Enterobacter cloacae complex: clinical impact and emerging antibiotic resistance.

Species of the Enterobacter cloacae complex are widely encountered in nature,but they can act as pathogens. The biochemical and molecular studies onE. cloacae have shown genomic heterogeneity, comprising six species :Enterobacter cloacae, Enterobacter asburiae, Enterobacter hormaechei,Enterobacter kobei, Enterobacter ludwigii and Enterobacter nimipressuralis,E. cloacae and E. hormaechei are the most frequently isolated in human clinicalspecimens. Phenotypic identification of all species belonging to this taxon isusually difficult and not always reliable; therefore, molecular methods are oftenused. Although the E. cloacae complex strains are among the most commonEnterobacter spp. causing nosocomial bloodstream infections in the last decade,little is known about their virulence-associated properties. By contrast, much hasbeen published on the antibiotic-resistance features of these microorganisms.In fact, they are capable of overproducing AmpC b-lactamases by derepressionof a chromosomal gene or by the acquisition of a transferable ampC gene onplasmids conferring the antibiotic resistance. Many other resistance determinantsthat are able to render ineffective almost all antibiotic families have beenrecently acquired. Most studies on antimicrobial susceptibility are focused onE. cloacae, E. hormaechei and E. asburiae; these studies reported small variationsbetween the species, and the only significant differences had no discriminatingfeatures.