Differential Stains (Gram stain & Acid Fast Stain) Abdelraheem BA Microbiology Lab (3) Differential Stains (Gram stain & Acid Fast Stain) Abdelraheem BA

Objectives To learn how to stain bacteria with gram stain & acid-fast stain. To determine the form, gram reaction, shape and arrangement of stained bacteria.

Differential stains A stain that is used to differentiate between certain groups of bacteria. This techniques requires the use of: At least 3 chemical reagents. Heat fixation of prepared bacterial smears. Examples: Gram stain: Gram +ve Vs Gram –ve bacteria. Acid-fast stain: Acid fast Vs Non-Acid fast.

Gram stain A technique that is used to stain bacteria and yeast. One of the most commonly performed procedures in microbiology lab. A very important step in the identification of bacteria.

Reagents in Gram stain Primary stain: Crystal violet. Mordant*: Gram’s Iodine. Decolorizing agent: 95% Alcohol or acetone- alcohol (50:50). Counter stain: Safranin *Mordant: A substance that forms a colored insoluble complex by binding the primary. CV-I complex hardly leave the cell. Result: intensified color & fixation of stain.

Steps to perform gram stain Allow the smear to completely air dry. Heat fixation. Place the slide on the staining tray. Flood the smear with the primary stain (Crystal violet) for 1 minute. Wash the smear with tap water to remove extra stain. Flood the smear with iodine for 1 minute. Wash the smear with tap water to remove extra iodine.

Steps – Cont’d Rinse with the decolorizing agent (a crucial and a sensitive step); Few drops for 10 seconds. Immediately wash the slide with tap water, to remove extra stain. Flood the smear with the counter stain (Safranin) for 1 minute. Wash with tap water. Use bibulous paper to dry the stained smear and examine with light microscope.

What is the principle? Bacterial cell wall review: G+ve: G-ve: Thick layer of peptidoglycan. Thin layer of lipids. Lipids will be dissolved by alcohol (forming minute pores). These pores will then shrink by the dehydrating effect of alcohol. CV-I complex will be prevented from leaving. G-ve: Thin layer of peptidoglycan. Thick layer of lipids. Due to alcohol, lipids will dissolve forming large pores. CV-I complex will leave, the bacteria become colorless.

Bacterial cell wall

Gram Stain - Summary

Precautions Decolorization is a crucial step; Over decolorization; loss of primary stain (False gram – ve bacteria) Insufficient decolorization; failure to remove CV-I complex (Fals gram +ve bacteria). Fresh cultures must be used; Old cultures (in case of gram +ve bacteria); damage to cell wall which leads to failure to retain the CV-I complex. Gram variable result. Suitable fixation must be used; Too much heat will destroy cell wall; Gram +ve bacteria will lose the primary stain (False gram –ve bacteria). Freshness of reagents.

Acid Fast stain Most bacteria are stained by gram stain. Members of genus Mycobacterium and Nocardia have a special cell wall. Thick waxy layer (Lipoidal). This makes these bacteria resistant to many antiseptics and antibiotics. This is the reason for being ACID FAST.

Mycobacteria Cell wall is composed of: Examples: Fatty acids (mycolic acid). Complex waxes. Complex unique glycolipids. Examples: M. tuberculosis; the causative agent of TB. M. leprae; the causative agent of leprosy.

Membrane of Mycobacteria

What does “Acid Fast” mean? The cell wall keeps the color red of Carbol fuchsin (the primary stain), when decolorized with acid-alcohol. Other bacteria will be decolorized and stain blue, the color of Methylene blue (the counter stain).

Reagents of Ziehl-Neelsen Method Primary Stain. Carbol fuchsin; a phenolic dye and soluble in the waxy cell wall. Used with the aid of heating (figure). After this step, all bacteria will stain red. Decolorizer. Acid-alcohol. Before addition, the slide must be cooled to harden the waxy cell wall. After this step. Acid fast bacteria will remain red. Non-Acid fast bacteria will be colorless. Counter stain. Methylene blue. Acid fast bacteria remain red. Non-Acid fast bacteria will stain blue.

Good Luck