# Enterobacter cloacae
**Enterobacter cloacae** is a Gram-negative, facultatively anaerobic, rod-shaped bacterium of the family [Enterobacteriaceae](https://www.edgepedia.org/enterobacteriaceae) (order Enterobacterales) that lives in the human intestine and is an important cause of hospital-acquired infections, including bloodstream, urinary, respiratory, and wound infections. It is the type species of the Enterobacter cloacae complex, a group of closely related species that clinical laboratories often report collectively, and it carries an inducible chromosomal AmpC β-lactamase that makes antibiotic treatment unusually prone to failure during therapy.
| Key fact | Detail |
|---|---|
| Classification | Family Enterobacteriaceae, order Enterobacterales; NCBI taxonomy ID 550[1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=550&log_op=modifier_toggle&lvl=3&mod=1&mode=Info) |
| Intrinsic resistance | Resistant to ampicillin, amoxicillin, first-generation cephalosporins, and cefoxitin through inducible AmpC β-lactamase production[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4435039/) |
| Treatment hazard | Emergence of resistance to third-generation cephalosporins during therapy, reported in about 19% to 20% of infections in observational studies[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6763639/) • [4](https://www.idsociety.org/practice-guideline/amr-guidance?n=%40) |
| Reliable agents | Cefepime and carbapenems retain activity against AmpC producers lacking other β-lactamases[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6763639/) |
| Species complex | Includes E. asburiae, E. hormaechei, E. kobei, E. nimipressuralis, E. cloacae, and others; E. hormaechei is most often isolated from human clinical specimens[5](https://journals.asm.org/doi/10.1128/CMR.00002-19) |
| Clinical burden | In German tertiary-care surveillance, the most common specimen sources were swabs (42.7%), urine (17.5%), respiratory secretions (16.1%), and blood cultures (8.9%)[6](https://pubmed.ncbi.nlm.nih.gov/38554809/) |
| Emerging threat | Carbapenem-resistant E. cloacae complex has emerged globally with various carbapenemases, prompting a European genomic surveillance pilot survey planned for 2027[7](https://www.ecdc.europa.eu/en/publications-data/protocol-carbapenem-resistant-enterobacter-cloacae-complex-crecc-pilot-survey) |
## Taxonomy and the E. cloacae complex
What laboratories call E. cloacae is usually not a single species. DNA-[DNA hybridization](https://www.edgepedia.org/dna-hybridization) studies showed that organisms identified phenotypically as E. cloacae actually compose multiple hybridization groups, each a distinct genomospecies that cannot be separated by routine biochemical tests; the name therefore designates a "phenospecies" or complex rather than E. cloacae sensu stricto ('in the strict sense' — the species narrowly defined)[8](https://doi.org/10.1128/CMR.00174-20).
The Enterobacter cloacae complex includes [Enterobacter](https://www.edgepedia.org/enterobacter) asburiae, E. carcinogenus, E. cloacae, E. hormaechei, E. kobei, E. nimipressuralis, and E. mori, all genotypically close with more than 60% DNA-DNA homology[5](https://journals.asm.org/doi/10.1128/CMR.00002-19). A phylogenetic study based on four housekeeping genes resolved the complex into distinct clusters, including E. cloacae subsp. cloacae and subsp. dissolvens, E. hormaechei with three subspecies, and several unnamed clusters[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC6365427/). E. cloacae and E. hormaechei are the species most frequently isolated from human clinical specimens, and most clinical E. cloacae isolates are in fact E. hormaechei strains[5](https://journals.asm.org/doi/10.1128/CMR.00002-19) • [1](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=550&log_op=modifier_toggle&lvl=3&mod=1&mode=Info). Species-level identification errors within the complex have no practical impact on therapy, because members share the same antibiotic resistance profiles[5](https://journals.asm.org/doi/10.1128/CMR.00002-19).
