Klebsiella pneumoniae

Klebsiella pneumoniae is a complex, rod-shaped bacterium that plays many roles in the natural world. It is classified as a Gram-negative, non-motile, and encapsulated organism. This means it has a specific cell structure and a protective outer layer. It is also a facultative anaerobe, which allows it to live in environments with or without oxygen. While it is part of the normal flora in the human mouth, skin, and intestines, it can act as a dangerous pathogen. When it moves into the lungs, it can cause destructive changes to the alveoli, the tiny air sacs where breathing happens.

The biological mechanisms of this bacterium allow it to thrive in diverse settings. In the soil, about 30% of strains are diazotrophs, meaning they can fix nitrogen. This process allows them to take nitrogen from the environment and use it for growth under anaerobic conditions. This ability is of great interest to agriculture because it can increase crop yields. Inside a human host, the bacteria can cause various illnesses through specific pathways. For example, aspiration can lead to pneumonia, which often results in a characteristic "currant jelly" sputum. This sputum is a thick, bloody, brownish, or yellow jelly-like substance coughed up by the patient.

Scientists categorize different types of Klebsiella based on their specific traits. K. pneumoniae is the most significant member of the Enterobacteriaceae family in clinical settings. Other species include K. oxytoca and K. rhinoscleromatis. Recently, a new variant known as hypervirulent Klebsiella pneumoniae (hvKp) has emerged. While classical strains (cKp) usually affect immunocompromised patients, hvKp is more concerning because it can infect healthy individuals. These hypervirulent strains often overproduce capsule components and siderophores, which help them acquire iron. They can also cause serious issues in the central nervous system or the eyes.

The history of this organism is tied to important scientific discoveries. The genus was named after the German microbiologist Edwin Klebs, who lived from 1834 to 1913. It is also known as Friedländer's bacillum. This name honors Carl Friedländer, a German pathologist. He proposed that this bacterium was the etiological factor, or the cause, of pneumonia in certain people. This includes those who are immunocompromised, such as individuals with chronic diseases or alcoholism. Because of his work, community-acquired pneumonia caused by this germ is sometimes called Friedländer's pneumonia.

Klebsiella pneumoniae has a significant impact on global health statistics. It ranks as the second most common cause of urinary tract infections in older people. In some regions, the mortality rate is quite high. A 2016 study in Turkey showed that bacteremia, or infection in the blood, had a death rate of around 50% even with treatment. The bacteria are also a major cause of nosocomial infections, which are infections caught while staying in a hospital. They can spread through medical devices like urinary catheters, respiratory support equipment, or neonatal ward devices.

One of the most challenging aspects of Klebsiella is its ability to develop antibiotic resistance. The bacteria often use plasmids, which are small DNA molecules, to share resistance genes. Some strains produce beta-lactamase, an enzyme that makes them resistant to ampicillin. Other strains have acquired extended-spectrum beta-lactamases (ESBL), which resist drugs like amoxicillin and ceftazidime. In 2009, scientists discovered the NDM-1 gene in India and Pakistan. This gene provides resistance against carbapenems, which are very important intravenous antibiotics.

Finally, the bacterium is connected to broader environmental and systemic issues. Research at King's College, London suggests a link between Klebsiella surface molecules and ankylosing spondylitis through molecular mimicry. Beyond humans, hvKp has been found in various environmental reservoirs. It has been isolated from cucumbers in China, pork in Chinese markets, and water sources in Brazil. It was even found in fresh oysters in Egypt in 2024. These findings show how the bacterium can move between the environment, food, and human populations.