Pre-Laboratory Questions 1. Why do bacterial cultures need to be incubated, usua

Question

Pre-Laboratory Questions 1. Why do bacterial cultures need to be incubated, usually for at least 24 hours, before we can see evidence of bacterial growth that can be observed with the unaided eye? In what ways are the media used in this investigation selective? 2. 3. 4. In what ways are the media used in this investigation differential? Consider a mixture of bacterial species from the same genus, such as Staphylococcus. Some members of this genus are harmful pathogens, while others are not. Would a selective medium alone be a good tool to separate the harmful species from the non-harmful species? Explain your answer.

Explanation / Answer

Q1).The bacterial growth cycle of bacteria can be divided into four phases. They are,

1). Lag phase

2). Log phase or exponential phase

3). Stationary phase

4). Death phase

The organisms do not reproduce during the lag phase, but synthesize the components necessary for the rapid growth.

During the stationary phase the population size remain constant, means they stop growing. Stationary phase occurs due to the limiting factors such as restricted amount of nutrients essential for growth, or formation of growth inhibitory factors.

During the death phase, the population size decreases due to the increased rate of death than the rate of reproduction.

So, the bacterial growth cannot be observed immediately after inoculating the bacteria into the bacterial culture because the bacteria remain in the lag phase for certain period of time although they are incubated at the right temperature. The growth can only be observed after the bacteria enters into the log phase or exponential phase, which usually takes about 24 hours.

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