Which genus comprises bacteria that naturally lack a rigid cell wall, rendering them intrinsically resistant to beta-lactam antibiotics such as penicillin?

Introduction / Context:
Antibiotic targets determine intrinsic susceptibility. Beta-lactam antibiotics inhibit transpeptidation in peptidoglycan synthesis; organisms without a rigid cell wall are inherently resistant. This question tests matching an organism's envelope biology to drug action.

Given Data / Assumptions:

• Penicillin and related beta-lactams inhibit peptidoglycan cross-linking by targeting penicillin-binding proteins.
• Organisms that lack peptidoglycan (rigid cell wall) will not be affected by these drugs.
• We must identify the genus that naturally lacks a rigid cell wall.

Concept / Approach:
Mycoplasma species lack a cell wall and rely on sterols in their cell membrane for structural stability. Consequently, they are not susceptible to cell wall synthesis inhibitors. Treponema (spirochete), Staphylococcus (Gram-positive), and Klebsiella (Gram-negative) possess peptidoglycan and are, in principle, targetable by beta-lactams (subject to resistance mechanisms).

Step-by-Step Solution:

Verification / Alternative check:
Clinical guidance for atypical pneumonia (Mycoplasma pneumoniae) recommends macrolides or tetracyclines rather than beta-lactams due to lack of a cell wall.

Why Other Options Are Wrong:
Treponema: Has peptidoglycan and can be treated with penicillin clinically. Staphylococcus/Klebsiella: Possess cell walls; beta-lactam susceptibility depends on specific resistance factors (beta-lactamases, altered PBPs).

Common Pitfalls:
Assuming all bacteria have peptidoglycan; Mycoplasma are notable exceptions.

Final Answer:
Mycoplasma