Diseases
These organisms are usually opportunistic pathogens that cause nosocomial infections, especially pneumonia and urinary tract infections. K. pneumoniae is an important respiratory tract pathogen outside hospitals as well.
Important Properties
K. pneumoniae, Enterobacter cloacae, and Serratia marcescens are the species most often involved in human infections. They are frequently found in the large intestine but are also present in soil and water. These organisms have very similar properties and are usually distinguished on the basis of several biochemical reactions and motility. K. pneumoniae has a very large polysaccharide capsule, which gives its colonies a striking mucoid appearance. S. marcescens produces red-pigmented colonies (Figure 1).
Fig1. Serratia marcescens—red-pigmented colonies. Arrow points to a red-pigmented colony of S. marcescens. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
Pathogenesis & Epidemiology
Of the three organisms, K. pneumoniae is most likely to be a primary, nonopportunistic pathogen; this property is related to its antiphagocytic capsule. Although this organism is a primary pathogen, patients with K. pneumoniae infections frequently have predisposing conditions such as advanced age, chronic respiratory disease, diabetes, or alcoholism. The organism is carried in the respiratory tract of about 10% of healthy people, who are prone to pneumonia if host defenses are lowered.
Enterobacter and Serratia infections are clearly related to hospitalization, especially to invasive procedures such as intra venous catheterization, respiratory intubation, and urinary tract manipulations. In addition, outbreaks of Serratia pneumonia have been associated with contamination of the water in respiratory therapy devices. Prior to the extensive use of these procedures, S. marcescens was a harmless organism most frequently isolated from environmental sources such as water.
Serratia also causes endocarditis in users of injection drugs. As with many other gram-negative rods, the pathogenesis of septic shock caused by these organisms is related to the endo toxins in their cell walls.
Clinical Findings
Urinary tract infections and pneumonia are the usual clinical entities associated with these three bacteria, but bacteremia and secondary spread to other areas such as the meninges and liver occur. It is difficult to distinguish infections caused by these organisms on clinical grounds, with the exception of pneumonia caused by Klebsiella, which produces a thick, mucoid, bloody sputum (currant-jelly sputum) and can progress to necrosis and abscess formation.
There are two other species of Klebsiella that cause unusual human infections rarely seen in the United States. Klebsiella ozaenae is associated with atrophic rhinitis, and Klebsiella rhinoscleromatis causes a destructive granuloma of the nose and pharynx.
Laboratory Diagnosis
Organisms of this group produce lactose-fermenting (colored) colonies on differential agar such as MacConkey’s or EMB, although Serratia, which is a late (slow) lactose fermenter, can produce a negative reaction. These organisms are differentiated by the use of biochemical tests.
Treatment & Prevention
Because the antibiotic resistance of these organisms can vary greatly, the choice of drug depends on the results of sensitivity testing. Isolates from hospital-acquired infections are frequently resistant to multiple antibiotics. Strains of K. pneumoniae that produce extended-spectrum β-lactamases (ESBL) are an important cause of hospital-acquired infections and are resistant to almost all known antibiotics. A cephalosporin (e.g., ceftriaxone) is used empirically until the results of testing are known.
Some hospital-acquired infections caused by gram-negative rods can be prevented by such general measures as changing the site of intravenous catheters, removing urinary catheters when they are no longer needed, and taking proper care of respiratory therapy devices. There is no vaccine.
