Gram-positive bacteria are injected under the skin of experimental animals. Three days later, the animals are treated with antibiotic therapy. Aspirates taken from the infection site are plated on nutrient-rich agar. The isolated bacteria assume a spherical configuration when placed in an isotonic solution and disintegrate rapidly when placed in a hypotonic solution. Which of the following antibiotics was most likely administered to the animals in this experiment?

Question

Gram-positive bacteria are injected under the skin of experimental animals.  Three days later, the animals are treated with antibiotic therapy.  Aspirates taken from the infection site are plated on nutrient-rich agar.  The isolated bacteria assume a spherical configuration when placed in an isotonic solution and disintegrate rapidly when placed in a hypotonic solution.  Which of the following antibiotics was most likely administered to the animals in this experiment?

Quizplus · Accepted Answer

11ec160b_3bf4_c137_9249_9f41288ff4d9_MD0001_00 The bacterial cell wall is composed of rigid, cross-linked peptidoglycan pentads on a glucosamine backbone. Its primary purpose is to provide external structure to the bacterium and prevent disfigurement or lysis in changing osmotic gradients. In this experiment, the assumption of a spherical shape in an isotonic solution and lysis in a hypotonic solution indicate disruption of the bacterial cell wall. Several classes of antibiotics target cell wall synthesis, including fosfomycin, vancomycin, penicillins, and cephalosporins (eg, cefuroxime). (Choices A and C) Azithromycin and chloramphenicol inhibit the bacterial 50S ribosomal subunit. They have no effect on the cell wall and would not alter bacterial shape in different osmotic gradients. Side effects of chloramphenicol include dose-related myelosuppression and aplastic anemia. (Choice D) Fluoroquinolones (eg, ciprofloxacin) inhibit bacterial DNA topoisomerase and gyrase, which results in bacterial cell death due to DNA damage. They do not affect the cell wall. (Choice E) Doxycycline disrupts bacterial protein synthesis by inhibiting the bacterial 30S ribosomal subunit. It has no effect on the rigidity of the cell wall. Educational objective: The bacterial cell wall protects the organism from osmotic stress. Antibiotics that target the cell wall result in bacterial lysis in hypotonic solutions. Fosfomycin, vancomycin, penicillins, and cephalosporins all disrupt bacterial cell wall synthesis.

Gram-Positive Bacteria Are Injected Under the Skin of Experimental Animals