Question

Think about the results you recorded. a. Growth on the PEA and NA plates was recorded as "good growth," "p oor growth," or “no are qualitative and, at least for the first two, subjective terms. What did you use to establish what These constituted "good growth?" bhy wouldn't it be advisable to compare growth of the organisms on each plate to each other? There are at least two answers to this question!

Answer 1

PEA stands for phenylethyl alcohol agar and NA stands for nutrient agar. PEA is used as a selective medium and it inhibits the growth of gram negative bacteria. It does so by inhibiting the structure of the cell wall of the gram negative organisms. At higher concentrations of PEA, both gram positive and negative will be inhibited.

Many gram negative bacteria show growth on PEA agar whike their numbers and sizes are reduced. Prolonged incu ation might lead to visible colonies. Good growth means the bacterial plate forms colonies in the soecified incubation time and correct temperature for growth. The size of the colonies are not too large nor too small and numbers are also not too few.

Some gram positive bacteria may also show less growth on the PEA plate initially but upon longer incubation time, dor he colonies to be visible.

Nutrient agar on the otjer hand supports the growth of a wide variety of microbes. These are non selective as most organisms grow on this.

No growth means tje growth is totally inhibited on the plate. Good growth tells us that the colony size and number is fine. The incubation period was fine and not under or over incubated

Poor growth could be a result of bacteria not being able to grow on PEA agar but if incubation time is increased, tje growth will resume or that amount of incubation time eas not enough.

We cannot compare the growth of microbes on each plate witj each other as the plates are not constant. Within a plate also, the colonies differ from each other. They differ in colony size and number. The differential and selective media cannot be used to compare as they allow growth of one ans inhibit tje growth of the other or on the same plate, different bacteria will show different growth patterns.