Gram-Negative Bacilli Overview

Study Notes

Gram-negative bacilli are a diverse group of bacteria that can be found in various environments including the gastrointestinal tract of humans and animals.
They are classified based on their morphology, growth requirements, and biochemical characteristics.
Enteric rods are a major group of Gram-negative bacilli that infect the gastrointestinal tract of humans and animals.
- Facultative anaerobes are enteric rods that can grow in the presence or absence of oxygen. Examples include:
  - Escherichia coli
  - Klebsiella pneumoniae
  - Proteus sp.
  - Enterobacter
  - Serratia sp.
  - Salmonella sp.
  - Shigella sp.
  - *Vibrio sp. *
  - Campylobacter jejuni
  - Helicobacter pylori
- Obligate aerobes are enteric rods that require oxygen to survive. Examples include:
  - Pseudomonas aeruginosa
- Obligate anaerobes are enteric rods that cannot survive in the presence of oxygen. Examples include:
  - Bacteroides sp.
  - Fusobacterium sp.
Non-enteric rods are Gram-negative bacilli that are not typically found in the gastrointestinal tract. Examples include:
- Haemophilus sp.
- Bordetella sp.
- Brucella sp.
- Pasteurella sp.
- Yersinia sp.
- Francisella sp.

Enterobacteriaceae is a large family of enteric organisms that contains many pathogens and commensal bacteria.
- Salmonella, Shigella, and enteropathogenic E. coli are examples of pathogens within the Enterobacteriaceae family.
- Enterobacter, Providencia, and Proteus are examples of commensal bacteria.
Many Enterobacteriaceae can cause wound infections even if they are commensal bacteria.

To distinguish between different Gram-negative bacilli, a presumptive identification method is employed.
The ability to ferment lactose is used as a primary means of separation.
Lactose fermenters include:
- E. coli
- Klebsiella sp
- Serratia
Non-lactose fermenters include:
- Salmonella
- Shigella
- Proteus
- Pseudomonas

Vibrio is a Gram-negative, cocci-bacilli (comma-shaped) bacteria with a single terminal flagellum, making it highly motile.
Vibrio is responsible for causing diseases like cholera.
Laboratory diagnosis is achieved through:
- Wet preparations that reveal the characteristic "darting" movement of Vibrio
- Growth on alkaline media such as Thiosulphate Citrate Bile Sucrose (T.C.B.S) agar.

Haemophilus are pleomorphic Gram-negative bacilli that require X (Heam) and V (co-enzyme I or II / Nicotinamide adenine dinucleotide) factors for growth.
Laboratory diagnosis is achieved through:
- Boiled blood agar which contains readily available X and V factors for the growth of Haemophilus.
- Blood agar with X factor only, where superimposing with Staphylococcus aureus provides the V factor.
- Nutrient agar which lacks both X and V factors. X and V discs are placed on the agar to identify the requirement for the specific factor.

Pseudomonas is a Gram-negative bacilli with a single polar flagellum and an obligate aerobic lifestyle.
It produces pigments, including:
- Pyocyanin (blue-green)
- Pyoverdine (yellow-green)
- Pyorubin (red-brown)
Pseudomonas is commonly found in soil and water.
It is a multi-drug resistant pathogen and a frequent cause of nosocomial infections.
Pseudomonas often infects external wounds, particularly in burn victims.

API 20E is a commercially available test system used to identify different species of Enterobacteriaceae and related Gram-negative bacilli.
The test involves a strip with various biochemical tests.
Preparation of test organism: A bacterial suspension is prepared in distilled water and used to inoculate the different compartments on the API 20E strip.
Inoculation of sample: Each compartment on the strip is inoculated with the suspension, and anaerobic conditions are created for specific tests.
Reading and interpretation of results: After incubation, the reactions observed in each test compartment are recorded and interpreted to identify the specific bacterium.

TDA test: A TDA reagent is added to the TDA test compartment. A reddish-brown color indicates a positive reaction.
IND test: A JAMES reagent is added to the IND test compartment. A pink color developed in the whole cupule indicates a positive reaction.
VP test: VP 1 and VP 2 reagents are added to the VP test compartment. A pink or red color indicates a positive reaction.
The indole production test (IND) should be performed last because it releases gaseous products that can interfere with other tests.