Question

A fecal sample from a patient with diarrhea was streaked onto MacConkey lactose agar medium, a medium that selectively prevents the growth of Gram-positive bacteria but will differentiate Gram-negative bacteria that can ferment lactose from those that cannot. The next day, several different types of bacterial colonies were visible on the agar plate. Some were red; others were white. The white colonies led the clinical microbiologist to suspect that the patient was infected with the Gram-negative bacillus Salmonella enterica serovar typhimurium .
MacConkey lactose agar medium selectively prevents the growth of Gram-positive bacteria but will differentiate Gram-negative bacteria that can ferment lactose from those that cannot. The use of these types of media, called selective and differential media, are extremely helpful when microbiologists already have an idea of the pathogen present. What led the microbiologist to believe that the white colonies were Salmonella enterica serovar typhimurium?

Answer 1

MacConkey agar is used for isolation, identification and enumeration of lactose fermenting and lactose non fermenting enteric bacteria. MacConkey agar is the earliest selective and differential medium for cultivation of enteric microorganisms from a varietu of clinical specimens.

When bacteria ferment a sugar, the pH of the medium becomes acidic. The microbiologist evaluated lactose fermentation on individual colonies on solid MacConkey media. The microbiologist incorporated lactose directly into the agar. Changes in pH attributable to fermentation were observed by taking advantage of the knowledge that bile acids precipitate in an acidic environment. In this way, lactose- fermenting colonies were surrounded by a haze of precipitated bile.

The microbiologist saw the growth of two kinds of colonies white and red. He examined the plates for typical colony morphology. On MacConkey agar, lactose fermenters forms red colonies.

Since the microbiologist already had an idea of what the pathogen is, he ruled out the red colonies and confirmed that the white colonies are non lactose- fermenters, which is Salmonella enterica serovar typhimurium.