Enterobacteriaceae Family

The Enterobacteriaceae family contains a large number of genera that are biochemically and genetically related to one another. Taxonomically, the Enterobacteriaceae family currently has 53 genera and over 170 named species, of these 26 genera are known to be associated with infections in humans. Many of the familiar bacteria are found in this family e.g. Escherichia coli, Shigella, Salmonella, Enterobacter, Proteus, Yersinia, etc.

LF and NLF colonies in MacConkey Agar
LF and NLF colonies in MacConkey Agar

Common Characteristics

  • They are gram-negative, short rods
  • They are non-sporulating, facultative anaerobes
  • These organisms have simple nutritional requirements and MacConkey agar is used to isolate and differentiate organisms of the Enterobacteriaceae family (pink-colored colonies of lactose fermenter-coliforms and pale-colored colonies of the non-lactose fermenter)
  • Motility if present is by means of peritrichous (lateral) flagella, except Shigella and Klebsiella which are non-motile.
  • Most of the species are catalase-positive (there are few exceptions that can be utilized for the identification/differentiation of organisms)
  • Cytochrome C oxidase negative (enteric always negative-separates enterics from oxidase-positive bacteria of genera Pseudomonas, Aeromonas, Vibrio, Alcaligenes, Achromobacter, Flavobacterium, Cardiobacterium which may have similar morphology.)
  • Usually reduces nitrate to nitrite (distinguishes enteric bacteria from bacteria that reduce nitrate to nitrogen gas, such as Pseudomonas and many other oxidase-positive bacteria).
  • Produces acid from glucose; ability to ferment lactose- distinguishes enteric from obligately aerobic bacteria.
  • Sodium is neither required nor stimulatory for the growth
  • The cell contains a characteristic antigen, called the enterobacterial common antigen.
Antigens of Enterobacteriaceae family
Antigens of the Enterobacteriaceae family

Antigens of Enterobacteriaceae are:

  1. O: Outer membrane
  2. H: Flagella
  3. K: Capsule
  4. Vi: Capsule of Salmonella

Medically Important Genera

  1. Citrobacter species
  2. Enterobacter species
  3. Escherichia species
  4. Hafnia species
  5. Klebsiella species
  6. Morganella species
  7. Plesiomonas shigelloides
  8. Proteus species
  9. Providencia species
  10. Salmonella species
  11. Serratia species
  12. Shigella species
  13. Yersinia species

List of lactose fermenter and non-lactose fermenter

Lactose fermenters: (CEEK)

Mnemonic Lactose Fermenter and NLF
  1. Citrobacter
  2. Escherichia
  3. Enterobacter
  4. Klebsiella

Non lactose fermenter (ShYPS)

  1. Shigella
  2. Yersinia
  3. Proteus
  4. Salmonella

Primary Isolation Media

  1. Blood Agar (BA)
  2. MacConkey (MAC) agar
  3. Cystine-lactose-electrolyte deficient (CLED) agar
  4. Desoxycholate citrate agar (DCA)
  5. Xylose-lysine-desoxycholate agar (XLD)
  6. Brilliant Green Agar (BGA)
  7. Cefixime-tellurite-sorbitol-MacConkey (CT-SMAC) agar
  8. Thiosulphate-citrate-bile salt (TCBS) agar
  9. Cefsulodin-Irgasan-novobiocin (CIN) agar
  10. Chromogenic media

Colonial Appearance

Name of the culture mediaColony morphology
Blood agarColonies are 2-3 mm in diameter, low, convex, grey, smooth, or mucoid, and may be hemolytic or swarming.
MACColonies may appear pink (lactose fermenting) or colorless (lactose non fermenting), size and shape vary with individual species
CLEDColonies may appear yellow (lactose fermenting) or blue (lactose nonfermenting), size and shape vary with individual species.
DCAColonies may appear pink (lactose fermenting) or colorless (lactose nonfermenting) and may have a black center (H2S producers).
XLDColonies may appear yellow (xylose, lactose, or sucrose fermenting) or pink (non-fermenting) and may have a black center (H2S producers).
BGAColonies appear as red-pink, 1-3mm in diameter, surrounded by brilliant red zones in the agar.
CT-SMACColonies may appear pink (sorbitol fermenting) or colorless (sorbitol nonfermenting).
TCBSColonies may appear yellow (sucrose fermenting) or blue-green (sucrose nonfermenting).
CINColonies may have deep-red centers (mannitol fermenting) surrounded by a translucent border giving the appearance of a “bull’s eye”.

Tests for the Identification of Enterobacteriaceae Family 

Members of the Enterobacteriaceae family are identified based on their biochemical properties. Commonly used biochemical tests are;

  1. Citrate utilization Test
  2. Indole Test
  3. Motility Test
  4. Methyl Red (MR) Test
  5. Voges–Proskauer (VP) Test
  6. Triple Sugar Iron (TSI) Agar Test
  7. Urease Test

Summary of biochemical reactions of Enterobacteriaceae

E.coliA/A, Gas+ve+ve-ve-ve-veMotile
Citrobacter freundiiA/A or K/A, Gas, H2S+ve+ve+ve-ve-veMotile
Klebsiella pneumoniaeA/A, Gas (++), H2S-ve-ve+ve+ve+veNon-motile
Enterobacter cloacaeA/A, Gas (++)-ve-ve+ve+ve+veMotile
Salmonella Typhik/A, H2S (weak)-ve+ve-ve+ve-veMotile
Shigella boydiiK/A, No Gas, No H2S-ve+ve-ve-ve-veNon-motile
Proteus mirabilisK/A, Gas, H2S-ve+ve-ve+ve+veMotile (swarming)

Antimicrobial Resistance

Certain members of the Enterobacteriaceae possess chromosomally determined inducible AmpC beta-lactamases. These include the so-called MYSPACE organisms: Morganella morgannii, Yersinia enterocolitica, Serratia marcescens, Providencia spp., Aeromonas spp. Citrobacter freundii complex, and Enterobacter spp.

Carbapenem-resistant or carbapenamase-producing Enterobacteriaceae have been reported worldwide and are major threats to global well-being. Carbapenem-resistant Enterobacteriaceae (CRE) are usually resistant to all β-lactam agents as well as most other classes of antimicrobial agents, which limits the available treatment options.

Carbapenem resistance in Enterobacteriaceae occurs when an isolate acquires a carbapenemase or when
an isolate produces an extended-spectrum cephalosporinase, such as an AmpC-type β-lactamase, in
combination with porin loss. Klebsiella pneumoniae carbapenemase (KPC) is one of the most common mechanism of carbapenem resistance.

Modified Hodge Test (MHT) is one of the recommended tests for the detection of carbapenemase production .


  1. Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
  2. Color Atlas and Textbook of Diagnostic Microbiology, Koneman, 5th edition
  3. Pelczar Jr., M., Chan, E., & Krieg, N. (2007). Microbiology (5th edition). Tata McGraw-Hill
  4. Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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