Enterobacter aerogenes

Enterobacter aerogenes, now known as Klebsiella aerogenes, is a Gram-negative bacterium recognized for its adaptability, hospital-associated infections, and multidrug resistance. As a member of the ESKAPE group, it poses a significant clinical threat due to its ability to evade antibiotics and thrive in vulnerable or immunocompromised patients.

What is Enterobacter aerogenes?

Enterobacter aerogenes, now reclassified as Klebsiella aerogenes, is a bacterium in the Enterobacteriaceae family. It is considered part of the ESKAPE group—a group of bacteria known for their ability to "escape" the effects of antibiotics and cause serious hospital-acquired infections.

Enterobacter aerogenes: A Gram-Negative Bacterium

E. aerogenes is a Gram-negative, rod-shaped bacterium that can survive with or without oxygen. This makes it a facultative anaerobe, which means it is adaptable to a variety of environments.

Habitat

It is widely present in soil, water, sewage, and hospital environments and can also be found in the human gut as part of the normal flora. While typically harmless in healthy individuals, it can become dangerous in vulnerable patients.

When is Enterobacter aerogenes Detection Relevant?

Enterobacter aerogenes testing may be relevant in the following settings:

Diagnosis of Infections

E. aerogenes is an opportunistic pathogen, meaning it causes infections primarily in people who are already ill or immunocompromised. It is especially significant in hospital settings, where it has been linked to:

• Urinary tract infections (UTIs)
• Pneumonia
• Wound infections
• Bloodstream infections (bacteremia)
• Sepsis, particularly in ICU patients or those on mechanical ventilation

Microbiological Identification

It is identified through standard clinical cultures and biochemical tests. On MacConkey agar, it forms lactose-fermenting colonies, and its motility helps differentiate it from other Enterobacter species.

Detection is not part of routine screening. It becomes relevant when an infection is suspected based on symptoms and risk factors.

Testing and Identification of Enterobacter aerogenes

The following information describes general testing methods with concern for Enterobacter infections:

Sample Types

The following sample types may be used, depending on concern for infection location:

• Blood – Two sets of blood cultures (aerobic and anaerobic) are recommended for suspected bacteremia or sepsis.
• Urine – Used for urinary tract infections.
• Respiratory specimens – Such as sputum for pneumonia diagnosis.
• Wound or surgical site swabs – For localized infections.

Primary Diagnostic Method

• Culture is the gold standard for diagnosing Enterobacter infections.

Additional Supportive Tests

Additional tests that may be considered include:

• Urinalysis with culture
• Complete blood count (CBC)
• Comprehensive metabolic panel
• Imaging (e.g., chest x-ray, ultrasound) depending on the infection site.

Gut Microbiome Testing for Enterobacter aerogenes: Procedure and Interpretation

Microbiome testing is increasingly performed to assess microbial balance and gut health. 

Microbiome testing that includes Enterobacter aerogenes is typically done through stool sample analysis. This test can be conducted using various methods, such as PCR, next-generation sequencing (NGS), or culture-based techniques. 

A stool sample is collected and analyzed to quantify the presence of Enterobacter aerogenes and assess its relative abundance compared to other gut bacteria.

Always consult the ordering provider or laboratory company before sample collection, as special preparation, such as avoiding antibiotics or probiotics, may be necessary in the days leading up to sample collection.

Normal Reference Range

Normal reference ranges for Enterobacter aerogenes can vary based on the laboratory and the microbiome composition of the healthy population. Generally, the goal is to maintain a balanced ratio of Enterobacter aerogenes alongside other beneficial gut flora.

What Does the Presence of Enterobacter aerogenes in a Clinical Sample Mean?

Finding E. aerogenes in a sample (urine, blood, wound swab) usually indicates infection, especially if the patient has signs of illness. However, in some cases—such as in the gut—it may just be colonization without disease.

Importance of Susceptibility Testing

Because this organism is known for multidrug resistance (MDR), antibiotic susceptibility testing is essential to guide treatment. The organism can resist multiple classes of antibiotics, and selecting the right therapy is critical.

Context Matters

The meaning of a positive result depends on the type of sample, site of infection, and the patient’s overall condition. Not every detection warrants treatment—clinical judgment is key.

What Does the Absence of Enterobacter aerogenes in a Clinical Sample Mean?

The absence of E. aerogenes does not rule out infection. Other pathogens may be responsible; further investigation may be needed if symptoms persist.

Antibiotic Resistance in Enterobacter aerogenes

E. aerogenes is a model organism for antibiotic resistance. Its ability to resist treatment is due to several mechanisms:

• AmpC β-lactamase production: Provides resistance to first- and second-generation cephalosporins.
• Extended-spectrum β-lactamases (ESBLs): Especially TEM-24 and CTX-M types, which degrade newer cephalosporins.
• Carbapenemases: Enzymes like KPC, NDM, and OXA-48 make it resistant to carbapenems.
• Efflux pumps: These actively remove antibiotics from the bacterial cell.
• Porin mutations: Changes in outer membrane proteins (Omp35/36) reduce drug entry.
• Aminoglycoside-modifying enzymes: Limit the effect of drugs like gentamicin and amikacin.
• Fluoroquinolone resistance: Caused by mutations and plasmid-mediated mechanisms.

These features make E. aerogenes difficult to treat and a serious concern in hospitals.

Treatment Considerations

The following treatments are considered in an Enterobacter infection:

First-line Therapies

Carbapenems (e.g., meropenem) are often used but may be ineffective due to rising resistance.

Cefepime, a fourth-generation cephalosporin, may still be active in some cases.

Alternatives

New antibiotics like ceftazidime-avibactam, meropenem-vaborbactam, and cefiderocol are used when resistance is confirmed.

Aminoglycosides and fosfomycin may be options in urinary tract infections.

Challenges

Resistance to multiple drug classes is increasingly common.

In bloodstream infections, E. aerogenes is associated with higher complication rates and worse outcomes compared to similar bacteria like Enterobacter cloacae.

Cases of pan-resistant strains—resistant to all antibiotics, including colistin—have been reported.

Clinical Relevance Beyond Infections

Recent research suggests E. aerogenes may affect hormone levels and mental health. In one study, a gut strain of E. aerogenes was linked to lower estradiol levels and depression-like behavior in women. The bacterium's 3β-HSD enzyme breaks down estrogen in the gut, offering a new view of its role in human health.

Prevention and Infection Control

Hand hygiene, device care, and judicious antibiotic use are critical.

Genetic tracking helps monitor resistance and outbreaks.

Multidisciplinary teams, including infectious disease experts, pharmacists, and microbiologists are vital for complex cases.

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