A microbiologist isolates a novel bacterial species that consistently stains pink during Gram staining and exhibits significant resistance to penicillin, an antibiotic known to inhibit peptidoglycan cross-linking. Which of the following structural features of the bacterial cell wall is most likely responsible for both the observed Gram stain result and the high penicillin resistance?
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The answer
Correct answer: B. The presence of an outer membrane composed of lipopolysaccharide that restricts penicillin's access to the thin peptidoglycan layer.
Tested concept: Bacterial structure and the cell wall
Explanation
The observation that the bacterium stains pink with Gram stain indicates it is Gram-negative. Gram-negative bacteria possess a thin peptidoglycan layer located between the inner cytoplasmic membrane and an outer membrane. This outer membrane, composed of lipopolysaccharide (LPS), acts as a significant barrier, impeding the entry of many antibiotics, including penicillin, to the peptidoglycan layer. Penicillin targets peptidoglycan synthesis, so its inability to reach its target due to the outer membrane explains the observed resistance.
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