Why are the plates which you streak with supposedly pre colo

Solution

Microbiological examination of growth of microbes is based upon the fact that microbes require an optimum temperature for their sustained growth. For most of the bacterial cells, the most optimum temperature for growth ranges between 32-38 degree celcius. There is significantly stunted growth of bacteria below and above the range of this temperature.

According to the information, the media plates are streaked and these plates are supposed to contain colonies of E. coli and S. marcescens already in them. Thus, further streaking of the media plates will lead to multiplication of the microbes. However, in order to obtain a log-phase of growth, the plates must be incubated at a temperature most favourable for their growth.

Here, it should be noted that the optimum growth temperatures for both of these microbes is different. Whereas E. coli shows log phase growth in the range of 30-37 degree celcius, S. marcescens shows a much broader range of temperature ranging from 5-40 degree celcius. Thus, if the plates are incubated at room temperature only, it is highly likely that growth of only one type of microbes i.e. S. marcescens will be favored. This will help in obtaining pure culture of S. marsescens from a mixed culture of these two types of microbes by observing differential growth of these microbes under different temperatures.