## The AmpC β-lactamase problem
E. cloacae carries a chromosomal ampC gene encoding a class C β-lactamase, an enzyme with a serine residue at its active site. At baseline, the regulatory protein AmpR holds expression to very low levels, which explains the bacterium's intrinsic resistance to ampicillin, amoxicillin, first-generation cephalosporins, and cefoxitin[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4435039/) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6763639/).
**Induction during therapy.** Exposure to certain β-lactams increases production of cell-wall degradation products that compete for binding to AmpR, lifting repression and raising AmpC output; after drug exposure stops, production generally returns to baseline[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6763639/). Mutations in regulatory genes, most commonly ampD, produce stable derepression, meaning the bacterium overproduces AmpC even without a β-lactam trigger[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6763639/) • [9](https://pmc.ncbi.nlm.nih.gov/articles/PMC6365427/). Because β-lactam therapy eliminates susceptible subpopulations, it selects for these stably derepressed mutants[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6763639/).
The clinical consequence is that an isolate reported susceptible at the start of treatment can become resistant within days. In a landmark 1991 study, 19% of patients treated with ceftriaxone for initially susceptible Enterobacter bloodstream infections developed resistance during therapy[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6763639/). The 2026 IDSA guidance on antimicrobial-resistant gram-negative infections states that clinically significant ampC expression during treatment is most frequently documented for the E. cloacae complex, [Klebsiella](https://www.edgepedia.org/klebsiella) aerogenes, and [Citrobacter freundii](https://www.edgepedia.org/citrobacter-freundii), and that resistance emergence after exposure to agents such as ceftriaxone may occur in about 20% of infections caused by these organisms[4](https://www.idsociety.org/practice-guideline/amr-guidance?n=%40). For this reason, most experts advise avoiding third-generation cephalosporins for invasive E. cloacae complex infections, except in uncomplicated urinary tract infections[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6763639/).
**Drug-specific behavior.** Aminopenicillins, amoxicillin-clavulanate, first-generation cephalosporins, and cephamycins are potent inducers of the pathway, but E. cloacae hydrolyzes them even at basal expression levels. Third-generation cephalosporins, piperacillin-tazobactam, and aztreonam are weak inducers but can be hydrolyzed when enough enzyme is made. Cefepime is both a weak inducer and stable against AmpC hydrolysis, and carbapenems such as imipenem, though potent inducers, also resist hydrolysis[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC6763639/). Fourth-generation cephalosporins retain reasonable activity against derepressed strains, unless the strain also produces an extended-spectrum β-lactamase (ESBL)[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4435039/). ESBL and CTX-M producers represented roughly 5% of isolates in recent studies[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4435039/).
## Clinical significance
E. cloacae complex organisms are increasingly recognized as major opportunistic pathogens in healthcare-associated infections, particularly in intensive care settings[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC12674772/). In the EC-COMPASS passive surveillance study across four German tertiary-care hospitals, analysis of 31,193 ECC datasets from 14,311 patients found a higher incidence in male patients, with swabs, urine, respiratory secretions, and blood cultures as the leading specimen sources[6](https://pubmed.ncbi.nlm.nih.gov/38554809/).
**Bloodstream infection outcomes.** Studies comparing outcomes with the closely related Klebsiella aerogenes (formerly [Enterobacter aerogenes](https://www.edgepedia.org/enterobacter-aerogenes)) have yielded conflicting results. One retrospective study found 30-day mortality of 24.1% for E. cloacae complex bacteremia versus 10.6% for K. aerogenes bacteremia, with E. cloacae complex an independent risk factor for death[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC8364985/). A large Australian cohort, by contrast, found nearly identical 30-day case-fatality (13.6% versus 13.5%), though E. cloacae patients had almost four times the risk of recurrent bloodstream infection (relative risk 3.59)[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC10545569/). A Spanish prospective cohort likewise found similar mortality (19.4% versus 20.2%) and low rates of multidrug resistance or ESBL production[13](https://pubmed.ncbi.nlm.nih.gov/33964492/).
