The bacterium Lactobacillus lectis that causes souring of milk is :

Options

A. a facultative aerobe
B. an obligate aerobe
C. a facultative anaerobe
D. an obligate anaerobe

Answer

To determine the classification of the bacterium Lactobacillus lactis, which causes the souring of milk, we need to understand the definitions of the terms related to oxygen requirements for bacteria.

### Step-by-Step Solution:

1. **Understand the Terms**:
- **Facultative Aerobe**: These organisms primarily grow in anaerobic conditions but can also survive in aerobic conditions (with oxygen).
- **Obligate Aerobe**: These organisms require oxygen to survive and cannot grow in its absence.
- **Facultative Anaerobe**: These organisms prefer aerobic conditions but can also survive in anaerobic conditions.
- **Obligate Anaerobe**: These organisms can only survive in anaerobic conditions and are harmed by oxygen.

2. **Identify the Characteristics of Lactobacillus lactis**:
- Lactobacillus lactis is known for its role in fermentation, particularly in dairy products like yogurt and sour milk.
- It primarily functions in environments where oxygen is limited, such as in the fermentation of milk.

3. **Determine the Correct Classification**:
- Since Lactobacillus lactis can perform its functions in both the presence and absence of oxygen, it fits the definition of a **facultative anaerobe**. This means it can thrive in aerobic conditions but does not strictly require oxygen for survival.

4. **Select the Correct Option**:
- Based on the definitions and the characteristics of Lactobacillus lactis, the correct answer is **facultative anaerobe**.

### Final Answer:
The bacterium Lactobacillus lactis that causes souring of milk is a **facultative anaerobe**.

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