Microbiology Laboratory


Written by MicroDok

The microscopical examination of CSF specimen has 2 basic aspects to it, viz:

  • Gram smear.
  • Cell count.
  • Differential count.

They are very important in the analysis of CSF specimen and each has its own role/significance in deciphering the presence of bacteria in a CSF. Their methods are expanded below. Before embarking on any of the above methods by which a CSF specimen can be analyzed microscopically, it is very important to also observe and report the appearance of CSF specimens first. The appearance of a CSF specimen can be reported based on whether the CSF specimen is:

  • Clear – normal CSF appears clear, bright, and colourless.
  • Cloudy or purulent – indicates the presence of pus cells that is suggestive of pyogenic bacterial meningitis.
  • Bloody – blood in CSF specimen may be due to a bloody/traumatic lumbar puncture.
  • Contains clots – this indicates a high protein concentration with increased fibrinogen.

AIM: To detect the presence of pus cells and bacteria in cerebrospinal fluid (CSF) specimen.

MATERIAL/APPARATUS: CSF specimen, Gram staining reagents, microscope, glass slide, immersion oil, Bunsen burner.


  1. Place a drop of the CSF specimen on a clean glass slide.
  2. Make a thin smear of the CSF specimen.
  3. Allow to dry in a secure place to prevent it from dust.
  4. Heat fix the smear by passing it thrice over the blue flame of a Bunsen burner.
  5. Perform the Gram staining technique.
  6. Allow the Gram stained slide to dry.
  7. Place a drop of immersion oil on the slide.
  8. View the slide under the microscope using the ×100/oil immersion objective lens


Look for Gram negative intracellular diplococcic, Gram negative rods, Gram positive diplococcic, and pus cells and report. If the Gram smear contains bacteria and pus cells, inform the physician about it immediately. Culture the CSF specimen once the Gram smear proves positive for bacteria and pus cells. In cases of an emergency treatment when the patient is given antibiotic treatment before the CSF specimen is collected, it is possible and more difficult to detect bacteria in a Gram stained smear and to isolate bacteria from culture.

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