**Outbreaks and reservoirs.** Outbreaks are often traced to contaminated water systems, medical devices, or healthcare workers' hands. In a 2023 outbreak at a trauma center in northern India, whole-genome sequencing linked 19 clinical infections to 52 environmental isolates (60% from healthcare worker hand swabs, 40% from hospital water) with more than 99.8% identity between clinical and environmental strains; after hand hygiene reinforcement, water decontamination, and central-line care bundles, incidence fell from 15.8 to 6.7 cases per 1,000 patient-days[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC12674772/).
## Carbapenem resistance
Carbapenems have been the most reliable treatment for AmpC-hyperproducing strains, but carbapenem-resistant E. cloacae complex has emerged globally and is associated with various carbapenemases. Several European countries have reported increasing cases, outbreaks, and dissemination of carbapenemase-producing strains, including a cross-border outbreak of E. hormaechei ST1344 carrying blaNDM-5 traced by the European Antimicrobial Resistance Genes Surveillance Network. At a 2025 network meeting, participants selected carbapenem-resistant E. cloacae complex as the most relevant carbapenem-resistant Enterobacterales for a European-level genomic surveillance pilot survey planned for 2027[7](https://www.ecdc.europa.eu/en/publications-data/protocol-carbapenem-resistant-enterobacter-cloacae-complex-crecc-pilot-survey). In Japan, K. aerogenes and the E. cloacae complex were the major species among non-carbapenemase-producing carbapenem-resistant Enterobacterales, where resistance arises through other mechanisms such as AmpC hyperproduction combined with reduced permeability[14](https://www.sciencedirect.com/science/article/am/pii/S2213716522000820).
## References
1. [Taxonomy Browser: Enterobacter cloacae, NCBI](https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=550&log_op=modifier_toggle&lvl=3&mod=1&mode=Info)
2. [Enterobacter aerogenes and Enterobacter cloacae; versatile bacterial pathogens confronting antibiotic treatment, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4435039/)
3. [Tamma PD et al. A Primer on AmpC β-Lactamases. Clinical Infectious Diseases 2019, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6763639/)
4. [IDSA 2026 Guidance on the Treatment of Antimicrobial Resistant Gram-Negative Infections](https://www.idsociety.org/practice-guideline/amr-guidance?n=%40)
5. [Enterobacter spp.: Update on Taxonomy, Clinical Aspects, and Emerging Antimicrobial Resistance, Clinical Microbiology Reviews](https://journals.asm.org/doi/10.1128/CMR.00002-19)
6. [The EC-COMPASS: Long-term, multi-centre surveillance of Enterobacter cloacae complex, PubMed](https://pubmed.ncbi.nlm.nih.gov/38554809/)
7. [Protocol for the carbapenem-resistant Enterobacter cloacae complex (CRECC) pilot survey, ECDC](https://www.ecdc.europa.eu/en/publications-data/protocol-carbapenem-resistant-enterobacter-cloacae-complex-crecc-pilot-survey)
8. [The Changing Face of the Family Enterobacteriaceae, Clinical Microbiology Reviews](https://doi.org/10.1128/CMR.00174-20)
9. [Multidrug-Resistant Enterobacter cloacae Complex Emerging as a Global, Diversifying Threat, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6365427/)
10. [Multi-source environmental reservoirs drive Enterobacter cloacae complex transmission, JAC-Antimicrobial Resistance, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12674772/)
11. [Difference in the Clinical Outcome of Bloodstream Infections Caused by Klebsiella aerogenes and Enterobacter cloacae Complex, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8364985/)
12. [Significant clinical differences but not outcomes between Klebsiella aerogenes and Enterobacter cloacae bloodstream infections, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10545569/)
13. [Clinical characteristics and outcome of bacteraemia caused by Enterobacter cloacae and Klebsiella aerogenes, PubMed](https://pubmed.ncbi.nlm.nih.gov/33964492/)
14. [Clinical characteristics and treatment outcomes of carbapenem-resistant Enterobacterales infections in Japan, ScienceDirect](https://www.sciencedirect.com/science/article/am/pii/S2213716522000820)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Bacteria › Medically important pathogenic bacteria*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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