Medical Pharmacology Chapter 35  Antibacterial Drugs

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  • The Aminopenicillins:  Ampicillin and Amoxicillin

    • Therapeutic Uses

      • Urinary Tract Infections

        Urinary Tract Infections Audio Overview

        • The majority of uncomplicated urinary tract infections (UTIs) are due to Enterobacterales with the most common species isolated from UTI patients being E. coli.1

        • Ampicillin was historically one of the main antibiotics for urinary tract infections (UTIs) in the 1970s, as it covers E. coli and Proteus mirabilis, the predominant UTI pathogens, as well as Enterococcus faecalis.2 

        • Amoxicillin likewise can treat these infections if the organism is susceptible.3 

          • Amoxicillin is FDA-approved for UTIs caused by beta-lactamase negative Escherichia coli, Proteus mirabilis, or Enterococcus faecalis.3,4 

            • However, due to increasing resistance, neither amoxicillin nor ampicillin is recommended as empiric therapy for uncomplicated UTIs in most regions.2 

            • IDSA guidelines (2011) for acute uncomplicated cystitis state that amoxicillin or ampicillin should not be used empirically because of their poor efficacy and the high prevalence of E. coli resistance to these drugs worldwide.5

              • Presently, >30–50% of community E. coli UTIs may be resistant to ampicillin in many areas, rendering it unreliable unless culture results confirm susceptibility.5  

            • Therefore, first-line agents for uncomplicated UTIs are typically nitrofurantoin, trimethoprim-sulfamethoxazole (if local resistance <20%), or fosfomycin, with beta-lactams (e.g. amoxicillin-clavulanate or oral cephalosporins) as alternatives.6

        • There are circumstances where aminopenicillins are still used in UTIs.

          • Pregnancy

            • Amoxicillin is a category B drug and has been commonly used for asymptomatic bacteriuria or cystitis in pregnant women if the causative organism is susceptible. Treatment is guided by results of urine culture and sensitivity.8

              • However, both ampicillin and amoxicillin should not be used as empiric therapy given the high antibiotic resistance to these agents from E. coli.7

          • Many Enterococcus faecalis isolates from UTIs (particularly hospital-associated or catheter-associated UTIs) remain ampicillin-susceptible, so ampicillin is a drug of choice for serious enterococcal UTIs (IV administration for pyelonephritis or urosepsis).9 

          • In Enterococcus faecalis bacteriuria or cystitis, amoxicillin can be an effective oral therapy, as E. faecalis is usually sensitive (and aminoglycosides or other typical UTI drugs don’t cover enterococci as reliably).9  

          • For complicated UTIs , IV ampicillin is sometimes included in regimens, particularly to cover enterococci, in combination with other drugs that cover Gram-negatives.9   

          • In summary, aminopenicillins can treat some UTIs caused by susceptible organisms, but due to resistance and relatively poor efficacy they are seldom first-line empiric choices in non-pregnant adults today.5

September, 2025

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References

  1. MacDougall C Chapter 58 Cell Envelope Disruptors: In Goodman & Gilman's The Pharmacological Basis of Therapeutics (Brunton LL Knollman BC eds) McGraw HIil LLC (2023).

  2. Jancel T Dudas V Management of uncomplicated urinary tract infections. West J Med. 2002 January;176(1): 51-55. https://pmc.ncbi.nlm.nih.gov/articles/PMC1071654/#

  3. Akhavan B Khanna N Vijhani P Amoxicillin. StatPearls. National Library of Medicine. (Last update: November 17, 2023). https://www.ncbi.nlm.nih.gov/books/NBK482250/#

  4. FDA labeling. Therapeutic Indications and Usage. https://www.accessdata.fda.gov/drugsatfda_docs/label/2015/50542s02950754s01950760s01950761s016lbl.pdf (Revised: September 2015).

  5. Gupta K Hooton T Naber K Wullt B Colgan R Miller l Moran G Nicolle L Raz R Schaeffler A Soper D International Clinical Practice Guidelines for the Treatment of Acute Uncomplicated Cystitis and Pyelonephritis in Women: A 2010 Update by the Infectious Diseases Society of America and the European Society for Microbiology and Infectious diseases. Clinical Infectious Diseases 2011:52(5): e103-e120. https://asap.nebraskamed.com/wp-content/uploads/sites/3/2017/07/Acute-uncomplicated-cystitia-and-pyelonephritis-in-women-IDSA.pdf#

  6. Bono lM Leslie S Uncomplicated Urinary Tract Infections. StatPearls. National Library of Medicine. (Last update: February 21, 2025). https://www.ncbi.nlm.nih.gov/books/NBK470195/

  7. Habak P Carlson K Griggs Jr R Urinary Tract Infection in Pregnancy. StatPearls. National Library of Medicine. (Last update: April 20, 2024). https://www.ncbi.nlm.nih.gov/books/NBK537047/

  8. Givler D Givler A Asymptomatic Bacteremia. StatPearls. National Library of Medicine. (Last update: July 17, 2023). https://www.ncbi.nlm.nih.gov/books/NBK441848/#

  9. Said M Tirthani E Lesho E Enterococcus Infections. StatPearls. National Library of Medicine. (Last update: February 12, 2024). https://www.ncbi.nlm.nih.gov/books/NBK567759/

 

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