Klebsiella pneumoniae

Klebsiella pneumoniae is a gram-negative bacillus distinguished on the bench by three features: it is non-motile, it ferments lactose (producing pink colonies on MacConkey agar), and it is surrounded by a thick polysaccharide capsule that gives its colonies a distinctly mucoid, glistening appearance. It is also urease positive and oxidase negative. The organism lives harmlessly in the gastrointestinal tract of many healthy people and becomes pathogenic when it reaches a site where it does not belong.

The capsule is the dominant virulence factor. It resists phagocytosis and complement-mediated killing, which is why Klebsiella infections tend to be severe and destructive. The classic presentation is a lobar pneumonia following aspiration, producing a thick, blood-tinged sputum described as currant jelly sputum, with a tendency toward tissue necrosis, cavitation, and abscess formation — an unusual outcome for a bacterial pneumonia. Risk associations are commonly summarized as aspiration, abscess, alcoholics, and diabetics.

Beyond pneumonia, Klebsiella is a leading cause of urinary tract infections, especially catheter-associated ones, and of hospital-acquired bloodstream infections, surgical site infections, and ventilator-associated pneumonia. Its clinical importance today rests heavily on resistance. Many strains produce extended-spectrum beta-lactamases that inactivate penicillins and cephalosporins, and some produce carbapenemases — including KPC, the Klebsiella pneumoniae carbapenemase named for this organism — which hydrolyze carbapenems and place the isolate in the carbapenem-resistant Enterobacteriaceae category. These resistance genes sit on plasmids and can transfer to other gram-negative organisms.

USMLE Step 1 covers Klebsiella in the gram-negative bacilli material and revisits it under mechanisms of antibacterial resistance in bacteria. High-yield points for the exam are the lactose-fermenting, encapsulated, non-motile profile; the currant jelly sputum and cavitary upper-lobe pneumonia vignette in a patient with alcohol use disorder or diabetes; its role as a nosocomial and catheter-associated pathogen; and its status as a prototype for extended-spectrum beta-lactamase and carbapenemase production.