Question

1. Why was it important in this case to identify Salmonella Typhi in the feces of the restaurant worker?

Wouldn’t the discovery of any bacterium be adequate?

2. Which of the three isolation techniques in this exercise would have been least suited to the isolation of

Salmonella Typhi in this case? Why?

3. MacConkey agar is a selective medium that only allows certain types of bacteria to grow. How could

the use of MacConkey agar has simplified the isolation of Salmonella Typhi in this case?

4. Thinking through the case, other than the restaurant workers, when else was microbiological

sampling likely used?

Answer 1

1. Salmonella typhi infects the intestinal tract and the blood. It causes the disease known as the typhoid fever. It is commonly present in the stools of the infected persons. Hence the feces of the restaurant worker was examined to see whether he was affected with this bacterium.

2. Basically this bacterium is an intestinal microbe and it is isolated from feces. The bacteria is commonly grown on selective or differential medium to differentiate it from other enteric microbes. Hence any other techniques apart from this will be least suited for isolation.

3. The macconkey agar is a selective medium which allows only the growth of enteric gram negative bacteria. Salmonella typhi is a gram negative enteric bacteria which is a non lactose fermenting bacteria. On macconkey agar the bacteria that ferment lactose will be producing acid and they show as yellow color colonies, which non lactose fermentors will be appearing as pale colonies. Salmonella appears as pale colonies on the agar.

4. The microbiological sampling is likely used to test the diseased conditions of persons who are going to join in hostels or to check the spread of a pathogen in a highly populated slum areas.

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