Identify the bacterium that appears violet after Gram staining

Options

A. Salmonella enterica
B. Escherichia coli
C. Mycobacterium tuberculosis
D. Rhizobium meliloti

Answer

To identify the bacterium that appears violet after Gram staining, we can follow these steps:

### Step 1: Understand Gram Staining
Gram staining is a method used to classify bacteria into two groups: Gram-positive and Gram-negative. This classification is based on the structure of their cell walls.

### Step 2: Identify Characteristics of Gram-Positive Bacteria
Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which allows them to retain the crystal violet stain during the Gram staining process. As a result, they appear violet or purple under a microscope.

### Step 3: Identify Characteristics of Gram-Negative Bacteria
In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane. They initially appear violet but lose this color when a decolorizer (like ethanol or acetone) is applied. They then take up a counterstain (safranin) and appear red or pink.

### Step 4: Analyze the Given Options
The options provided are:
1. Salmonella enterica
2. Escherichia coli (E. coli)
3. Mycobacterium tuberculosis
4. Rhizobium melliloti

Now, we analyze each option:
- **Salmonella enterica**: This is a Gram-negative bacterium.
- **Escherichia coli (E. coli)**: This is also a Gram-negative bacterium.
- **Mycobacterium tuberculosis**: This is classified as a Gram-positive bacterium, but it does not retain the deep purple color due to its high lipid content and is typically stained using the acid-fast method.
- **Rhizobium melliloti**: This is another Gram-negative bacterium.

### Step 5: Conclusion
Based on the analysis, none of the options are ideal candidates for appearing violet after Gram staining. However, Mycobacterium tuberculosis is the only Gram-positive bacterium listed, although it does not exhibit the typical deep purple color. Therefore, the answer is:

**Correct Answer: Mycobacterium tuberculosis**