Streptococcal pneumonia

Streptococcus pneumoniae accounts for more than 80% of all the bacterial pneumonias. The organism is a gram-positive, nonmotile coccus that is found singly, in pairs (called diplococci), and in short chains (Figure 15-2). The cocci are enclosed in a smooth, thick polysaccharide capsule that is essential for virulence. There are more than 80 different types of S. pneumoniae. Serotype 3 organisms are the most virulent. Streptococci are generally transmitted by aerosol from a cough or sneeze of an infected individual. Most strains of S. pneumoniae are sensitive to penicillin and its derivatives. S. pneumoniae also is commonly cultured from the sputum of patients having an acute exacerbation of chronic bronchitis.

Staphylococcal pneumonia

There are two major groups of Staphylococcus: (1) Staphylococcus aureus, which is responsible for most “staph” infections in humans, and (2) Staphylococcus albus and Staphylococcus epidermidis, which are part of the normal skin flora. The staphylococci are gram-positive cocci found singly, in pairs (called diplococci), and in irregular clusters (Figure 15-3). Staphylococcal pneumonia often follows a predisposing virus infection and is seen most often in children and immunosuppressed adults. S. aureus is commonly transmitted by aerosol from a cough or sneeze of an infected individual and indirectly via contact with contaminated floors, bedding, clothes, and the like. Staphylococci are a common cause of hospital-acquired pneumonia and are becoming increasingly antibiotic resistant—thus the term multiple drug–resistant S. aureus (MDRSA) organisms (some centers shorten this acronym to MRSA).

Haemophilus influenzae

Haemophilus influenzae is a common inhabitant of human pharyngeal flora. H. influenzae is one of the smallest gram-negative bacilli, measuring about 1.5 mm in length and 0.3 mm in width. It appears as coccobacilli on Gram stain. There are six types of H. influenzae, designated A to F, but only type B is commonly pathogenic. Pneumonia caused by H. influenzae type B is seen most often in children aged 1 month to 6 years old. H. influenzae type B is almost always the cause of acute epiglottitis. The organism is transmitted via aerosol or contact with contaminated objects. It is sensitive to cold and does not survive long after expectoration. H. influenzae is commonly cultured from the sputum of patients having an acute exacerbation of chronic bronchitis. Additional risk factors for H. influenzae infection include COPD, defects in B-cell function, functional and anatomic asplenia, and human immunodeficiency virus (HIV) infection.

Klebsiella pneumoniae (Friedländer’s Bacillus)

K. pneumoniae organisms have long been associated with lobar pneumonia, particularly in men older than 40 years and in chronic alcoholics of both genders. Klebsiella is a gram-negative bacillus that is found singly, in pairs, and in chains of varying lengths. It is a normal inhabitant of the human gastrointestinal tract. The organism can be transmitted directly by aerosol or indirectly by contact with freshly contaminated articles. K. pneumoniae is a common nosocomial, or hospital-acquired, disease. It is typically transmitted by routes such as clothing, intravenous solutions, foods, and the hands of health-care workers. The mortality of patients with K. pneumoniae is quite high because septicemia is a frequent complication.

Pseudomonas aeruginosa (Bacillus Pyocyaneus)

P. aeruginosa is a highly motile, gram-negative bacillus. It colonizes the gastrointestinal tract, burns, and catheterized urinary tract and is a contaminant in many aqueous solutions. Risk factors include neutropenia, HIV infection, preexisting lung disease, endotracheal intubation, and prior antibiotic use. P. aeruginosa frequently is cultured from the respiratory tract of chronically ill, tracheostomized patients and is a leading cause of hospital-acquired pneumonia. This makes P. aeruginosa a particular problem for the respiratory care practitioner. Because the Pseudomonas organism thrives in dampness, it is frequently cultured from contaminated respiratory therapy equipment. The organism is commonly transmitted by aerosol or by direct contact with freshly contaminated articles. The sputum from patients with Pseudomonas infection is frequently green and sweet-smelling.

Moraxella catarrhalis

M. catarrhalis is a natural inhabitant of the human pharynx. After S. pneumoniae and H. influenzae, M. catarrhalis is the third most common cause of acute exacerbation of chronic bronchitis.

Escherichia coli

E. coli is a normal inhabitant of the intestinal tract. It is sometimes the cause of nosocomial pneumonia (see the discussion of hospital-acquired pneumonia, page 230).

Serratia species

Serratia species cause about 7% of the cases of nosocomial pneumonia (see discussion of hospital-acquired pneumonia, page 230).

Enterobacter species

Enterobacter species (Enterobacter cloacae and Enterobacter aerogenes) are sometimes the cause of pneumonia.

Mycoplasma pneumoniae

M. pneumoniae is a common cause of mild pneumonia. These organisms cause symptoms similar to both bacterial and viral pneumonia, although the symptoms develop more gradually and are often milder. A common symptom of mycoplasma pneumonia is a cough that tends to come in violent attacks, producing only a small amount of white mucus. Chills and fever are early symptoms. Some patients experience nausea or vomiting. Some patients may experience a profound weakness that lasts for a long time.

The mycoplasma are tiny, cell wall–deficient organisms. They are smaller than bacteria but larger than viruses. The pneumonia caused by the mycoplasmal organism is described as primary atypical pneumonia—atypical because the organism escapes identification by standard bacteriologic tests. M. pneumoniae is most frequently seen in people younger than 40 years of age during the late summer and early fall months. This type of pneumonia spreads easily in areas where people congregate, such as child-care centers, schools, and homeless shelters. Patients with M. pneumoniae often are said to have “walking pneumonia” because the condition is mild (i.e., slight fever, fatigue, and a characteristic dry, hacking cough) and the patient is usually ambulatory.

Legionella pneumophila

In July 1976, a severe pneumonia-like disease outbreak occurred at an American Legion convention in Philadelphia. The causative agent eluded identification for many months, despite the concerted efforts of the nation’s top epidemiologic experts. When the organism finally was recovered from a patient, it was found to be an unusual and fastidious gram-negative bacillus with atypical concentrations of certain branched-chain lipids. The initial isolate was designated as Legionella pneumophila. More than 20 Legionella species have now been identified.

Most of the species are free-living in soil and water, where they act as decomposer organisms. The organism also multiplies in standing water such as contaminated mud puddles, large air-conditioning systems, and water tanks. The organism is transmitted when it becomes airborne and enters the patient’s lungs as an aerosol. No convincing evidence suggests that the organism is transmitted from person to person. The organism can be detected in pleural fluid, sputum, or lung tissue by direct fluorescent antibody microscopy. Although it is rarely found outside the lungs, the organism may be found in other tissues. The disease is most commonly seen in middle-aged men who smoke.

Chlamydia psittaci (Psittacosis)

C. psittaci is a small gram-negative bacterium in the respiratory tract and feces of a variety of birds (e.g., parrots, parakeets, lorikeets, cockatoos, chickens, pigeons, ducks, pheasants, turkeys). C. psittaci is transmitted from birds to humans by aerosol or direct contact. The clinical manifestations of C. psittaci closely resemble those caused by M. pneumoniae.

Chlamydia pneumoniae

C. pneumoniae recently has been identified as a cause of pneumonia in adults. It has been detected in schools, military institutions, and families. It is associated with meningoencephalitis, myocarditis, endocarditis, coronary artery disease, and Guillain-Barré syndrome.