Penicillin is effective against:

PENICILLIN-G (BENZYL PENICILLIN) Antibacterial spectrum: PnG is a narrow spectrum antibiotic; activity is limited primarily to gram-positive bacteria, few gram negative ones and anaerobes. Cocci: Streptococci (except viridans, group D or enterococci) are highly sensitive, so are many pneumococci. Staph. aureus, though originally very sensitive, has acquired resistance to such an extent that it must be counted out of PnG spectrum. Gram negative cocci-Neisseria gonorrhoeae and N. meningitidis are susceptible to PnG, though increasing number of gonococci have developed partial and other high degree resistance. Bacilli: Gram-positive bacilli – majority of B. anthracis, Corynebacterium diphtheriae, and practically all Clostridia (tetani and others), listeria are highly sensitive, so are spirochetes (Treponema pallidum, Leptospira, and others), but Bacteroides fragilis is largely resistant. Actinomyces israelii is only moderately sensitive. Majority of aerobic gram-negative bacilli, Mycobacterium tuberculosis, rickettsiae, chlamydiae, protozoa, fungi and viruses are totally insensitive to PnG. Gonorrhea: PnG has become unreliable for the treatment of gonorrhoea. Syphilis: T. pallidum has not shown any resistance and PnG is the drug of choice. Early and latent syphilis is treated either with daily i.m. injection of 1.2 MU of procaine penicillin for 10 days or with 1-3 weekly doses of 2.4 MU benzathine penicillin. For late syphilis, benzathine penicillin 2.4 MU weekly for 4 weeks is recommended. Cardiovascular and neurosyphilis requires sod. PnG 5 MU i.m 6 hourly for 10-14 days followed by the above regimen. Reference: Essentials of Medical Pharmacology Eighth Edition KD TRIPATHI  page no 768,770