Cell Shape and Arrangement

Introduction Results Methods Photos


The three main shapes of bacterial cells are spheres (cocci (plural) or coccus (singular)), rods (bacilli (plural) or bacillus (singular), and helical (spirilla (plural) or spirillum (singular). The main arrangements are single cells, diplo- (pairs), staphylo- (clusters), and strepto- (chains).



Cell Shape Cell Arrangements
C = Cocci Single
B = Bacilli Chains = strepto- arrangement
C-B = Oval (not quite spheres or very short rods) Clusters = staphylo- arrangement

NOTE: In practice, it is usually difficult to determine whether Gram-positive cocci are in clusters or chains when viewed in a Gram-stain. This is because chains are broken apart during the production of a smeared slide. So streptococci end up looking like they are in clusters.


1. Clean a microscope slide with Bon Ami cleanser followed by a 1 minute soak in isopropyl alcohol.

2. Dry the slide with paper towels.

3. Make a bacterial smear.

a. From an agar stock culture, put a SMALL drop of water on the slide and smear a SMALL amount of cells into the water.

b. From a broth stock culture, smear several (4 to 6) loopfuls onto the slide.

4. Allow slide to air dry (so that there is no visible moisture on the slide).

5. Heat fix the slide by passing it (cell side up) three times through the cool (upper) part of the flame.

6. Flood with Gram's Crystal Violet for 60 seconds. Rinse with distilled water.

7. Flood with Gram's Iodine for 60 seconds. Rinse with distilled water.

8. Flood with Acetone-Alcohol for no more than 5 seconds. Rinse with distilled water.

9. Flood with Safranin for 60 to 90 seconds. Rinse with distilled water.

10. Dry in bibulous paper.

11. View under 4x, 10x, 40x, and oil immersion. A Gram-stain can only be validly evaluated using immersion oil and the oil immersion lens!



Gram-Positive Diplococci


Gram-Positive Staphylococci


Gram-Positive Streptococci


Gram-Negative Bacilli
Gram-Negative Bacilli