Unknown bacteria flow chart gram negative

Here is the eight steps identification of gram negative bacteria in microbiology.

  1. Start: Unknown Bacteria
    • Perform a Gram Stain to confirm the bacteria is Gram-negative (pink cells).
  1. Step 1: Cell Morphology and Arrangement
    • Observe under a microscope:
      • Bacilli (Rod-shaped): Continue to Step 2.
      • Cocci (Spherical): Test for oxidase activity to differentiate Neisseria and Moraxella (oxidase-positive) from other cocci.
  1. Step 2: Oxidase Test
    • Oxidase-positive: Likely Pseudomonadaceae or Vibrionaceae.
      • Perform extra tests like motility and glucose fermentation.
    • Oxidase-negative: Enterobacteriaceae family; proceed to Step 3.
  1. Step 3: Lactose Fermentation (MacConkey Agar or TSI Test)
    • Lactose fermenters (pink on MacConkey agar): Possible genera:
      • Escherichia coli: Confirm with indole and methyl red (positive).
      • Klebsiella: Non-motile, urease-positive.
      • Enterobacter: Motile, ornithine decarboxylase-positive.
    • Non-lactose fermenters (no color change on MacConkey):
      • Proceed to Step 4.
  1. Step 4: H2S Production (TSI Slant or SIM Test)
    • H2S-positive: Likely Proteus, Salmonella, or Edwardsiella:
      • Proteus: Swarming motility, urease-positive.
      • Salmonella: Lysine decarboxylase-positive, citrate-positive.
      • Edwardsiella: Indole-positive, found in aquatic environments.
    • H2S-negative: Proceed to Step 5.
  1. Step 5: Urease Test
    • Urease-positive: Likely Proteus or Klebsiella.
      • Proteus: Swarming motility, phenylalanine deaminase-positive.
    • Urease-negative: Proceed to citrate utilization test.
  1. Step 6: Citrate Utilization Test
    • Citrate-positive: Citrobacter, Salmonella.
    • Citrate-negative: Likely Shigella (non-motile, lactose-negative).
  1. Step 7: Motility Test
    • Motile: Differentiates motile genera (e.g., E. coli, Proteus) from non-motile (e.g., Shigella, Klebsiella).

Final Identification

Combine the results from all tests, including:

  • Gram stain
  • Cell morphology
  • Oxidase test
  • Lactose fermentation
  • H2S production
  • Urease activity
  • Citrate utilization
  • Motility test

Leave a Comment

Discover more from Medical Lab Technology

Subscribe now to keep reading and get access to the full archive.

Continue reading