Microbiology

Microbial diseases of the respiratory system may occur in the upper or lower regions. The upper region consists of the nose, pharynx, and other structures such as the middle ear and sinuses. Although many defensive mechanisms exist in this part of the body, such as ciliated hairs and mucous membranes, infections are common because of the proximity to the external environment. The lower portion of the system consists of the respiratory tubes and alveoli of the lungs. Infection occurs here because of the excessive moisture and rich supply of nutrients.

Strep throat. The common strep throat is due to a group A betahemolytic streptococcus known as Streptococcus pyogenes. This Gram-positive organism is encapsulated and produces streptokinase, which breaks down fibrin clots and permits the organism to spread to other tissues. The disease is accompanied by enlarged lymph nodes, inflamed tissues, and pus found on the tonsils. Diagnosis can be performed by obtaining throat swabs and combining the bacteria present with specific antibodies coated to beads. If the beads clump together, then S. pyogenes is presumably present. Cases of strep throat are treated with penicillin antibiotics.

Scarlet fever. Scarlet fever is caused by Streptococcus pyogenes , the same organism that causes strep throat. In scarlet fever, the betahemolytic streptococci produce an erythrogenic toxin, which causes a skin rash. The fever is usually high, the throat tissues are inflamed, and the tongue exhibits a strawberrylike appearance (“strawberry tongue”). Penicillin antibiotics are normally used in therapy.

Diphtheria. Diphtheria is caused by a club-shaped, Gram-positive rod calledCorynebacterium diphtheriae. The disease is characterized by sore throat, neck swelling, and blockage of the respiratory passageways with membranelike accumulations. These accumulations are due to the effects of a bacterial exotoxin, which destroys cells of the epithelial lining. Antibiotic therapy is augmented by administration of antitoxins to neutralize the toxins. Immunization is imparted in theDPT vaccine, in which diphtheria toxoid is employed.

Otitis media. Otitis media is infection of the middle ear accompanied by earache. Numerous bacteria may cause this problem including Streptococcus pneumoniae, Haemophilus influenzae , and Staphylococcus aureus. Antibiotic therapy is usually indicated.

Pertussis (whooping cough). Pertussis is caused by the Gram-negative bacteriumBordetella pertussis. Transmitted by airborne droplets, this organism multiplies in the trachea and bronchi and causes paroxysms of cough. A rapid inrush of air following a paroxysm results in the high-pitched whooping sound. Treatment is rendered with erythromycin and other antibiotics, and immunization may be performed with killed pertussis bacilli in the DPT vaccine or acellular bacterial fragments in the DTaP vaccine.

Tuberculosis. Tuberculosis is caused by Mycobacterium tuberculosis , an acid-fast rod. The bacteria have large amounts of mycolic acid in their cell walls, which permits them to retain carbolfuchsin stain despite a washing with acid-alcohol. The bacteria are acquired in respiratory droplets and infect the lung tissues.

Tuberculosis is accompanied by the formation of tubercles, which are nodules on the lung tissue. The tubercle has a soft, cheeselike center and is surrounded by layers of macrophages and T-lymphocytes. When the lesions heal as calcified bodies, they are called Ghon complexes. In some individuals, the tubercles continue to grow, and the lesion may rupture to release microorganisms into the bloodstream for spread to other body organs. This condition is called miliary tuberculosis. Sometimes the disease is called consumption.

Tuberculosis may be treated over a period of months with several drugs, including isoniazid (INH), rifampin, streptomycin, pyrazinamide, ethambutol, and others. Thetuberculin skin test is based on a type of cellular (delayed) hypersensitivity and is used to determine whether a person has had a previous exposure to tuberculosis antigens. One variation, the Mantoux test, uses dilutions of antigen called PPD(purified protein derivative), which are injected superficially to the skin to induce a reaction. A vaccine called BCG (bacille Calmette Guerin) is prepared from bovine tubercle bacilli and is available for immunization.

Pneumococcal pneumonia. Pneumococcal pneumonia is caused by Streptococcus pneumoniae , the pneumococcus. This organism is a Gram-positive pair of cocci occurring in chains. There are almost 100 serological types of the organism, and the vaccine currently available provides protection against approximately 25 of them. The disease involves the lung tissues and is accompanied by fever, consolidation of the lung (filling of the air spaces with bacteria, fluid, and debris), and severe chest pains, with blood in the sputum. Aggressive penicillin therapy is used in treatment. Many individuals are healthy carriers of the bacterium.

Mycoplasmal pneumonia. Mycoplasmal pneumonia is caused by Mycoplasma pneumoniae , a species of mycoplasma. Mycoplasmas are exceptionally small, submicroscopic bacteria (about 0.15 μm) that have no cell walls. Penicillin is therefore useless as a therapeutic agent. Most cases are accompanied by a mild pneumonia, and erythromycin is generally recommended for therapy. The disease is sometimes calledprimary atypical pneumonia and is often described as walking pneumonia.

Legionnaires' disease. Legionnaires' disease, or legionellosis, was first recognized in 1976 when an outbreak occurred among American Legion members attending a convention in Philadelphia. The causative agent is a Gram-negative rod calledLegionella pneumophila. The organism exists where water collects and is airborne in wind gusts. Cases of Legionnaires' disease are accompanied by high fever, lung consolidation, and pneumonia. Erythromycin is used for therapy. A closely associated disease known as Pontiac fever is caused by the same organism.

Psittacosis. Psittacosis is caused by a species of chlamydia called Chlamydia psittaci. Chlamydiae are extremely tiny bacteria (0.25 μm) that cannot be seen with a light microscope. Psittacosis occurs in parrots, parakeets, and other psittacine birds, and when it is transferred to humans in airborne droplets, it manifests itself as a type of pneumonia with fever, headache, and lung consolidation. When the disease occurs in birds other than psittacines, it is known as ornithosis. Tetracyclines are effective drugs for therapy.

Chlamydial pneumonia. Chlamydial pneumonia is a recently recognized infection due to a species of chlamydia called Chlamydia pneumoniae. The infection resembles influenza and is treated successfully with tetracycline therapy.

Q fever. Q fever is due to a rickettsia known as Coxiella burnetii. The organism is transmitted by airborne droplets as well as by arthropods such as ticks. The infection resembles a form of pneumonia and is treated with tetracycline. Some cases are transmitted by contaminated or unpasteurized dairy products.