Question

74. Which of the following statements is incorrect?                                                                                                                     

A. β- hemolytic Streptococci show complete clearing around the colonies on Blood Agar.                        

B. Stapylococci can be isolated on Phenyl Ethanol Agar and always ferment mannitol on Mannitol Salt Agar.                                                                                                                                   

C. Chocolate agar provides NAD and hemin thus supports the growth of Haemophilus influenzae                                                                                                                                                  

D. Salmonella appears as yellow colonies with black centers on Salmonella-Shigella agar

75. Which of the following statements is incorrect?                                                                                          

A. Typhoid fever is a systemic disease causing septicemia.                                                                     

B. Subacute bacterial endocarditis is caused by the oropharynx inhabitant, Streptococcus mutans.                                                                                                           

C. Killing the Lactobacilli in the female genital tract leads to fungal diseases.                                                                            

D. Lactobacilli are not part of the intestinal normal flora.

76. A microbiology student conducted the tube-dilution method using a series of tubes containing sterile medium and various concentrations of a variety of antibiotics. The tubes were inoculated with a test bacterium. If penicillin G had a MIC of 25µg/µl, erythromycin had a MIC of 50µg/µl, and doxycycline has a MIC of 10µg/µl, which of the following correctly ranks these antibiotics according to their antimicrobial activity against the bacterium, from the highest activity to lower?                                                                                                                                 

A. Erythromycin, penicillin G, doxycycline                                                                                                             

B. Erythromycin, doxycycline, penicillin G                                                                                                           

C. Doxycycline, penicillin G, erythromycin                                                                                                            

D. Doxycycline, erythromycin , penicillin G                                                                                                                

E. Penicillin G, erythromycin, doxycycline

Answer 1

Answer74- B. Staphylococci can be isolated on Phenyl Ethanol Agar and always ferment mannitol on Mannitol Salt Agar.

Staphylococci will not always ferment mannitol. Mannitol Salt Agar is a type of selective and differential medium and only pathogenic staphylococci like staphylococcus aureus ferment mannitol while non-pathogenic strains of staphylococci do not ferment mannitol. Fermentation of mannitol by pathogenic strains changes the color of the medium to yellow, thus it is used to distinguish between pathogenic and non-pathogenic staphylococci.

Answer75- D- Lactobacilli are not part of the intestinal normal flora.

Answer 76- The correct rank of antibiotics according to their antimicrobial activity against the bacterium, from the highest activity to lower is- Doxycycline, penicillin G, erythromycin

MIC (MINIMUM INHIBITORY CONCENTRATION) is the lowest concentration of drug required to prevent the growth of microbes or bacteria. Doxycycline has the least MIC that means it shows the highest activity against prevention of bacterial growth while erythromycin has the highest MIC, thus it is least potent as compared to others.