Question

1. emb plate is selective but not differential

a. true.
b. false

1. the targets of antibiotics include the

following, except:
a. dna replication
b. protein synthesis
c. glucose synthesis
d. cell wall synthesis

1. the presumptive mpn test looks for the

presence of:
a. escherichia coli
b. enterobacter aerogenes
c. enteric bacteria
d. coliforms

1. metallic sheen on colonies growing on an

emb plate indicates that the organism:
a. is a gram negative
b. uses metal in metabolism
c. ferments lactose

Explanation:

• Question 6: EMB (Eosin - Methylene Blue) agar is both a selective and differential medium. It is selective as it inhibits the growth of Gram - positive bacteria (due to the presence of dyes like eosin and methylene blue). It is differential as it differentiates between bacteria based on lactose fermentation. For example, Escherichia coli (a lactose fermenter) produces colonies with a metallic sheen, while non - lactose fermenters have different colony appearances. So the statement "EMB plate is selective but not differential" is false.
• Question 7: Antibiotics target various bacterial processes. Many antibiotics target DNA replication (e.g., quinolones inhibit DNA gyrase), protein synthesis (e.g., tetracyclines bind to the ribosome), and cell wall synthesis (e.g., penicillins inhibit peptidoglycan synthesis). Glucose synthesis is not a common target of antibiotics. Bacteria can obtain glucose from the environment or synthesize it through normal metabolic pathways, and antibiotics do not typically interfere with this general anabolic process.
• Question 8: The Most Probable Number (MPN) test is a statistical method used to estimate the concentration of bacteria in a sample. The presumptive MPN test is mainly used to detect coliforms. Coliforms are a group of Gram - negative, facultative anaerobic, non - endospore - forming, rod - shaped bacteria that ferment lactose with gas production within 48 hours at 35 - 37°C. While Escherichia coli and Enterobacter aerogenes are types of coliforms, and enteric bacteria is a broader category, the MPN test specifically looks for coliforms.
• Question 9: On an EMB plate, the metallic sheen is an indication of lactose fermentation. When bacteria ferment lactose, the acidic by - products react with the dyes (eosin and methylene blue) in the medium to produce the metallic sheen. Gram - negative nature is a characteristic of the bacteria that can grow on EMB (due to its selectivity), but the metallic sheen is specifically related to lactose fermentation. Resistance to bile (a selective factor in EMB) allows the bacteria to grow on the plate, but it does not cause the metallic sheen. And there is no relation to using metal in metabolism for the metallic sheen on EMB.

Answer:

1. B. False
2. C. Glucose synthesis
3. D. Coliforms
4. C. Ferments lactose