Enterobacterales

Klebsiella aerogenes (antiga Enterobacter aerogenes)

Gram-negative bacilli

Identification & Growth

Identification

Oxidase-negative, motile, ornithine-positive, lysine-positive, urease-negative
TSI A/A with gas

Growth Conditions

MacConkey agar: lactose-fermenting colonies, 35–37 °C, 24 h

Clinical Significance

Hospital-acquired respiratory, urinary and bloodstream infection

Intrinsic Resistance

Ampicillin, amoxicillin-clavulanate, cefalotin and cefoxitin (chromosomal AmpC, EUCAST Expected Resistant Phenotypes v1.2)

Bench Alerts

Inducible AmpC: a strain initially susceptible to ceftriaxone may become resistant during therapy
Prefer cefepime or a carbapenem in severe infection

Clinical Notes

Clinical presentations

Causes hospital infections such as ventilator-associated pneumonia, bacteremia, and complicated urinary tract infection.
Associated with surgical wound infection and intravascular device infection in patients with prolonged hospitalization.

Specimens and collection

Tracheal aspirate or bronchoalveolar lavage should be obtained in suspected ventilator-associated pneumonia, using technique that minimizes upper airway contamination.
Blood cultures should precede any empiric antibiotic adjustment in a hospitalized patient with signs of sepsis.

Epidemiology and at-risk populations

It was taxonomically reclassified from Enterobacter to Klebsiella based on genomic analyses but retains clinical behavior similar to the Enterobacter cloacae complex.
Included in carbapenem-resistant Enterobacterales (CRE) surveillance lists due to its capacity to acquire carbapenemases.
Predominantly affects ICU patients with prior broad-spectrum antimicrobial exposure.

Resistance and therapeutic implications

Carries an inducible chromosomal AmpC beta-lactamase, conferring expected resistance to ampicillin, amoxicillin-clavulanate, and first/second-generation cephalosporins.
Can develop high-level resistance to third-generation cephalosporins during therapy via ampC derepression, even with initial in vitro susceptibility.
Isolates with acquired carbapenemase require laboratory confirmation and pose a hospital dissemination risk similar to Klebsiella pneumoniae.

Bench and reporting notes

Gram-negative rod, lactose-fermenting, motility-positive, which helps phenotypically differentiate it from classic (nonmotile) Klebsiella pneumoniae.
The report should warn that initially susceptible third-generation cephalosporins may fail clinically due to AmpC derepression.
Correct species-level identification (MALDI-TOF) is important for CRE epidemiologic surveillance, given the recent taxonomic reclassification.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Klebsiella aerogenes (antiga Enterobacter aerogenes) in the lab?

Klebsiella aerogenes (antiga Enterobacter aerogenes) is identified through oxidase-negative, motile, ornithine-positive, lysine-positive, urease-negative, tsi a/a with gas.

What are the intrinsic resistances of Klebsiella aerogenes (antiga Enterobacter aerogenes)?

This organism is naturally resistant to ampicillin, amoxicillin-clavulanate, cefalotin and cefoxitin (chromosomal ampc, eucast expected resistant phenotypes v1.2). These drugs should not be reported as susceptible.

Where is Klebsiella aerogenes (antiga Enterobacter aerogenes) commonly found?

It is typically associated with hospital-acquired respiratory, urinary and bloodstream infection.

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