QUESTIONS 1 You were instructed to compare the growth on the NA and Columbia CNA

Question

QUESTIONS 1 You were instructed to compare the growth on the NA and Columbia CNA Agar plates. Because the two media used in this exercise were completely different and likely to produce differing amounts of growth, would it have been better to compare the organisms to each other on the CNA plate? Include what infor- mation is provided by the NA plate in your explanation 2 Earlier formulations of Columbia CNA Agar were made using a slightly higher concentration of colistin and nalidixic acid, but this was reduced to improve recovery of Gram-positive organisms. List some possi- ble reasons why the higher concentration was less effective at allowing growth of Gram-positive organism

Explanation / Answer

1. Yes, it is better to compare the organisms to each other on the CNA agar plate, rather then comparing organisms grown under different media composition. CNA agar plate is designed to suppress the growth of gram negative bacteria and specifically is used for the growth of gram positive bacteria.

On the other hand nutrient medium supports the growth of wide array of organisms including bacteria and fungi and is also an non selective media with lower morphology distinction of gram positive and negative bacteria. Organisms grown in CNA will have improved pigment production, colony morphology and sharply defined hemolytic reaction.

Hence taking the above mentioned features of both media into consideration, it is advisable to compare organisms grown in same plate rather then grown in different plate and composition.

2. Most probable reason for decreasing the concentrations of the antibiotics colictin and nalidixic acid to improve the recovery of gram positive bacteria is-

Generation time of the organisms were increasing and for few organisms like the streptococci variants were found to have thinner cell wall, since higher concentration of colistin is found to break the cell membrane of majority of gram negative but also some of the gram positive.

Further, increased concentration of the antibiotics lead to smaller colony size indicating slower cell division due to the presence of nalidixic acid which blocks the DNA replication.

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