Klebsiella aerogenes is a Gram-negative bacterium that has become a significant concern in healthcare settings due to its capacity to cause difficult-to-treat infections. This organism was previously known as Enterobacter aerogenes before a 2017 reclassification placed it within the Klebsiella genus. It is considered an opportunistic pathogen, typically causing disease only in individuals with weakened immune systems or those subjected to invasive medical procedures. Its growing ability to resist antibiotics makes managing infections in hospitals and long-term care facilities a major public health challenge.
Identifying the Organism and Its Habitat
Klebsiella aerogenes is a facultatively anaerobic bacterium, meaning it can grow with or without oxygen. It is Gram-negative, possessing a thin layer of peptidoglycan and an outer membrane that provides inherent protection. This bacterium is a natural and common inhabitant of the human gastrointestinal tract, where it generally exists as a harmless member of the gut microbiota.
K. aerogenes is also found widely in the environment, including in soil, water, and various waste materials. It transitions from a benign colonizer to a threat when it breaches the body’s natural barriers or reaches a normally sterile site, such as the bloodstream or lungs.
Spectrum of Clinical Infections
The infections caused by K. aerogenes are predominantly Healthcare-Associated Infections (HAIs), acquired during a patient’s stay in a medical facility. These infections frequently target the respiratory tract, leading to pneumonia, especially ventilator-associated pneumonia in intensive care units (ICUs).
The bacterium is also a common cause of urinary tract infections (UTIs), particularly those associated with the use of urinary catheters. When K. aerogenes enters the bloodstream, it causes bacteremia or sepsis. Furthermore, it can infect surgical sites and wounds, complicating patient recovery and prolonging hospital stays.
Mechanisms Driving Antibiotic Resistance
The primary concern surrounding K. aerogenes is its high propensity for developing multidrug resistance (MDR). One significant mechanism involves the production of extended-spectrum beta-lactamases (ESBLs). ESBLs are enzymes capable of inactivating many common antibiotics, including penicillins and broad-spectrum cephalosporins, which are typically used as first-line treatments.
A more severe resistance mechanism is the acquisition of carbapenemases, which renders the organism resistant to carbapenems, often considered the last resort for treating MDR Gram-negative infections. These enzymes, such as NDM-1 or KPC-2, dismantle the carbapenem molecule, making the drug ineffective. The genes encoding these resistance enzymes are often carried on mobile genetic elements called plasmids.
Plasmids allow the resistance genes to be easily transferred horizontally between different bacteria, rapidly spreading the MDR trait within a hospital environment. Beyond enzyme production, K. aerogenes can also express a chromosomal AmpC \(\beta\)-lactamase. Some strains develop resistance by physically altering their outer membrane, including the loss of porins or the overexpression of efflux pumps that actively push the drug out.
Treatment Protocols and Infection Control
Treating infections caused by multidrug-resistant K. aerogenes requires complex strategies due to limited therapeutic options. Clinicians frequently resort to combination therapy, administering two or more antibiotics simultaneously to increase efficacy. When resistance is confirmed, treatment may rely on last-resort drugs like colistin (a polymyxin), tigecycline, or newly developed \(\beta\)-lactam/\(\beta\)-lactamase inhibitor combinations, such as ceftazidime-avibactam.
The choice of antibiotic must be guided by susceptibility testing and consideration of the drug’s ability to penetrate the specific site of infection. Rigorous infection control measures are necessary to prevent the spread of the organism.
Infection Control Measures
Strict adherence to hand hygiene protocols by all healthcare personnel is a foundational step in limiting transmission. Hospitals implement contact precautions, often involving isolating infected patients to prevent spread. Environmental cleaning and disinfection are important, as the bacterium can survive on high-touch surfaces and medical equipment. Minimizing the use of invasive devices through antimicrobial stewardship programs reduces the opportunity for K. aerogenes to cause disease.