Which bacteria lack a rigid cell wall and are therefore intrinsically resistant to penicillin and other beta-lactam antibiotics?

Introduction / Context:
Drug mechanism knowledge guides empirical therapy. Beta-lactam antibiotics target peptidoglycan cross-linking. Bacteria that lack peptidoglycan entirely are not affected and require alternative drug classes.

Given Data / Assumptions:

• Penicillins and cephalosporins bind PBPs to block cross-linking in peptidoglycan.
• Mycoplasmas do not have a cell wall; their membranes contain sterols acquired from the host.
• Other listed groups possess cell walls with peptidoglycan (even if thin or unusual) and are not intrinsically wall-less.

Concept / Approach:
Mycoplasmas are the prototypical wall-less bacteria; thus beta-lactams are ineffective. Cyanobacteria, spirochetes, and predatory Bdellovibrio species all contain peptidoglycan and, absent resistance mechanisms, can be affected by beta-lactams.

Step-by-Step Solution:

Verification / Alternative check:
Clinical practice for atypical pneumonia (Mycoplasma pneumoniae) favors macrolides or tetracyclines, not beta-lactams.

Why Other Options Are Wrong:
Cyanobacteria: Have Gram-negative type envelopes. Bdellovibrios: Gram-negative predators with peptidoglycan. Spirochetes: Possess a thin peptidoglycan layer beneath an outer membrane.

Common Pitfalls:
Equating thin-walled or atypical envelope structures with absent peptidoglycan; only wall-less organisms are intrinsically beta-lactam resistant.

Final Answer:
Mycoplasmas