Question

Pretend you are a micrbiologist and need to grow E. coli strains of different genotypes. For the following bacterial genotypes, indicate which amino acids, carbohydrates, and antibiotics should be added to a bacterial growth media to provide both a media that will allow the bacteria to live and also a media that is the most selective media possibke for that particular strain of bacteria. Indicate whether an ingredient is required for survival of a particular strain, whether it js added to be selective for a particular strain, or both.

Available ingredients for media:

Amino Acids: histidine, tryptophan, leucine

Carbohydrates: galactose, lactose, glucose

Antibiotics: ampicillin, tetracyclin

1. his+, gal+, ampr, tets, trp-, lac-, leu+

2. his+, gal-, ampr, tetr, trp-, lac-, leu-

3. his-, gal-, ampr, tetr, trp+, lac+, leu+

4. his+, gal-, amps, tets, trp-, lac-, leu+

Answer 1

glucose is the primary source of energy for bacterial cells. in the presence of glucose and lactose, bacterial cells will use glucose. and if there is scarsity of glucose then only cells will use lactose

for first strain of bacteria, it can syynthesize histidine, galactose and leucine. so this strain will grow evenif this ingredients are not there in the medium. but it will not grow without lactose AND TRYPTOPHAN. because this strain is not able to syntesize it.

this strain is sensitive to tetracycline but resistant to ampicllin. so if we want to make selective medium that medium should contain tetracyclin. so on this medium only tetracycline resistant cells will grow.

2. for second strain it can only synthesize only histidine. so for the growth of cells we must add other amino acids and carbohydrates to the growth medium. as this strain is resistant to both of the above antibiotics therefore we can use antibiotic in a selective medium. for this selective medium should lack the histidine and can contain other growth nutrients. so the cells which will synthesize will grow on this medium

3. the third strain is resistant to both the antibiotics, so they cannot be used for selection purpoose.

this strain can synthesize trypthophan, but cannot synthesize other two amino acids. it cannot synthsize galactose , but there is no need of providing latose. so the selective medium should not contain tryptophan and lactose.

4. this strain is sensitive to both of the above antibiotics. so any of this can be used in medium. the cells which will grow will be resistant to this antibiotics.

as this strain can only synthesize only histidine and leucine, it can grow on the medium that lack this component, but that medium must contain all other nutrients  for the growth of the bacterial cells