Microbiology

The following conditions can lead to the presence of small amounts of occult blood in the faeces:

• Hemorrhoids: Swollen veins in the rectal area can bleed and cause occult blood in the stool.
• Bleeding gums: Blood swallowed from bleeding gums can appear in the stool as occult blood.
• Peptic ulcers: Ulcers in the stomach or small intestine may cause minor bleeding, resulting in occult blood.

However, iron supplements do not directly cause occult blood in faeces but can make the stool dark, which is often mistaken for blood.

Isotonic saline (0.85% NaCl) is used as a diluent for RBC counts because it maintains the integrity of the red blood cells, preventing hemolysis. Other diluents, such as dilute acetic acid or Turk’s solution, are used for WBC counts as they lyse red blood cells to better visualize white blood cells.

Darkfield microscopy employs a special condenser that directs light onto the specimen at oblique angles. This setup causes light to scatter when it hits the specimen, making bright objects appear against a dark background. It is particularly useful for observing transparent or unstained specimens like live microorganisms.

Fluorescence microscopy is based on light absorption at one wavelength and emission at a longer wavelength, ideal for tagging biological structures.

Fluorescence microscopy is based on the principle that certain substances absorb light (usually ultraviolet or blue light) at one wavelength and then emit it at a longer wavelength (visible light). This property of fluorescence allows specific structures or molecules in a specimen to be tagged with fluorescent dyes, enabling detailed visualization.

Interference contrast microscopy, such as Differential Interference Contrast (DIC), is capable of producing three-dimensional images and performing optical sectioning. This technique enhances contrast in transparent specimens and provides a pseudo-3D appearance by exploiting differences in the optical path length.

If the speck remains visible regardless of changes in the objective or specimen movement, it indicates that the speck is located on the eyepiece. This is because the eyepiece remains constant in the optical path and does not change as the objectives or specimen are adjusted.