Rashes with Ampicillin
Michael J. Kraemer MD1
Arnold L. Smith MD2
1 The Department of Pediatrics, University of Washington School of Medicine, Seattle
2 The Division of Infectious Disease, Children's Orthopedic Hospital and Medical Center, Seattle
Ampicillin, first introduced in 1961, has probably become the most widely used penicillin in clinical pediatrics.
STRUCTURE ACTIVITY RELATIONSHIPS
All penicillins contain the 6-amino penicillanic acid moiety (Fig 1). Its structure includes a thiazolidine ring (A), a 
-lactam ring (B), the source of antibacterial activity, and an acyl side chain (R), containing a variety of substitutions creating the family of semisynthetic penicillins. The only difference between ampicillin and penicillin G is the presence of an amino group in the acyl side chain (Fig 1).
PHARMACOLOGY AND BACTERIOLOGY
Ampicillin is a semisynthetic penicillin, active against Streptococus pneumoniae and certain Gram-negative bacteria, including most Haemophilus influenzae, Escherichia coli, and certain Proteus species. Compared to penicillin G, it has increased stability in acid solutions: a property facilitating oral administration and absorption. It penetrates into most body tissues; effective entry into CSF, however, occurs only with inflamed meninges. The serum half-life with normal renal function varies from four hours in newborns1 to 1.3 hours in adults.2 Ampicillin can cause an allergic, or nonallergic skin rash (Fig 2).
ALLERGY
Allergy (for the purposes of this discussion) is defined as a specific immunologic interaction, between either antigen and antibody, or antigen with a sensitized lymphocyte, resulting in a clinically deleterious effect. Implicit is a prior contact with the antigen.
