Which of this bacteria is resistant to penicillin as it lacks a cell wall?

The correct option is Mycoplasmas.

Step-by-Step Explanation:

1. Understanding Penicillin's Mechanism of Action:
Penicillin is a beta-lactam antibiotic that targets and inhibits the synthesis of peptidoglycan, a key structural component of the bacterial cell wall. By preventing the cross-linking of peptidoglycan, penicillin weakens the cell wall, eventually leading to cell lysis and death of the bacterium.

2. Analyzing the Characteristics of Mycoplasmas:
Mycoplasmas are a unique genus of bacteria that naturally lack a cell wall entirely. Instead, their cell membrane is stabilized by sterols, which they typically acquire from their environment. Because they do not possess a peptidoglycan cell wall, they lack the target site that penicillin acts upon.

3. Evaluating Other Options:
• Spirochetes: These are Gram-negative, spiral-shaped bacteria that have a cell wall containing peptidoglycan.
• Cyanobacteria: These are photosynthetic bacteria with a complex, peptidoglycan-containing cell wall structurally similar to Gram-negative bacteria.
• Bdellovibrios: These are predatory Gram-negative bacteria that also possess a peptidoglycan cell wall.

4. Conclusion:
Since Mycoplasmas completely lack a cell wall, penicillin has no molecular target to disrupt, rendering these bacteria naturally resistant to penicillin and other beta-lactam antibiotics.