Cell wall stress and osmotic protection: Bacterial cells exposed to penicillin can be protected from lysis if the surrounding medium has which property?

Introduction / Context:
Penicillins inhibit transpeptidation during peptidoglycan synthesis, weakening the bacterial cell wall. In hypotonic environments, the compromised wall cannot counteract turgor, leading to osmotic lysis. Understanding how external osmolarity modifies this outcome explains classic laboratory observations.

Given Data / Assumptions:

• Penicillin acts primarily on actively dividing bacteria by disrupting cell-wall cross-linking.
• Osmotic balance across the cytoplasmic membrane determines whether wall-defective cells lyse.
• Protective media can prevent immediate cell rupture even when cell-wall synthesis is inhibited.

Concept / Approach:
In a hypertonic (high osmotic pressure) medium containing solutes such as sucrose, water does not rush into the cell, and wall-defective forms (spheroplasts or protoplasts) can persist without bursting. Conversely, hypotonic media favor water influx and lysis when the wall is weakened by β-lactams.

Step-by-Step Solution:
Relate penicillin’s mechanism (cell-wall inhibition) to structural integrity of dividing cells.Consider osmotic gradients: hypertonic surroundings reduce inward water movement.Conclude that high osmotic pressure protects against lysis; select option ‘‘high osmotic pressure (hypertonic medium)’’.

Verification / Alternative check:
Laboratories create L-forms/spheroplasts using cell-wall inhibitors plus osmoprotective media, demonstrating survivability without lysis.

Why Other Options Are Wrong:

• Low osmotic pressure (hypotonic): Promotes lysis.
• Isotonic moderate pressure: Less protective than hypertonic, lysis still occurs.
• High surface tension/alkaline pH only: Do not provide osmotic protection against wall failure.

Common Pitfalls:
Assuming any non-thermal environmental change prevents lysis; osmolarity specifically counteracts turgor-driven rupture.

Final Answer:
high osmotic pressure (hypertonic medium).