Enterobacter species
MICROBIOLOGY
- Gram-negative, aerobic, motile bacilli of the Enterobacteriaceae family that ferments lactose and forms mucoid colonies. Twenty-two species belong to the Enterobacter genus.[11]Enterobacter spp. are commensals of the human gut and are commonly found in water, sewage, and soil.
- Opportunistic human pathogens include E. cloacae (most common), E. aerogenes (renamed Klebsiella aerogenes), E. gergoviae[20] , and Pantoea agglomerans.
- E. sakazakii is now classified as Cronobacter.[13]
- High levels of drug resistance are often seen and are due to:
- AmpC β-lactamases - Ambler class C
- Chromosomal AmpC β-lactamases can be constitutive (always active) or inducible (variably active) and are not inhibited by β-lactam β-lactamase inhibitors.
- Enterobacter spp. have intrinsic resistance to ampicillin, amoxicillin, first-generation cephalosporins, and cefoxitin due to expression of constituitive AmpC β-lactamase.[11]
- β-lactams must be present to activate inducible β-lactamases, so initial susceptibility reports may not detect resistance that can emerge during therapy. In the absence of β-lactams, AmpR, a regulatory protein, reduces or represses AmpC β-lactamase expression to very low levels.[12]
- Certain β-lactams induce the production of cell wall degradation products and decrease AmpR repression of AmpC, which results in increased ampC transcription and AmpC expression.
- ’Potent inducers’ of AmpC production include aminopenicillins, amoxicillin-clavulanate, narrow-spectrum cephalosporins, and cephamycins. AmpC producers, such as E. cloacae, can hydrolyze these agents and are intrinsically resistant.[12]
- Imipenem is a potent inducer but remains stable against hydrolysis due to forming an acyl-enzyme complex.
- ’Weak inducers’ of AmpC production include piperacillin-tazobactam, aztreonam, and extended-spectrum cephalosporins, and these antibiotics can be hydrolyzed to a variable extent depending on the amount of β-lactamase production. This results in increased drug-specific MICs.[12]
- Cefepime is a weak inducer and can withstand hydrolysis by AmpC β-lactamases due to forming a stable acyl-enzyme complex.
- ’Potent inducers’ of AmpC production include aminopenicillins, amoxicillin-clavulanate, narrow-spectrum cephalosporins, and cephamycins. AmpC producers, such as E. cloacae, can hydrolyze these agents and are intrinsically resistant.[12]
- Plasmid-mediated
- Phenotypic assays cannot distinguish between AmpC β-lactamase production due to derepression of chromosomal versus plasmid-associated ampC gene.
- Chromosomal AmpC β-lactamases can be constitutive (always active) or inducible (variably active) and are not inhibited by β-lactam β-lactamase inhibitors.
- Plasmid-encoded extended-spectrum β-lactamases (ESBLs)
- ESBL genes include blaCTX-M, blaSHV, and blaTEM. Commercially available molecular platforms limited to detection blaCTX-M.
- Most often, such organisms demonstrate elevated MICs to cefepime.
- ESBLs inactivate most penicillins, cephalosporins, and aztreonam.[1]
- Carbapenemases[16]
- Ambler class A - Most common are Klebsiella pneumoniae carbapenemases (KPC) that any Enterobacterales can produce.
- Ambler class B - Metallo-β-lactamases include New Delhi (NDMs), Verona integron-encoded (VIM), and imipenem-hydrolyzing (IMPs)
- Ambler class D - Oxacillinase (OXA-48-like) carbapenemases
- AmpC β-lactamases - Ambler class C
- Ceftriaxone MIC ≥ 2 is used as a proxy for ESBL production.
- Other resistance mechanisms
- Alterations in the active site of penicillin-binding protein
- Defects in outer membrane permeability that reduce diffusion of β-lactams into the cell
- The presence of efflux pumps that move β-lactams out of the cell
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Last updated: February 10, 2024
Citation
Spacek, Lisa A. "Enterobacter Species." Johns Hopkins ABX Guide, The Johns Hopkins University, 2024. Pediatrics Central, peds.unboundmedicine.com/pedscentral/view/Johns_Hopkins_ABX_Guide/540201/all/Enterobacter_species.
Spacek LA. Enterobacter species. Johns Hopkins ABX Guide. The Johns Hopkins University; 2024. https://peds.unboundmedicine.com/pedscentral/view/Johns_Hopkins_ABX_Guide/540201/all/Enterobacter_species. Accessed November 1, 2024.
Spacek, L. A. (2024). Enterobacter species. In Johns Hopkins ABX Guide. The Johns Hopkins University. https://peds.unboundmedicine.com/pedscentral/view/Johns_Hopkins_ABX_Guide/540201/all/Enterobacter_species
Spacek LA. Enterobacter Species [Internet]. In: Johns Hopkins ABX Guide. The Johns Hopkins University; 2024. [cited 2024 November 01]. Available from: https://peds.unboundmedicine.com/pedscentral/view/Johns_Hopkins_ABX_Guide/540201/all/Enterobacter_species.
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