Medical Microbiology Past Paper PDF (2024-2025)

Prof. Dr. Hanaa J. Al- Kabee Medical Microbiology (2024 – 2025) NEISSERIA spp (Gram negative cocci) NEISSERIA The genus Neisseria contains two important human pathogens: Neisseria meningitidis and Neisseria gonorrhoeae. N. meningitidis mainly causes meningitis and meningococcemia (Figure - 1...

Prof. Dr. Hanaa J. Al- Kabee Medical Microbiology (2024 – 2025) NEISSERIA spp (Gram negative cocci) NEISSERIA The genus Neisseria contains two important human pathogens: Neisseria meningitidis and Neisseria gonorrhoeae. N. meningitidis mainly causes meningitis and meningococcemia (Figure - 1, purpuric lesions on leg caused by endotoxin-mediated disseminated intravascular coagulation (DIC)). FIGURE -1, Meningococcemia. Note purpuric lesions on leg caused by endotoxin-mediated disseminated intravascular coagulation (DIC). N. gonorrhoeae causes gonorrhea. (Tables 1 and 2). It also causes neonatal conjunctivitis (ophthalmia neonatorum) (Figure –2) and pelvic inflammatory disease (PID). Note that Neisseria meningitidis is also known as the meningococcus (plural, meningococci), and Neisseria gonorrhoeae is also known as the gonococcus (plural, gonococci). Figuer - 2, Neonatal conjunctivitis (ophthalmia neonatorum) cause by N. gonorrhoeae Important Properties of Neisseriae 1. Neisseriae are gram-negative cocci that resemble paired kidney beans 2. The growth of both organisms is inhibited by toxic trace metals and fatty acids found in certain culture media (e.g., blood agar plates). They are therefore cultured on “chocolate” agar containing blood heated to 80°C, which inactivates the inhibitors. 3. Neisseriae are oxidase-positive (they possess the enzyme cytochrome c). This is an important laboratory diagnostic test in which colonies exposed to phenylenediamine turn purple or black as a result of oxidation of the reagent by the enzyme. 4. N. meningitidis (meningococcus) has a prominent polysaccharide capsule (Table - 3). Meningococci are divided into at least 13 serologic groups on the basis of the antigenicity of their capsular polysaccharides. Five serotypes cause most cases of meningitis and meningococcemia: A, B, C, Y, and W-135. Serotype A is the leading cause of epidemic meningitis worldwide. TABLE–3 Properties of the Polysaccharide Capsule of the Meningococcus 5. N. gonorrhoeae (gonococcus) has no polysaccharide capsule but has multiple serotypes based on the antigenicity of its pilus protein, more than 100 serotypes are known. Gonococci have three outer membrane proteins (proteins I, II, and III). Protein II plays a role in attachment of the organism to cells and varies antigenically as well.  LPS(LOS) 6. Neisseriae are contain endotoxin in their outer membrane consist of lipooligosaccharide (LOS) 1. Neisseria meningitidis Pathogenesis and Epidemiology 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). About 5% of people become chronic carriers and serve as a source of infection for others. The carriage rate can be as high as 35% in people who live in close quarters. 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. (3) An immunoglobulin A (IgA) protease that helps the bacteria attach to the membranes of the upper respiratory tract. Laboratory Diagnosis The principal laboratory procedures are smear and culture of blood and spinal fluid samples. Immunofluorescence can also be used to identify these species. Treatment Penicillin G is the treatment of choice for meningococcal infections. A third generation cephalosporin such as ceftriaxone can also be used. Prevention Chemoprophylaxis and immunization are both used to prevent meningococcal disease. Either rifampin or ciprofloxacin can be used for prophylaxis There are three forms of the meningococcal vaccine all of which contain the capsular polysaccharide of groups A, C, Y, and W-135 as the immunogen. 2. Neisseria gonorrhoeae Pathogenesis and Epidemiology 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 favors 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 infect primarily the mucosal surfaces (e.g., the urethra and vagina), but dissemination occurs. Laboratory Diagnosis 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. 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. Prevention The prevention of gonorrhea involves the prompt treatment of symptomatic patients A major problem is the detection of asymptomatic carriers. Gonococcal conjunctivitis in newborns is prevented most often by the use of erythromycin ointment. No vaccine is available