`10^(9 )` bacteria were spread on an agar plate containing penicillin. After incubation overnight at `37^(@)C`, 10 bacterial colonies were observed on the plate. That the colonies are likely to be resistant to penicillin can be tested by

Options

A. measuring their growth rate
B. observing the colour of the colonies
C. checking their ability to grow on another plate containing penicillin
D. checking their ability to cause disease

Answer

To determine whether the bacterial colonies observed on the agar plate containing penicillin are resistant to the antibiotic, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Experiment**:
- We started with \(10^9\) bacteria spread on an agar plate that contains penicillin. After incubation at \(37^\circ C\), only 10 colonies were observed. This suggests that most of the bacteria were killed by the penicillin.

2. **Identifying the Purpose**:
- We need to test if the 10 surviving bacterial colonies are resistant to penicillin.

3. **Setting Up a Test**:
- To test for penicillin resistance, we can take the 10 bacterial colonies and inoculate them onto a new agar plate that also contains penicillin.

4. **Observing Growth**:
- After incubating the new agar plate with the colonies in the presence of penicillin, we will observe whether any of the colonies can grow.

5. **Interpreting Results**:
- If any of the colonies grow on the new plate containing penicillin, this indicates that those bacteria possess resistance to penicillin. If they do not grow, it suggests that they are susceptible to the antibiotic.

### Conclusion:
- The correct method to test if the colonies are resistant to penicillin is to check their ability to grow on another plate containing penicillin.