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Klebsiella pneumoniae is a Gram-negative, non-motile, encapsulated, lactose fermenting, facultative anaerobic,
rod shaped bacterium found in the normal flora of the mouth, skin, and intestines. It is clinically the most important
member of the Klebsiella genus of Enterobacteriaceae; it is closely related to K. oxytoca from which it is distinguished by
being indole-negative and by its ability to grow on both melezitose and 3-hydroxybutyrate. It naturally occurs in
the soil and about 30% of strains can fix nitrogen in anaerobic condition. As a free-living diazotroph, its
nitrogen fixation system has been much studied.
This large, non-motile
bacterium produces large sticky colonies when plated on nutrient media. Klebsiella's pathogenicity can be attributed to its
production of a heat-stable enterotoxin. K. pneumoniae infections are common in hospitals where they cause pneumonia
(characterized by emission of bloody sputum) and urinary tract infections in catheterized patients. In fact, K.
pneumoniae is second only to E. coli as a urinary tract pathogen. Klebsiella infections are encountered far more
often now than in the past. This is probably due to the bacterium's antibiotic resistance properties. Klebsiella
species may contain resistance plasmids (R-plasmids) which confer resistance to such antibiotics as ampicillin and
carbenicillin. To make matters worse, the R-plasmids can be transferred to other enteric bacteria not necessarily
of the same species.
New antibiotic resistant strains of K. pneumoniae are appearing, and it is increasingly found as a nosocomial infection.
K. pneumoniae can cause bacterial pneumonia, typically due to aspiration by alcoholics, though it is more commonly implicated in hospital-acquired urinary tract and wound infections, particularly in immunocompromised individuals. Klebsiella ranks second to E. coli for urinary tract infections in older persons. It is also an opportunistic pathogen for patients with chronic pulmonary disease, enteric pathogenicity, nasal mucosa atrophy, and rhinoscleroma. Feces are the most significant source of patient infection, followed by contact with contaminated instruments.
Members of the Klebsiella genus typically express 2 types of antigens on their cell surface. The first, O antigen, is a lipopolysaccharide of which 77 varieties exist. The second is K agent, a capsular polysaccharide with 9 varieties. Both contribute to pathogenicity and form the basis for subtyping
Research conducted at King's College, London has implicated molecular mimicry between between HLA-B27 and two molecules in Klebsiella
microbes as the cause of ankylosing spondylitis.
The bacteria overcome innate host immunity through several means. They possess a polysaccharide capsule, which is the
main determinant of their pathogenicity. The capsule is composed of complex acidic polysaccharides. Its massive layer
protects the bacterium from phagocytosis by polymorphonuclear granulocytes. In addition, the capsule prevents bacterial
death caused by bactericidal serum factors. This is accomplished mainly by inhibiting the activation or uptake of
complement components, especially C3b. The bacteria also produce multiple adhesins. These may be fimbrial or
nonfimbrial, each with distinct receptor specificity. These help the microorganism to adhere to host cells,
which is critical to the infectious process.
Klebsiella possesses a chromosomal class A beta-lactamase giving it inherent resistance to ampicillin. Many strains have acquired an extended-spectrum beta-lactamase with additional resistance to carbenicillin, ampicillin, quinolones, and increasingly to ceftazidime. The bacteria remain largely susceptible to aminoglycosides and cephalosporins. Varying degrees of inhibition of the beta-lactamase with clavulanic acid have been reported.
We all have millions of bacteria in our gastrointestinal tracts, primarily in the colon (or "large" bowel). These bacteria are important for normal bowel
health and function. Klebsiella is the genus name for one of these bacteria found in the respiratory, intestinal, and urinogenital tracts of animals and
man. When Klebsiella bacteria get outside of the gut, however, serious infection can occur.
K. pneumoniae is second only to E. coli as a urinary tract pathogen. Klebsiella infections are encountered far more often now than in the past. This is
probably due to the bacterium's antibiotic resistance properties.
