Microbiology Lecture Notes PDF

Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 10 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــ...

Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 10 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Neisseria The genus Neisseria contains two important human pathogens: Neisseria meningitidis and Neisseria gonorrhoeae. Neisseria meningitidis mainly causes meningitis and meningococcemia. In the United States, it is the leading cause of death from infection in children. Neisseria gonorrhoeae causes gonorrhea, the second most common bacterial disease in the United States. It also causes neonatal conjunctivitis (ophthalmia neonatorum) and pelvic inflammatory disease (PID). Note that N. meningitides is also known as the meningococcus (plural, meningococci), and N. gonorrhoeae is also known as the gonococcus (plural, gonococci). Important Properties Neisseriae are gram-negative cocci that resemble paired kidney beans. (1) Neisseria meningitidis (meningococcus) has a prominent polysaccharide capsule that enhances virulence by its antiphagocytic action. The capsule also is the immunogen in the vaccine that induces protective antibodies. 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. Serotype B accounts for most disease in the United States. This is because the group B polysaccharide is not immunogenic in humans and therefore is not part of the vaccines that contain the capsular polysaccharide of the other four groups. (2) Neisseria gonorrhoeae (gonococcus) has no polysaccharide capsule but has multiple serotypes based on the antigenicity of its pilus protein. There is marked antigenic variation in the gonococcal pili as a result of chromosomal rearrangement; 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. 1 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 10 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ Neisseriae are gram-negative bacteria and contain endotoxin in their outer membrane. Note that the endotoxin of Neisseriae consist of lipooligosaccharide (LOS), in contrast to the lipopolysaccharide (LPS) found in enteric gramnegative rods. Both LPS and LOS contain lipid A, but the oligosaccharide part of LOS contains few sugars, whereas the polysaccharide part of LPS contains a long repeating sugar side chain. The genus Neisseria is one of several in the family Neisseriaceae. A separate genus contains the organism Moraxella catarrhalis, which is part of the normal throat flora but can cause such respiratory tract infections as sinusitis, otitis media, bronchitis, and pneumonia. Neisseria meningitides Pathogenesis & 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). Meningococci have four important virulence factors: (1) A polysaccharide capsule that enables the organism to resist phagocytosis by polymorphonuclear leukocytes (PMNs). The capsule is the immunogen in several commonly used vaccines against meningococci. (2) Endotoxin, which causes fever, shock, and other pathophysiologic changes (in purified form, endotoxin can reproduce many of the clinical manifestations of meningococcemia). (3) An immunoglobulin A (IgA) protease that helps the bacteria attach to the membranes of the upper respiratory tract by cleaving secretory IgA. (4) Factor H binding protein (FHBP) on meningococci binds Factor H, an inhibitor of complement factor C3b. The presence of Factor H on the surface of meningococci 2 Assist. Prof. Dr. Ameer Mezher Hadi Al-Zahrawi University College Lec. 10 Microbiology Department of Dentisy ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬ reduces the opsonizing activity of C3b and reduces the amount of membrane attack complex produced. Moraxella Moraxella species are gram-negative coccobacillary rods that resemble the diplococcal appearance of the Neisseriae. Moraxella catarrhalis is the major pathogen in this genus. It causes otitis media and sinusitis, primarily in children, as well as bronchitis and pneumonia in older people with chronic obstructive pulmonary disease. It is found only in humans and is transmitted by respiratory aerosol. Trimethoprim-sulfamethoxazole or amoxicillinclavulanate can be used to treat these infections. Most clinical isolates produce β-lactamase. Moraxella nonliquefaciens is one of the two common causes of blepharitis (infection of the eyelid); Staphylococcus aureus is the other. The usual treatment is local application of antibiotic ointment, such as erythromycin. Veillonella Veillonella are Gram-negative bacteria (Gram stain pink) anaerobic cocci, unlike most Bacillota, which are Gram-positive bacteria. This bacterium is well known for its lactate fermenting abilities. It is a normal bacterium in the intestines and oral mucosa of mammals. In humans they have been implicated in cases of osteomyelitis and endocarditis, for example with the species Veillonella parvula. Veillonella dispar is the most nitrate-reducing bacterium in the oral cavity, which is beneficially anti-bacterial. When Veillonella is responsible for clinical infections in humans, it should be kept in mind that more than 70% of the strains are resistant to penicillin. 3