Microbial Diseases Of The Respiratory System


Infections of the upper respiratorysystem are the most common type of infection.

Pathogens that enter the respiratory system can infect other parts of the body.

Structure and Function of the Respiratory System

The upper respiratory system consists of the nose, pharynx, and associated structures, such as the middle ear and auditory tubes.

Coarse hairs in the nose filter large partilces from air entering the respiratory tract.

The ciliated mucous membranes of the nose and throat trap airborne particles and remove them from the body.

Lympyhoid tissue, tonsils, and adenoids provide immunity to certain infections.

The lower respiratory system consists of the larynx, trachea, bronchial tubes, and alveoli.

The ciliary escalator of the lower respiratory system helps prevent microorganisms from reaching the lungs.

Microbes in the lungs can be phagocytized by alveolar macrophages.

Respiratory mucus contains IgA antibiodies.

Normal Microbiota of the Respiratory System

The normal flora of the nasal cavity and throat can include pathogenic microorganisms.

Nasal Cavity:


The lower respiratory system is usually sterile because of the action of the ciliary escalator.

Microbial Diseases of the Upper Respiratory System

Specific areas of the upper respiratory system can become infected to produce pharyngitis, laryngitis, tonsillitis, sinusitis, and epiglottis.

These infections may be caused by several bacteria and viruses, often in combination.

Most respiratory tract infections are self-limiting.

H. influenzae type b can cause epiglottitis.

Bacterial Diseases of the Upper Respiratory System

Streptococcal Pharyngitis (Strep Throat)

Caused by S. pyogenes (group A beta-hemolytic streptococcus). S. pyogenes is resistant to phagocytosis and produces streptokinase, DNAse and hemolysins.

Symptoms are inflammation of the mucous membrane and fever; tonsillitis and otitis media may also occur.

Preliminary rapid diagnosis by indirect agglutination tests. Definitive diagnosis is based on a rise in IgM antibodies.

Penicillin is used to treat streptococcal pharyngitis.

Immunity does not usually develop (it is type-specific).

Transmitted by respiratory route but at one time was associated with unpasteurized milk.

Scarlet Fever

Strep throat caused by erythrogenic toxin producing strains of Streptococcus pyogenes results in scarlet fever.

Erythrogenic toxin production is caused by a phage.

Symptoms inclued a red rash, high fever, red enlarged tongue.

Penicillin is the drug of choice.

Immunity is developed to the toxin not the organism.


Diphtheria is caused by an exotoxin-producing gram-positive rod, Corynebacterium diphtheriae.

A potent exotoxin is produced when the bacteria are lysogenized by a phage.

Pseudomembrane formation occurs in the throat. The membrane contains fibrin and dead human and bacterial cells and can block the passage of air.

The exotoxin inhibits protein synthesis and may damage the heart, kidney and/or nerves.

Laboratory diagnosis is based on isolation and identification.

Treatment: Administer antitoxin to neutralize the toxin and antibiotics to stop growth of the bacteria.

Prevention: Immunization with DPT.

Cutaneous diphtheria is characterized by slow healing skin ulcerations.

There is minimal dissemination of exotoxin in the bloodstream.

Otitis Media

Often a complication of nose and throat infections but can also be caused by direct inoculation.

Pus accumulation causes pressure on the tympanic membrane.

Common bacterial causes:

Viral Diseases of the Upper Respiratory System

The Common Cold

Caused by any one of around 200 viruses; rhinoviruses are the most common cause (about 50%).

Symptoms include sneezing, nasal secretions, and congestion.

Complications include sinus infections, lower respiratory tract infections, laryngitis, and otitis media.

Colds are most often transmitted by indirect contact.

The increased incidence during winter is probably due to increased interpersonal indoor contact or physiological changes.

Microbial Diseases of the Lower Respiratory System

Many of the same microorganisms that infect the upper respiratory system also infect the lower respiratory system.

Diseases of the lower respiratory system include bronchitis and pneumonia.

