Prescription Drug Information: Ampicillin

AMPICILLIN- ampicillin sodium injection, powder, for solution
A-S Medication Solutions

(For Intramuscular or Intravenous Injection)

To reduce the development of drug-resistant bacteria and maintain the effectiveness of ampicillin and other antibacterial drugs, ampicillin should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria.


Ampicillin for Injection, USP the monosodium salt of [2S-[2α,5α,6β(S*)]]-6-[(aminophenylacetyl)amino]-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid, is a synthetic penicillin. It is an antibacterial agent with a broad spectrum of bactericidal activity against both penicillin-susceptible Gram-positive organisms and many common Gram-negative pathogens.

Ampicillin for Injection, USP is a white to cream-tinged, crystalline powder. The reconstituted solution is clear, colorless and free from visible particulates.

Each vial of Ampicillin for Injection, USP contains ampicillin sodium equivalent to 125 mg, 250 mg, 500 mg, 1 gram or 2 grams ampicillin. Ampicillin for Injection, USP contains 65.8 mg [2.9 mEq] sodium per gram ampicillin.

It has the following molecular structure:

(click image for full-size original)

The molecular formula is C16 H18 N3 NaO4 S, and the molecular weight is 371.39. The pH range of the reconstituted solution is 8 to 10.


Ampicillin for Injection, USP diffuses readily into most body tissues and fluids. However, penetration into the cerebrospinal fluid and brain occurs only when the meninges are inflamed. Ampicillin is excreted largely unchanged in the urine and its excretion can be delayed by concurrent administration of probenecid. Due to maturational changes in renal function, ampicillin half-life decreases as postmenstrual age (a sum of gestational age and postnatal age) increases for infants with postnatal age of less than 28 days. The active form appears in the bile in higher concentrations than those found in serum. Ampicillin is the least serum-bound of all the penicillins, averaging about 20% compared to approximately 60 to 90% for other penicillins. Ampicillin for Injection, USP is well-tolerated by most patients and has been given in doses of 2 grams daily for many weeks without adverse reactions.


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 demonstrated.

Antibacterial Activity

The following bacteria have been shown in in vitro studies to be susceptible to Ampicillin for Injection, USP:

Gram-positive Bacteria

Hemolytic and nonhemolytic streptococci

Streptococcus pneumoniae

Nonpenicillinase-producing staphylococci

Clostridium spp.

B. anthracis

Listeria monocytogenes

Most strains of enterococci.

Gram-negative Bacteria

H. influenzae

N. gonorrhoeae

N. meningitidis

Proteus mirabilis

Many strains of Salmonella , Shigella , and E. coli.

AMPICILLIN does not resist destruction by penicillinase.

Susceptibility Test Methods

Diffusion Techniques

Quantitative methods that require measurement of zone diameters provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure1,2 that has been recommended for use with disks to test the susceptibility of microorganisms to ampicillin, uses the 10 mcg ampicillin disk. Interpretation involves correlation of the diameter obtained in the disk test with the minimum inhibitory concentration (MIC) for ampicillin. Reports from the laboratory providing results of the standard single-disk susceptibility test with a 10 mcg ampicillin disk should be interpreted according to the criteria provided in Table 1.

Dilution Techniques

Quantitative methods that are used to determine minimum inhibitory concentrations (MICs) provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure1,3 uses a standardized dilution method (broth or agar) or equivalent with ampicillin powder. The MIC values obtained should be interpreted according to the criteria provided in Table 1.

Susceptibility Test Result Interpretive Criteria
Pathogen Disk diffusion
(Zone diameter in mm)
Minimal Inhibitory
(MIC in mcg/mL)



14 to 16





Enterococcus spp.







Haemophilus influenzae and Haemophilus parainfluenzae


19 to 21





Streptococcus spp.

(beta-hemolytic group)







Streptococcus spp.

(viridans group)


0.5 to 4


Neisseria meningitidis





0.25 to 1


Non-meningitidis S. pneumoniae isolates may be considered susceptible to ampicillin if the isolate has a penicillin MIC of ≤0.06 mcg/mL.

Susceptibility of staphylococci to ampicillin may be deduced from testing only penicillin and either cefoxitin or oxacillin.

A report of “Susceptible”(S) indicates that the pathogen is likely to be inhibited by usually achievable concentrations of the antimicrobial compound in the blood. A report of “Intermediate” (I) indicates that the result should be considered equivocal, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where high dosage of the drug can be used. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “Resistant” (R) indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable; other therapy should be selected.

Quality Control

Standardized susceptibility test procedures require the use of laboratory control microorganisms1,2,3.

The 10 mcg ampicillin disk and the standard ampicillin powder should provide respectively the following zone diameters and MIC values in these laboratory test quality control strains:

Acceptable Quality Control Ranges
Microorganism Disk diffusion
(Zone diameter ranges in mm)
Minimal Inhibitory
Concentration Range
(MIC in mcg/mL)

Enterococcus faecalis

ATCC® 29212

0.5 to 2

Escherichia coli

ATCC® 25922

16 to 22

2 to 8

Escherichia coli

ATCC® 35218



Haemophilus influenzae

ATCC® 49247

13 to 21

2 to 8

Staphylococcus aureus

ATCC® 25923

27 to 35

Staphylococcus aureus

ATCC® 29213

0.5 to 2

Streptococcus pneumoniae

ATCC® 49619

30 to 36

0.06 to 0.25

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