laboratory Diagnosis: A presumptive diagnosis of meningococcal meningitis can be made if gram-negative cocci are seen in a smear of spinal fluid. The differentiation between N. meningitidis and N. gonorrhoeae is made on the basis of sugar fermentation: meningococci ferment maltose, whereas gonococci do not (both organisms ferment glucose). Immunofluorescence can also be used to identify these species. Tests for serum antibodies are not useful for clinical diagnosis. However, a procedure that can assist in the rapid diagnosis of meningococcal meningitis is the latex agglutination test, which detects capsular polysaccharide in the spinal fluid.
Treatment: Penicillin G is the treatment of choice for meningococcal infections. A third-generation cephalosporin such as ceftriaxone can also be used. Strains resistant to penicillin have rarely emerged, but sulfonamide resistance is common.
Prevention: Chemoprophylaxis and immunization are both used to prevent meningococcal disease. Either rifampin or ciprofloxacin can be used for prophylaxis in people who have had close contact with the index case. These drugs are preferred because they are efficiently secreted into the saliva, in contrast to penicillin G. There are three forms of the meningococcal vaccine for use in the United States, all of which contain the capsular polysaccharide of groups A, C, Y, and W-135 as the immunogen. There are two forms of the conjugate vaccine: Menactra contains the four polysaccharides conjugated to diphtheria toxoid as the carrier protein, whereas Menveo contains the four polysaccharides conjugated to a nontoxic mutant of diphtheria toxin as the carrier protein. Menomune, the unconjugated vaccine, contains only the four polysaccharides (not conjugated to a carrier protein).
Neisseria Gonorrheae - I
Gonococci, like meningococci, cause disease only in humans. The organism is usually transmitted sexually; newborns can be infected during birth. Because gonococcus is quite sensitive to dehydration and cool conditions, sexual transmission favours its survival. Gonorrhea is usually symptomatic in men but often asymptomatic in women. Genital tract infections are the most common source of the organism, but anorectal and pharyngeal infections are important sources as well.
Pili constitute one of the most important virulence factors, because they mediate attachment to mucosal cell surfaces and are antiphagocytic. Piliated gonococci are usually virulent, whereas nonpiliated strains are avirulent. Two virulence factors in the cell wall are endotoxin (lipooligosaccharide, LOS) and the outer membrane proteins. The organism’s IgA protease can hydrolyze secretory IgA, which could otherwise block attachment to the mucosa. Gonococci have no capsules.
Gonococci cause both localized infections, usually in the genital tract, and disseminated infections with seeding of various organs. Gonococci reach these organs via the bloodstream (gonococcal bacteremia). Gonorrhea in men is characterized primarily by urethritis accompanied by dysuria and a purulent discharge. Epididymitis can occur in women, infection is located primarily in the endocervix, causing a purulent vaginal discharge and intermenstrual bleeding (cervicitis). The most frequent complication in women is an ascending infection of the uterine tubes (salpingitis, PID), which can result in sterility or ectopic pregnancy as a result of scarring of the tubes.
Neisseria Gonorrheae - II
The diagnosis of urogenital infections depends on Gram staining and culture of the discharge. However, nucleic acid amplification tests are widely used as screening tests. In men, the finding of gram-negative diplococci within PMNs in a urethral discharge specimen is sufficient for diagnosis. In women, the use of the Gram stain alone can be difficult to interpret; therefore, cultures should be done. Gram stains on cervical specimens can be falsely positive because of the presence of gram-negative diplococci in the normal flora and can be falsely negative because of the inability to see small numbers of gonococci when using the oil immersion lens. Cultures must also be used in diagnosing suspected pharyngitis or anorectal infections.
Treatment: Ceftriaxone is the treatment of choice in uncomplicated gonococcal infections. Azithromycin or ciprofloxacin should be used if the patient is allergic to penicillins or cephalosporins. Because mixed infections with C. trachomatis are common, azithromycin or doxycycline should be prescribed also A follow up culture should be performed after 1 week of treatment to confirm whether or not gonnococcal infection is still present. Treatment of complicated gonococcal infections, such as PID, typically requires hospitalization.
Treatment for Resistant Strains: Isolates emerged with low-level resistance to penicillin and to other antibiotics such as tetracycline and chloramphenicol. This type of resistance is encoded by the bacterial chromosome and is due to reduced uptake of the drug or to altered binding sites rather than to enzymatic degradation of the drug. Isolates resistant to fluoroquinolones, such as ciprofloxacin, have become a significant problem, and fluoroquinolones are not recommended as treatment.
Prevention: Use of condoms and the prompt treatment of symptomatic patients and their contacts. Cases of gonorrhea must be reported to the public health department to ensure proper follow-up, A major problem is the detection of asymptomatic carriers, Gonococcal conjunctivitis in newborns is prevented most often by the use of erythromycin ointment, Silver nitrate drops are used less frequently and no vaccines are available.