Agent

Mode of Action

Antibacterial Spectrum of Clinical Importance

Dosage

Penicillin G

Bactericidal; interferes with bacterial cell wall synthesis

Streptococci, Pneumococci Clostridia, Neisseriae, Corynebacteria, Pasteurella multocida, Actinomyces, Treponema, Listeria

30 mil units IV/day every 2-4 hours

Ampicillin

Bactericidal; same as above

Hemophilus influenzae, Proteus mirabilis, Salmonallae, Shigellae and some E. coli. Gram-positive organisms as with Penicillin G

8 gm IV/day every 6 hours

Gentamicin

Bacteriostatic; inhibition of bacterial protein synthesis

Klebsiella sp., Aerobacter sp., Pseudomonas aeruginosa, Serratia, indole positive Proteus sp., some Methicillin-resistant Staphylococci

5 mg/kg/day every 8-12 hours

Metronidazole

Bacteriocidal

Gram-negative anaerobes especially Bacteriodes fragilis. Also effective against several protozoa.

2 gm IV/day every 4 hours

Cefoxitin (2nd generation cephalosporin)

Cell wall synthesis inhibitors; stable to staphylococcal beta lactamases

Same as above plus Bacteroides fragilis; not active against Enterobacter spp.

12 gm IV/day every 6 hours

Clindamycin

Bacteriostatic; inhibits protein synthesis

Gram-positive and Gram-negative anaerobes, Gram-positive aerobic cocci, Streptococcus faecalis, Clostridia

1200-2700 mg IV/day every 6-8 hours