All are used against anaerobic streptococci except

Penicillin, clindamycin and metronidazole are the most active drugs against anaerobes. Carbenicillin is a bactericidal antibiotic belonging to the carboxypenicillin subgroup of the penicillins.It has Gram-negative coverage which includes Pseudomonas aeruginosa but limited Gram-positive coverage. Streptococci are Gram-positive. Reference: Textbook of Microbiology; Anathanarayan and paniker's; 10th edition; Pg:277