Penicillin would be LEAST effective in treating:

Mycoplasma organisms do have a cell wall and are therefore resistant to penicillin. Other forms of bacteria that lack a cell wall are spheroplasts and protoplasts, which are formed from gram-negative and gram-positive bacteria, respectively, through the action of penicillin or by other procedures that remove the cell wall or interfere with its formation. The other organisms listed in the question are all susceptible to the action of penicillin. Tetracyclines, erythromycin and the aminoglycosides are effective antibiotics for the treatment of mycoplasmal infections. Ref: Infection and Immunity By Huw Davies, D. H. Davies, Page 34