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Ampicillin

Ampicillinum

For medical students2 min readUpdated 2026-10-10

Ampicillin is a semi-synthetic antibacterial agent belonging to the aminopenicillin group of beta-lactam antibiotics. It features an extended spectrum of activity and is acid-stable.

Pharmacological groupBeta-lactam antibiotics, aminopenicillins
FormulationsTablets, capsules (0.25 g)
Duration of action6 hours (normal half-life ~1.3 hours)
PermeabilityHydrophilic molecule, easily passes through porins of Gram-negative ($G-$) bacteria

Mechanism of Action and Pharmacokinetics

Ampicillin is a hydrophilic, broad-spectrum antibiotic. Due to its amino group side chain and hydrophilic nature, it readily penetrates the outer membrane porins of Gram-negative ($G-$) bacteria, while remaining highly effective against Gram-positive ($G+$) microorganisms.

The drug is stable in the acidic environment of the stomach, allowing oral administration. However, its gastrointestinal absorption is incomplete (30–40%) and is affected by food intake (food decreases bioavailability). Plasma protein binding is low (15–20%), and it poorly crosses the blood-brain barrier (BBB) under normal physiological conditions. It is excreted primarily unchanged by the kidneys and via bile, achieving high local concentrations in these excretion tracts.

Pharmacological Effects

The drug exerts a pronounced bactericidal antimicrobial effect by inhibiting bacterial cell wall synthesis. It is characterized by an expanded spectrum of activity covering both Gram-positive and Gram-negative flora (unlike the narrow spectrum of natural penicillins).

Due to incomplete absorption in the gastrointestinal tract, ampicillin achieves high concentrations directly within the intestinal lumen, rendering it effective against enteric infections.

Indications for Use

Ampicillin is used for various infectious and inflammatory conditions:

Adverse Effects and Risks

Under standard dosing regimens in the presence of excretory organ pathology, drug accumulation occurs, significantly increasing the risk of toxicity.

Renal function is of critical importance: in renal impairment, elimination is delayed, and the half-life ($t_{1/2}$) of ampicillin can increase from 1.3 hours normally up to 13–20 hours in renal failure. Because ampicillin is excreted unchanged, renal dysfunction mandates strict dosage adjustment.

Administration and Prescription

The drug is available as 0.25 g tablets and capsules. The standard oral dosage regimen is 0.25–0.5 g four times daily. It can also be administered parenterally (intravenously or intramuscularly).

To overcome resistance from $eta$-lactamase-producing bacteria, ampicillin is often combined with $eta$-lactamase inhibitors, such as sulbactam.

Mnemonic

AMPIcillin = AMPlified spectrum (broader than natural penicillins), but absorption is limp (only 30–40% absorbed orally, leaving plenty in the intestinal lumen).

Frequently asked questions

Which specific Gram-positive and Gram-negative bacteria are included in the spectrum of ampicillin?

Ampicillin's spectrum includes Gram-positive and Gram-negative flora, as well as anaerobes.

  • Gram-positive bacteria: Streptococci, pneumococci, enterococci, and Listeria monocytogenes.
  • Gram-negative bacteria: Haemophilus influenzae, and certain strains of Escherichia coli and Proteus.
  • Other microorganisms: Anaerobes.

Ampicillin is generally inactive or less active against staphylococci, Pseudomonas aeruginosa, Klebsiella, Moraxella, Enterobacter, and intracellular bacteria.

What adverse effects (allergic, gastrointestinal, etc.) can ampicillin cause?

Ampicillin can cause allergic, dyspeptic, and neurologic adverse reactions.

  • Allergic reactions: Ranging from skin rashes to angioedema and anaphylactic shock.
  • Specific cutaneous reactions: In patients with infectious mononucleosis, administration frequently triggers a maculopapular 'ampicillin rash,' which can become hemorrhagic.
  • Gastrointestinal: Dyspeptic symptoms are typical.
  • Neurologic disorders: High doses can cause neurotoxicity manifested as lowered seizure threshold.

Overall, the drug is well tolerated, and severe toxic reactions are rare.

How does ampicillin cross the blood-brain barrier under normal conditions versus meningeal inflammation?

Under normal conditions, ampicillin penetrates the blood-brain barrier (BBB) poorly. When purulent meningitis is suspected or confirmed, maximum doses of antibacterial agents must be used, accounting for BBB penetration. Systemic administration of high doses is necessary to achieve adequate therapeutic concentrations within the intrathecal space.

What is the primary route of elimination for ampicillin, and why is renal failure dangerous?

Ampicillin is excreted unchanged by the kidneys. In renal impairment, its half-life increases from 1.3 to 13–20 hours, leading to drug accumulation and toxicity at standard doses.

Why is ampicillin effective for intestinal infections?

The drug is incompletely absorbed in the gastrointestinal tract (30–40%), resulting in high antibiotic concentrations within the intestinal lumen that provide a therapeutic local effect.

What condition is treated with ampicillin as the drug of choice?

Intravenous ampicillin is the drug of choice for listeriosis (meningitis, endocarditis, neonatal sepsis) and invasive enterococcal infections.

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