ampicillin ămˌpĭsĭlˈĭn [key], a penicillin-type antibiotic that is effective against both gram-negative microorganisms and gram-positive microorganisms such as Escherichia coli. It is often used in the treatment of urinary tract infections, but resistant organisms are increasingly common (see drug resistance). Like other penicillin antibiotics, ampicillin acts against bacteria by inhibiting the synthesis of bacterial cell wall components.

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