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Gram Negative Bacilli

Background and Introduction

ENTEROBACTERIACEAE is a large family of bacteria that consists of more than two dozen genera. As a group they are commonly called enterics. They typically inhabit the colon of warm-blooded animals as normal flora. However, various strains of these bacterial groups can cause severe gastrointestinal disease. As a group they have the following characteristics:

  1. Straight Gram-negative rods

  2. Oxidase negative

  3. Usually have simple nutritional needs (non-fastidious)

  4. Facultative anaerobes

  5. If motile - flagella are peritrichous (all around the cell)

The most common genera include...

  • Escherichia: normal gut flora, fecal indicator, can be an opportunistic pathogen. Escherichia coliis the most medically relevant species and is associated with diseases such as meningitis and urinary tract infections.

  • Proteus: opportunistic pathogen causing urinary tract infections (renal stones)

  • Salmonella: typhoid fever, gastroenteritis

  • Shigella: shigellosis. The exotoxin producing S. dysenteriae causes bacillary dysentery.

  • Klebsiella: normal gut flora, can be an opportunistic pathogen. The most common species is K. pneumoniae, which causes pneumonia in alcoholics and people with compromised pulmonary function.

            Members of Enterobacteriaceae that are capable of causing disease have virulence factors such as adhesins and exotoxins. For example, the adhesins of E. coli allow it to adhere to tissues in the urinary tract and keep it from being voided with the urine.


PSEUDOMONAS: is a genus of Gram-negative bacilli commonly found in the environment and some species are successful opportunistic pathogens. Because these Gram-negative bacilli survive well in many environments in nature and in homes and hospitals, they are commonly encountered in the clinical laboratory. As a group they have the following characteristics:

  1. Straight or slightly curved Gram-negative rods

  2. Oxidase positive

  3. Nonfermentative

  4. Strict aerobes

  5. Many species produce a water-soluble pigment and appear mucoid

  6. Motile - one or several polar flagella

A few of the more important species include...

  • Pseudomonas aeruginosa– has the most clinical significance

  • Pseudomonas fluorescens

  • Pseudomonas putida

Pseudomonas aeruginosa produces several enzymes and toxins that account for its virulence. The most important toxin blocks protein synthesis.

Although Pseudomonas spp. and the enterics, and many other bacteria, are Gram-negative bacilli, they differ in their biochemical activities. In the next couple of labs, the key biochemical tests used to identify the Enterobacteriaceae members and Pseudomonas spp. will be introduced.

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