Flash card

Key Characteristics of the Family Enterobacteriaceae

General Overview

  • Enterobacteriaceae consists of diverse organisms commonly referred to as enterics.

  • They are all gram-negative bacilli and coccobacilli.

  • Characterized by their ability to ferment glucose.

  • Notably, they do not produce cytochrome oxidase (except for Plesiomonas).

Colony Morphology

  • Colony morphology provides crucial clues for identifying these organisms:

    • Colonies typically appear large, moist, and gray on nonselective media like sheep blood agar (SBA) and chocolate agar.

    • Exception: Some species (e.g., Klebsiella and certain Enterobacter) produce larger and mucoid colonies.

  • On smears, they can appear as coccobacilli or straight rods.

Biochemical Identification

  • Common biochemical tests used for identification include:

    • Glucose fermentation: All members ferment glucose.

    • Nitrate reduction: All reduce nitrate to nitrite except for Photorhabdus and Xenorhabdus.

    • Motility: Most are motile at body temperature (exceptions include Klebsiella, Shigella, and Yersinia).

    • Colony morphology on selective media (like eosin-methylene blue (EMB), MacConkey (MAC), Hektoen enteric (HE), and xylose-lysine-desoxycholate (XLD) agar) can aid in preliminary identification based on sugar fermentation abilities.

Key Terms Related to Enterobacteriaceae

  • Buboes

  • Enterotoxigenic Escherichia coli (ETEC)

  • Diffusely adherent Escherichia coli (DAEC)

  • H antigen

  • K antigen

  • Enteroaggregative Escherichia coli (EAEC)

  • Shiga toxin (Stx)

  • Enterohemorrhagic Escherichia coli (EHEC)

  • Verotoxin

  • Enteroinvasive Escherichia coli (EIEC)

  • Enteropathogenic Escherichia coli (EPEC)

  • Vi antigen

Clinical Relevance

  • Clinical isolates mainly include Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis.

  • Awareness of other species is critical since they can also cause significant infectious diseases.

Classification Approach

  • Classification is regularly updated as new species are discovered and others may be removed.

  • Species are organized into tribes based on biochemical characteristics, enhancing clarity in identification.

Microscopic and Colony Morphology

  • Enterobacteriaceae are described as gram-negative, non-spore-forming, facultatively anaerobic bacilli.

  • Traditional classification has relied heavily on biochemical reactions and DNA homology to determine species-specific characteristics.

Important Biochemical Features

  • Biochemical features differentiating tribes are summarized in specific tables (e.g., Table 19.1 and Table 19.2).

  • The tribal classification approach has enabled effective grouping of organisms based on laboratory characteristics and genetic studies.

Conclusion

  • Understanding the taxonomy, morphology, and biochemical properties of Enterobacteriaceae is essential for diagnosing and treating infections caused by these bacteria.