Question

Microbiology - Gram Positive (ONLY answer)

[Q1] This organism is mainly associated with the resident flora of the mouth and gums.

- Streptococcus salivarius

- Staphylococcus aureus

- Streptococcus pneumonia

- Escherichia coli

[Q2] An example of a selective and differential medium is ...

- 6.5% NaCl

- Blood Agar

- Bile Esculin

- Tryptic Soy Agar (TSA)

[Q3] An organism is considered resistant to the antibiotic bacitracin if which occurs?

- There is no zone of clearing around the antibiotic A disc.

- There is a zone of clearing around the antibiotic A disc.

- There is no zone of clearing around the antibiotic P disc.

- There is a zone of clearing around the antibiotic P disc.

Answer 1

1) Streptococcus salivarius is the organism that is present in the oral cavity of humans. It does not harm the person until some circumstances hence can be considered opportunistic. It is a gram positive,and the organism is circular and present in long chains of groups. Sometimes it is known to inhibit other pathogens in the mouth by producing certain antimicrobial peptides.

2) A selective media is a media that is used for the selective growth of a particular organism of interest only by containing certain compounds that inhibit all other organisms from growing.

A differential media is a media that is used to differentiate between two different organisms growing on the same media. IT can be used to identify closely related organisms based on their visual appearance on the media that depends on their metabolism activities.

Bile esculin is a differential and selective media. IT is selective because it allows the growth of only enterococci and streptococci on the media. this is because the media contains bile salts that inhibit all other organisms. It is differential because it can differentiate between both streptococci and enterococci based on the organisms capability to hydrolyse esculin in the media. When esculin is hydrolysed the products formed react with ferric citrate in the media to form a blackish color due to iron deposition.

enterococcus is positive for esculin and streptococci is negative for esculin test. Hence it is easier to study and compare 2 characters on the same plate.

3) There is no zone of clearing around the antibiotic A disc. An organism is considered to be resistant to an antibiotic when there is no zone of inhibition around the disc. The antibiotic from the bacitracin disc will diffuse into the media and those bacteria that are sensitive to it will not grow around that area forming a zone of clearing.The bigger the diameter, stronger is the antibiotic.