Bacterial Diseases of the Lower Respiratory System

Pertussis (Whooping Cough)

Caused by Bordetella pertussis, which produces a very potent exotoxin.

The initial stage, or the catarrhal stage, presents like a cold.

The second, or paroxysmal stage, is characterized by mucus accumulations in the trachea and bronchi, which cause deep coughing.

The third, or convalescence stage, can last for months.

Laboratory diagnosis is based on isolation of bacteria on Bordet-Gengou medium and serological tests.

Prevention: Immunization with DPT


Caused by Mycobacterium tuberculosis.

The bacterium is acid-fast due to lipid in the cell wall, which makes it very resistant to drying and disinfectants.

The bacteria may survive phagocytosis and may reproduce in macrophages.

Transmission is by droplet nuclei (respiratory aerosols).

Lesions are called tubercles; dead macrophages and bacteria form caseous lesions that may calcify and appear on X ray as a Ghon complex.

Liquefaction of the caseous lesion results in a tuberculous cavity in whichM. tuberculosis can grow.

New foci of infection can develop when a caseous lesion ruptures and releases bacteria into blood or lymph vessels; this is called miliary tuberculosis.

Miliary tuberculosis is characterized by weight loss, coughing, and loss of vigor.

Chemotherapy usually involves two drugs taken for 1 - 2 years; multidrug-resistant M. tuberculosis is becoming prevalent.

A positve tuberculin test is used to determine previous exposure; it is not diagnostic because a positive test may indicate an active case, a previous infection, or vaccination and immunity.

Laboratory diagnosis is based on isolation and identification - takes up to 8 weeks.

Mycobacterium bovis causes bovine tuberculosis and can be transmitted to humans by unpasteurized milk.

M. bovis infections usually affect the bones or lymphatic system.

Prevention is by case finding and treatment.

The BCG vaccine is a live, avirulent culture of M. bovis; It is of variable utility and is not used in the U.S.

A related group of organisms, M. avium-intracellulare complex, infects patients in the late stages of HIV infection.

Bacterial Pneumonias

Mostly caused by normal flora from the mouth and throat. The most common causes are:

Typical pneumonia is caused by S. pneumoniae.

Atypical pneumonias are caused by other microorganisms.

Pneumococcal Pneumonia

Pneumococcal pneumonia is caused by encapsulated Streptococcus pneumoniae.

Symptoms are rust colored sputum, fever, difficult breathing, and chest pain.

The lungs have reddish appearance due to dilation of blood vessels. Alveoli fill with erythrocytes and fluid.

Initial diagnosis is made by X-rays. Laboratory diagnosis is by isolation and identification based on production of alpha-hemolysins, inhibition by optochin, bile solubility, and through serological tests.

The treatment of choice is penicillin.

A purified capsular vaccine consisting of capsular material from 23 serotypes of S. pneumoniae is available.

Haemophilus influenzae Pneumonia

Alcoholism, poor nutrition, cancer, and diabetes are predisposing factors for H. influenzae pneumonia.

H. influenzae is a gram-negative coccobacillus.

Mycoplasmal Pneumonia

Mycoplasmal pneumonia (primary atypical pneumonia or walking pneumonia) is caused by the pleomorphic rod Mycoplasma pneumoniae.

Mycoplasma pneumoniae produces characteristic fried egg colonies after 2 weeks' incubation on enriched media containing horse serum and yeast extract.

Symptoms are low grade fever, cough, and headache. Mortality rate is less than 1%.

Diagnosed by PCR or complement fixation.

The treatment of choice is tetracycline or erythromycin.


Caused by the aerobic gram-negative rod Legionella pneumophila.

The organism grows in water and is spread through the air; it is resistant to chlorine.

Person to person spread is not likely.

High risk groups are males over 50, heavy smokers, alcohol abusers, and those with chronic illness.

Bacterial culture, FA tests and DNA probes are used for laboratory diagnosis.

The treatment of choice is erythromycin.

