During microbial limit testing MacConkey broth and MacConkey agar are two media used for identification of E.Coli which is a pathogen. But MacConkey broth is incubated at 42-44°C and MacConkey agar is incubated at 30-35°C temperature for the identification of same pathogen. Why elevated temperature of 42-44°C is used initially and then 30-35°C is used for incubation? The reason is that there are different strain of E.coli present in the environment. E.coli is gram negative bacteria and is an indicator of fecal contamination. Such contamination could arise from poor hygiene of operators, contamination from animals like cats, birds or a low quality water supply. E.coli is capable of causing diarrhoea and sickness also. So initially, 42-44°C temperature is provided for the incubation which is suitable for growth of fecal E.coli strain which we need to identify but other strains of E.coli couldn't grow at this high temperature. Once the enrichment of fecal E.coli has been performed in MacConkey broth then sample is streaked on MacConkey agar media and then incubate at 30-35°C which is suitable temperature for the growth of fecal E.Coli. That's why initially elevated temperature is used for the selection of fecal E.coli.