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.
Table 1: Susceptibility Test Interpretive Criteria
| Susceptibility Test Result Interpretive Criteria |
Pathogen | Disk diffusion (Zone diameter in mm) | Minimal Inhibitory Concentration (MIC in mcg/mL) |
| S | I | R | S | I | R |
Enterobacteriaceae | ≥17 | 14-16 | ≤13 | ≤8 | 16 | ≥32 |
Enterococcus spp. | ≥17 | - | ≤16 | ≤8 | - | ≥16 |
Haemophilus influenzae and Haemophilus parainfluenzae | ≥22 | 19-21 | ≤18 | ≤1 | 2 | ≥4 |
Streptococcus spp. (beta-hemolytic group) | ≥24 | - | - | ≤0.25 | - | - |
Streptococcus spp. (viridans group) | - | - | - | ≤0.25 | 0.5-4 | ≥8 |
Neisseria meningitidis | - | - | - | ≤0.12 | 0.25-1 | ≥2 |
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:
Table 2: Acceptable Quality Control Ranges
| Acceptable Quality Control Ranges |
Microorganism | Disk diffusion (Zone diameter ranges in mm) | Minimal Inhibitory Concentration Range (MIC in mcg/mL) |
Enterococcus faecalis ATCC® 29212 | - | 0.5-2 |
Escherichia coli ATCC® 25922 | 16-22 | 2-8 |
Escherichia coli ATCC® 35218 | 6 | >32 |
Haemophilus influenzae ATCC® 49247 | 13-21 | 2-8 |
Staphylococcus aureus ATCC® 25923 | 27-35 | - |
Staphylococcus aureus ATCC® 29213 | - | 0.5-2 |
Streptococcus pneumoniae ATCC® 49619 | 30-36 | 0.06-0.25 |