If E. coli bacteria stains pink when subjected to gram staining, what type of bacteria are they classified as?

• A. Gram Positive
• B. Gram Negative
• C. Gram Neutral
• D. Acid-Fast

Answer: (B)

When E. coli bacteria stain pink during a gram staining procedure, they are classified as gram-negative. The gram staining technique differentiates bacteria based on the characteristics of their cell walls.

Gram-negative bacteria, such as E. coli, possess a thin peptidoglycan layer surrounded by an outer membrane that contains lipopolysaccharides. During the staining process, the initial crystal violet dye is applied, followed by iodine, which helps form a complex with the dye. Then, a decolorizer (often alcohol or acetone) is applied, which disrupts the outer membrane of gram-negative bacteria, causing them to lose the crystal violet stain. Consequently, when a counterstain (safranin) is applied, these bacteria take up the pink color, indicating their gram-negative classification.

In contrast, gram-positive bacteria have a thicker peptidoglycan layer that retains the crystal violet stain, resulting in a purple appearance after the staining process. The gram-neutral and acid-fast classifications refer to other bacterial groups that do not fit within the standard gram staining outcomes, with acid-fast bacteria having a unique cell wall structure that requires special staining techniques. Thus, the pink coloration of E. coli is a clear indicator of their gram-negative