Streptococcus pneumoniae is a Gram-positive bacterium that colonizes the mucosal surface of the upper respiratory tract in humans. The presence of this bacterium in the nose and throat is widespread in the population, and in most people, colonization with Strep. pneumoniae is asymptomatic. Figure shows a comparison of in vitro growth curves of the wild-type strain of Strep. pneumoniae, as well as a Strep. pneumoniae mutant strain with a defect in one bacterial gene. The graph on the right shows the growth curve following addition of lysozyme during the logarithmic phase of bacterial growth.
Which statement could account for the data in these graphs?
A) Strain B is wild-type Strep. pneumoniae, and strain A is a mutant that cannot modify its peptidoglycan to be lysozyme-resistant.
B) Strain B is wild-type Strep. pneumoniae, and strain A is a mutant that that expresses increased levels of LPS.
C) Strain A is wild-type Strep. pneumoniae, and strain B is a mutant that cannot modify its peptidoglycan to be lysozyme-resistant.
D) Strain A is wild-type Strep. pneumoniae, and strain B is a mutant that secretes an enzyme that inactivates lysozyme.
E) Strain A is wild-type Strep. pneumoniae, and strain B is a mutant that cannot grow well in vitro.

Question

Streptococcus pneumoniae is a Gram-positive bacterium that colonizes the mucosal surface of the upper respiratory tract in humans. The presence of this bacterium in the nose and throat is widespread in the population, and in most people, colonization with Strep. pneumoniae is asymptomatic. Figure  shows a comparison of in vitro growth curves of the wild-type strain of Strep. pneumoniae, as well as a Strep. pneumoniae mutant strain with a defect in one bacterial gene. The graph on the right shows the growth curve following addition of lysozyme during the logarithmic phase of bacterial growth.  
Which statement could account for the data in these graphs?
A) Strain B is wild-type Strep. pneumoniae, and strain A is a mutant that cannot modify its peptidoglycan to be lysozyme-resistant. 
B) Strain B is wild-type Strep. pneumoniae, and strain A is a mutant that that expresses increased levels of LPS. 
C) Strain A is wild-type Strep. pneumoniae, and strain B is a mutant that cannot modify its peptidoglycan to be lysozyme-resistant. 
D) Strain A is wild-type Strep. pneumoniae, and strain B is a mutant that secretes an enzyme that inactivates lysozyme. 
E) Strain A is wild-type Strep. pneumoniae, and strain B is a mutant that cannot grow well in vitro.

Quizplus · Accepted Answer

These graphs suggest that strain A (the mutant strain) is more sensitive to lysozyme than the wild-type strain B. This suggests that strain A is unable to modify its peptidoglycan to be lysozyme-resistant. This would make sense, as lysozyme hydrolyzes the glycosidic bonds in peptidoglycan, and strains that can modify their peptidoglycan structure are better able to resist this action. Therefore, answer choice C is the best choice.

Streptococcus Pneumoniae Is a Gram-Positive Bacterium That Colonizes the Mucosal