Which type of bacterium does NOT have a cell wall?

• A. Staphylococcus
• B. Streptococcus
• C. Mycoplasma
• D. Pseudomonas

Answer: (C)

Most bacteria have a rigid cell wall made of peptidoglycan that gives shape and protection. A distinct group lacks this wall entirely: Mycoplasma. Without peptidoglycan, their cell envelope is just a cytoplasmic membrane, which in Mycoplasma contains sterols that stabilize it and allow the organism to survive osmotic stress. Because there’s no wall to stain or to target with typical cell-wall–inhibiting antibiotics, these organisms behave differently under Gram staining and are resistant to drugs like beta-lactams that disrupt wall synthesis. The other organisms listed do have cell walls: Staphylococcus and Streptococcus have thick peptidoglycan layers, while Pseudomonas has a cell wall with a thinner peptidoglycan layer plus an outer membrane.