Why do beta-lactam antibiotics (e.g., penicillins) show selective toxicity toward bacteria but not human cells?

Which Gram staining result corresponds to a bacterium with a thick peptidoglycan layer located outside a single cytoplasmic membrane?

An antibiotic that binds periplasmic beta-lactamases to inhibit them (a beta-lactamase inhibitor) is given in combination with a beta-lactam that targets transpeptidases. Why does this combination broaden effective coverage against some Gram-negative bacteria?

Which simple ecological concept best explains why resistant organisms can colonize after antibiotic therapy?

A patient on clindamycin develops profuse watery diarrhea and tests positive for C. difficile toxin. Which of the following management steps is the most appropriate immediate action?

A patient’s gut at baseline contains Pathogen R = 1.0 × 10³ cells (highly resistant) and Commensal S = 1.0 × 10⁷ cells (largely susceptible). Under antibiotic therapy, R replicates with net growth r_R = 0.7 day−1 and S has net growth r_S = 0.0 day−1 while therapy continues. Assuming exponential growth for R and no net growth for S during 5 days of therapy, calculate R(t) after 5 days and determine whether R would exceed S in absolute number. Show the calculation steps and final decision; use N(t) = N0·e^rt.

Which bacterial structural component must be present for polymyxins to bind and initiate their disruptive action?

Using a diagram of a Gram-negative envelope (outer membrane, periplasmic space with peptidoglycan, inner membrane), indicate the sequence of events after a polymyxin molecule binds the bacterial surface. Which sequence is correct?

Synthesize a monitoring plan for a patient started on systemic colistin that minimizes patient harm and resistance selection. Which combination of measures is most appropriate?