Klebsiella pneumoniae is known as a resident of the intestinal track in about 40% of man and animals. It is considered to be an opportunistic human
pathogen meaning that under certain conditions it may cause disease. For example, nosocomial infections are those that hospitalized patients pick up
because they are in a weakened state1,2.
Klebsiella pneumoniae is also well known in the environment and can be cultured from soil, water and vegetables. In fact, it is likely that we have K.
pneumoniae in our intestine from eating raw foods such as salads. Two research papers on surveys of bacteria in sprouts found K. pneumoniae to be a
predominant part of the microflora3. One research paper found 4% of the lettuce they tested contained Klebsiella pneumoniae4.
As a general rule, Klebsiella infections tend to occur in people with a weakened immune system. Many of these infections are obtained when a person
is in the hospital for some other reason. The most common infection caused by Klebsiella bacteria outside the hospital is pneumonia.
Klebsiella pneumonia tends to affect people with underlying diseases, such as alcoholism, diabetes and chronic lung disease. Classically, Klebsiella
pneumonia causes a severe, rapid-onset illness that often causes areas of destruction in the lung.
Infected persons generally get high fever, chills, flu-like symptoms and a cough productive of a lot of mucous. The mucous (or sputum) that is coughed
up is often thick and blood tinged and has been referred to as "currant jelly" sputum due to its appearance.
Mortality in Klebsiella pneumonia is around 50% due to the underlying disease that tends to be present in affected persons. While normal pneumonia
frequently resolves without complication, Klebsiella pneumonia more frequently causes lung destruction and pockets of pus in the lung (known as
abscesses). The mortality rate for untreated cases is around 90%.
There may also be pus surrounding the lung (known as empyema), which can be very irritating to the delicate lung tissue and can cause scar tissue to
form. At times, surgery may be needed to "rescue" a lung that is trapped in irregular pockets of pus and scar tissue.
Klebsiella can also cause less serious respiratory infections, such as bronchitis, which is usually a hospital-acquired infection. Other common
hospital-acquired infections caused by Klebsiella are urinary tract infections, surgical wound infections and infection of the blood. All of these infections
can progress to shock and death if not treated early in an aggressive fashion.
Many hospital-acquired infections occur because of the invasive treatments that are often needed in hospitalized patients. For example, intravenous
catheters used for fluid administration, catheters placed in the bladder for urine drainage and breathing tubes for people on a breathing machine can all
increase the susceptibility to infection.
While these devices may be needed in certain patients, they allow bacteria to bypass the natural barriers to infection and get into a personís body. The
result may be an infection if the personís immune system cannot fight the bacteria. Unfortunately, the people who need invasive treatments often have
weakened immune systems because of their underlying disease.
Unfortunately, once Klebsiella escapes the gut, it can be one nasty bacterium.
1Eickhoff, T. C. 1972. Klebsiella pneumoniae infection: a review with reference to the water-borne epidemiologic significance of K. pneumoniae
presence in the natural environment. national Council of the Paper Industry for Air and Stream Improvement, Inc. Technical Bulletin no. 254, New York,
2Martin, W. J., P., K. W. Yu, and J. A. Washington. 1971. Epidemiological significance of Klebsiella pneumoniae - a 3 month study. Mayo C.in. Proc.
46:785-793. 3) Selden, R., S. Lee, W. L. L. Wang, J. v. Bennett and T. C. Eickhoff. 1971. Nosocomial Klebsiella infections: intestinal colonization as
resevoir. Ann. Intern. Med. 74:657-664.]
3J.E. Patterson, and M. J. Woodburn, 1980. Journal of food Science 45:492-495 and D. F. Splittstoesser, D. T. Queale & B. W. Andaloro. 1983. The
microbiology of vegetable sprouts during commercial production. Journal of Food Safety 5:79-86.).
4Soriano JM, Rico H, Molto JC, Manes J., Int J Food Microbiol 2000 Jun 30;58(1-2):123-8, Assessment of the Microbiological Quality and Wash
Treatments of Lettuce Served in University Restaurants.