Enterobacter species are Gram-negative, rod-shaped bacteria belonging to the Enterobacteriaceae family. They are commonly found in the environment and the human intestinal tract, but they are clinically significant as opportunistic pathogens. Species like E. cloacae and E. aerogenes frequently cause hospital-acquired infections, affecting the bloodstream, urinary tract, and lower respiratory tract. Accurate identification is necessary because many strains exhibit multi-drug resistance, making effective treatment dependent on a precise diagnosis.
Initial Biochemical Screening Tests
Identifying Enterobacter begins by confirming its membership in the Enterobacteriaceae family and establishing its general metabolic profile. This requires a negative Oxidase test and a positive reaction for nitrate reduction. Most Enterobacter species are motile, a characteristic tested by observing growth spread in semi-solid agar. The core metabolic assessment uses the IMViC battery of tests (Indole, Methyl Red, Voges-Proskauer, and Citrate utilization). Enterobacter typically shows an IMViC pattern of Indole negative, Methyl Red negative, Voges-Proskauer positive, and Citrate positive, reflecting a preference for the butanediol pathway of glucose fermentation.
Key Reactions for Genus Differentiation
Comparative tests distinguish the Enterobacter genus from close relatives like Klebsiella, Citrobacter, and Serratia. Motility is a primary distinction, as Enterobacter is generally motile, unlike the non-motile Klebsiella pneumoniae. The Ornithine Decarboxylase (ODC) test is also important; most Enterobacter species are ODC positive, converting ornithine to an alkaline product, unlike K. pneumoniae. The Urease test helps exclude strongly positive Proteus species. While some Enterobacter strains may show a weak, delayed positive urease result, they are consistently negative for Phenylalanine Deaminase (PAD) and Hydrogen Sulfide (H2S) production. This negative pattern separates Enterobacter from the PAD-positive Proteus group and H2S-positive Salmonella and Citrobacter species.
Specialized Tests for Species Identification
The next phase focuses on distinguishing among clinically relevant species, primarily E. cloacae, E. aerogenes, and E. hormaechei. The decarboxylase activity for Lysine (LDC) and Ornithine (ODC) provides a discriminatory fingerprint. For example, E. aerogenes is LDC positive and ODC positive, while E. cloacae is LDC negative but ODC positive. Differentiation is further refined through specific carbohydrate fermentation patterns, testing the organism’s ability to utilize substrates like adonitol, sorbitol, and dulcitol. The E. hormaechei complex demonstrates unique sugar utilization profiles essential for precise identification. These numerous reactions, which form a species-specific metabolic profile, are often consolidated into commercial miniaturized test systems, such as API 20E strips, allowing for the simultaneous testing of multiple substrates.
Modern Confirmation Techniques
Modern clinical microbiology laboratories often incorporate automated and molecular methods for rapid confirmation. Automated systems, such as Vitek or Phoenix platforms, accelerate the process by simultaneously reading large panels of biochemical reactions. These systems use internal databases to interpret results and provide species-level identification within hours. Matrix-Assisted Laser Desorption/Ionization-Time of Flight (MALDI-TOF) Mass Spectrometry identifies bacteria by analyzing their unique protein signature. The resulting spectral pattern is matched against an extensive reference library, providing highly accurate identification within minutes. For challenging or novel strains, molecular methods, such as Polymerase Chain Reaction (PCR) or gene sequencing, remain the gold standard for definitive confirmation.