Enterobacter

Enterobacter, (genus Enterobacter), any of a group of rod-shaped bacteria of the family Enterobacteriaceae. Enterobacter are gram-negative bacteria that are classified as facultative anaerobes, which means that they are able to thrive in both aerobic and anaerobic environments. Many species possess flagella and thus are motile. Features such as motility, as well as certain biochemical properties, including the ability to synthesize an enzyme known as ornithine decarboxylase, are used to distinguish Enterobacter from the very similar and closely related Klebsiella bacteria. Enterobacter is named for the organisms’ predominant natural habitat, the intestines of animals (from Greek enteron, meaning “intestine”).

Enterobacter are ubiquitous in nature; their presence in the intestinal tracts of animals results in their wide distribution in soil, water, and sewage. They are also found in plants. In humans, multiple Enterobacter species are known to act as opportunistic pathogens (disease-causing organisms), including E. cloacae, E. aerogenes, E. gergoviae, and E. agglomerans. Pathogenic Enterobacter can cause any of a variety of conditions, including eye and skin infections, meningitis, bacteremia (bacterial blood infection), pneumonia, and urinary tract infections. In many instances, illness caused by E. cloacae or by E. aerogenes is associated with exposure to the organisms in nosocomial settings, such as hospitals or nursing homes.

The emergence of drug-resistant Enterobacter organisms has complicated treatment regimens, particularly within nosocomial settings, where such organisms have become increasingly common. Traditional approaches to treating Enterobacter infections involve single-agent antimicrobial therapy, typically with an aminoglycoside, a fluoroquinolone, a cephalosporin, or imipenem. In some instances, however, subpopulations of Enterobacter are capable of producing enzymes known as beta-lactamases, which cleave the central ring structure responsible for the activity of beta-lactam antibiotics, a group that includes imipenem (a type of carbapenem) and cephalosporins. Repeated exposure to these drugs selects for beta-lactamase-synthesizing Enterobacter, thereby giving rise to drug resistance, including against carbapenems, which were once highly effective against multidrug-resistant organisms. Newer approaches to Enterobacter infections have adopted combination-therapy regimens employing multiple antibiotics with different core structures, such as an aminoglycoside or a fluoroquinolone in combination with a beta-lactam agent. Despite the promise of this more diverse strategy, however, it has been associated with the selection of multidrug-resistant organisms.

Resistance of Enterobacter to non-beta-lactam antibiotics, including fluoroquinolones such as ciprofloxacin, involves distinct cellular and genetic mechanisms. Examples of bacteria utilizing such mechanisms include ciprofloxacin-resistant E. aerogenes and multidrug-resistant E. aerogenes, which in many instances is resistant to ciprofloxacin and imipenem. In Enterobacter organisms resistant to aminoglycosides, resistance has been associated with a bacterial genetic element known as an integron. Integrons contain genes that confer antibiotic resistance capabilities and are incorporated into bacterial genomes via genetic recombination. They are efficiently exchanged and disseminated among circulating bacterial populations, such as those occurring in nosocomial environments. In E. cloacae resistance to the aminoglycoside gentamicin has been attributed to the presence of integrons in the organism’s genome.

Free-living Enterobacter are capable of nitrogen fixation. Certain species, notably E. cloacae, are involved in symbiotic nitrogen fixation in plants and have been isolated from the root nodules of certain crops, such as wheat and sorghum, and from the rhizospheres of rice.