Question

1. Draw and label (or generate in excel) the standard growth curve for bacteria. Explain each of the stages of this curve fully. Conclude by explaining how and why this curve might look if it represented the growth of a lactose degrading bacteria in media that contains both glucose and lactose.

Answer 1

The growth cycle of a bacterial population in a sterile culture (closed system) is graphically represented which is known as a bacterial growth curve. A typical growth curve contains 4 phases which are lag, log, stationary and death.

Lag phase: When bacteria are introduced to a new culture, they don't show growth( increase in population by binary fission) immediately. During the lag phase, the bacterial cells start synthesizing required RNAs, proteins or enzymes to utilize energy (by degrading the contents of the new media) and to produce daughter cells. After this phase is over, the cells enter into the exponential phase or log phase. The lag phase is represented by a straight line.

Log phase: At this phase, the growth of the bacterial population is optimum. The cells rapidly divide and increase their number. At this phase, the death rate is lower than the new cell formation rate. This phase is represented by a slope of the line.

Stationary phase: As the bacterial population continues to grow, the media is gradually depleted of essential nutrients. Moreover, the bacterial cells produce several toxic by-products which are inhibitory to the growth of the bacterial population. Therefore, in this phase, the growth rate becomes nearly equal to the death rate of the cells. This phase is represented by a horizontal line.

Death phase: As the essential nutrients become almost depleted and toxic by-products accumulate in a high number, the cells start to die. New cell formation is almost ceased and ultimately, the whole population become dead. This is known as the death phase and represented by a declining line.

2nd part:

Bacteria tend to use simple sugar first(as breaking it requires less energy) and after the simple sugar is depleted in the media, they switch to the complex sugars present. Glucose is a simple sugar i.e monosaccharide whereas, lactose is a disaccharide made up of glucose and galactose. Therefore, if the media contains both glucose and lactose, the cells go through a log phase while utilizing glucose. After glucose is depleted, the cells enter into a lag phase where they synthesize essential enzymes, RNAs to utilize lactose. After that, the cells again into the log phase. This type of growth is known as the diauxic growth cycle.