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Ampicillin is bactericidal at low concentrations and is clinically effective not only against the gram-positive organisms usually susceptible to penicillin G but also against a variety of gram-negative organisms. It is stable in the presence of gastric acid and is well absorbed from the gastrointestinal tract. It diffuses readily into most body tissues and fluids; however, penetration into the cerebrospinal fluid and brain occurs only with meningeal inflammation. Ampicillin is excreted largely unchanged in the urine; its excretion can be delayed by concurrent administration of probenecid which inhibits the renal tubular secretion of ampicillin. In blood serum, ampicillin is the least bound of all the penicillins; an average of about 20 percent of the drug is bound to plasma proteins as compared to 60 to 90 percent of the other penicillins. The administration of 500 mg dose of ampicillin capsules results in an average peak blood serum level of approximately 3.0 mcg/mL; the average peak serum level for the same dose of ampicillin for oral suspension is approximately 3.4 mcg/mL.
While in vitro studies have demonstrated the susceptibility of most strains of the following organisms, clinical efficacy for infections other than those included in the INDICATIONS AND USAGE section has not been documented.
Gram-Positive: strains of alpha- and beta-hemolytic streptococci, Streptococcus pneumoniae, those strains of staphylococci, which do not produce penicillinase, Clostridium sp., Bacillus anthracis, Corynebacterium xeroses, and most strains of enteracocci.
Note: Ampicillin is inactivated by penicillinase and therefore is ineffective against penicillinase-producing organisms including certain strains at staphylococci, Pseudomonas aeruginosa, P. Vulgaris, Klebsiella pneumoniae, Enterobacter aerogenes, and some strains of E. coli. Ampicillin is not active against Rickettsia, Mycoplasma, and "large viruses" (Miyagawanella).
Testing for Susceptibility: The invading organism should be cultured and its susceptibility demonstrated as a guide to therapy. If the Kirby-Bauer method of disc susceptibility is used, a 10 mcg ampicillin disc should be used to determine the relative in vitro susceptibility.
Last reviewed on RxList: 1/29/2005
This monograph has been modified to include the generic and brand name in many instances.
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