Psittacosis (Ornithosis)

Caused by the gram-negative obligate intracellular parasite Chlamydia psittaci.

The organisms is transmitted by contact with bird droppings and causes a form of pneumonia.

Symptoms include fever, headaches, and chill.

Elementary bodies allow the bacteria to survive outside a host.

Because it is an obligate intracellular parasite the bacterium must be isolated in embryonated eggs, mice, or cell culture; identification is based on FA techniques or complement fixation.

The treatment of choice is tetracycline. No effective immunity is produced.

Chlamydial Pneumonia

Chlamydia pneumoniae causes pneumonia; it is transmitted from person to person.

Tetracycline is used for treatment.

Q Fever

Caused by the obligate intracellular parasite Coxiella burnetii.

It is usually transmitted by unpasteurized milk or inhalation of aerosols in dairy barns.

The infection is usually subclinical. Symptoms include fever, chills, chest pain, and headache.

Laboratory diagnosis by culture in embryonated eggs or cell culture.

The treatment of choice is tetracycline.

Other Bacterial Pneumonias

Gram positive bacteria that cause pneumonia include S. aureus and S. pyogenes.

Gram-negative bacteria that cause pneumonia include Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Branhamella catarrhalis, and Enterobacter species.

Klebsiella pneumonia

Caused by Klebsiella pneumoniae.

Klebsiella pneumonia results in lung abscesses and permanent lung damage; the mortality rate is 85%.

The treatment of choice is cephalosporins or gentamicin.

Viral Diseases of the Lower Respiratory System

Viral Pneumonia

Viral pneumonia often occurs as a complication of viral infections such as influenza. The etiology is usually not demonstrated because of the difficulty isolating and identifying viruses in clinical labs.

Respiratory Syncytial Virus (RSV)

RSV is the most common cause of pneumonia in infants.


Influenza is caused by the influenza virus (Influenzavirus) and is characterized by chills, fever, headache, and general muscular aches.

Hemagglutinin and neuraminidase spikes in the envelope are used for antigenic typing. The viral protein coat is also used to classify the virus as A, B, or C.

Viral isolates are identified by hemagglutination-inhibition tests and immunofluorescence testing with monoclonal antibodies.

Antigenic changes occur due to reasortment of the 8 segments of RNA and make natural immunity and vaccination of questionable value. Minor antigenic changes are caused by antigenic drift. Antigenic shift, or large changes, causes epidemics.

Deaths are usually due to secondary bacterial infections.

Multivalent vaccines may be indicated for high risk groups.

Amantadine and rimantadine are useful drugs against Influenzavirus A.

Fungal Diseases of the Lower Respiratory System

Fungal Diseases are easily inhaled; they may germinate in the lower respiratory tract.

The incidence of fungal diseases has been increasing in recent years.

The mycoses below can be treated with amphotericin B.


Histoplasma capsulatum causes a subclinical infectionthat only occasionally progresses to a severe, generalized disease.

The disease is acquired by inhalation of airborne conidia.

Isolation of the fungus or identification of the fungus in tissue samples is necessary for diagnosis.


Inhalation of the airborne arthrospores of Coccidioides immitis can result in coccidioidomycosis.

Most cases are subclinical but predisposing factors such as fatigue and poor nutrition can contribute to a progressive disease (about 1% of the cases) that resembles tuberculosis.

Symptoms include chest pain, fever, coughing, and weight loss.


Caused by Blastomyces dermatitidis.

The infection begins in the lungs and spreads to cause extensive abscesses.

Other Fungi Involved in Respiratory Disease

Occurs most often in immunosuppressed hosts.

Common causes:

Pneumocystis pneumonia

Caused by Pneumocystis jiroveci (carinii).

Pneumocystis jiroveci is an opportunistic pathogen that invades immunosuppressed or cancer patients.

The life cycle of organism is not well known.

Drug of choice: Trimethoprim-sulfamethoxazole

Untreated causes are usually fatal.