Question

Log number of bacteria

1. The graph below depicts the growth of a Gram-positive, spore-forming bacterium Bacillus thuringiensis (B. t.). This species normally lives in the soil as well as the alkaline gut of insects.

a) Label on the graph the phases of this growth cycle. Below, describe the important features of each phase.

b) The graph shown was obtained by culturing B. t. in media containing only minimal amounts of nutrients and salts. Draw a growth curve on the same graph that would result from growing this organism in media highly enriched with nutrients and salts. Label the curve “Rich Media”. Explain your curve.

c) Describe what technique you would use to determine the total number of cells present in the culture during the growth period.

Answer 1

Ans A.

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There are 4 distinct phases of bacterial cell growth:

1. The initial phase or the first phase is the lag phase in which the bacteria are metabolically active however they do not divide. Importantly, during the lag phase of the bacterial growth cycle, synthesis of RNA, enzymes and other molecules occurs.

2. The next phase is the phase of exponential growth termed as the log phase or the exponential phase. If growth is not limited, doubling will continue at a constant rate so both the number of cells and the rate of population increase doubles with each consecutive time period.

3. Then comes the stationary phase in which the growth reaches saturation phase and the number of cells dying equals the number of dividing cells.

4. The last phase is the decline phase, here there seems to be an exponential decrease in the number of viable cells. It is mainly due to the lack of nutrients in the medium as all of them gets used up.

Ans B. In the presence of rich media, the lag phase will be shorter and the exponential phase or the log phase will be longer as compared to the growth in minimal media. The graph is shown below:

Lo numbea Rich media Qn

Ans C. To determine the total number of cells present in the culture during the growth period, we can count the cells using a hemocytometer. The hemocytometer is a rectangular chamber which can give an accurate count of the total number of cells present. Another important approach can be taking a known diluted volume of the culture at a particular time point and pouring/spreading it on an agar Petri plate. We can then count the number of colonies that arise and then can calculate the concentration by multiplying the count by the volume spread on the agar plate.