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Orally administered erythromycin ethylsuccinate suspensions and Filmtab tablets are readily and reliably absorbed. Comparable serum levels (erythromycin ethylsuccinate) of erythromycin are achieved in the fasting and nonfasting states.
Erythromycin diffuses readily into most body fluids. Only low concentrations are normally achieved in the spinal fluid, but passage of the drug across the blood-brain barrier increases in meningitis. In the presence of normal hepatic function, erythromycin is concentrated in the liver and excreted in the bile; the effect of hepatic dysfunction on excretion of erythromycin by the liver into the bile is not known. Less than 5 percent of the orally administered dose of erythromycin is excreted in active form in the urine.
Erythromycin acts by inhibition of protein synthesis by binding 50 S ribosomal subunits of susceptible organisms. It does not affect nucleic acid synthesis. Antagonism has been demonstrated in vitro between erythromycin and clindamycin, lincomycin, and chloramphenicol.
Many strains of Haemophilus influenzae are resistant to erythromycin alone but are susceptible to erythromycin and sulfonamides used concomitantly.
Staphylocci resistant to erythromycin may emerge during a course of therapy.
Erythromycin has been shown to be active against most strains of the following microorganisms, both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.
The following in vitro data are available, but their clinical significance is unknown.
Erythromycin exhibits in vitro minimal inhibitory concentrations (MIC's) of 0.5 μg/mL or less against most ( ≥ 90%) strains of the following microorganisms; however, the safety and effectiveness (erythromycin ethylsuccinate) of erythromycin in treating clinical infections due to these microorganisms have not been established in adequate and well controlled clinical trials.
Viridans group streptococci
Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MIC's). These MIC's provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MIC's should be determined using a standardized procedure. Standardized procedures are based on a dilution method1 (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of erythromycin powder. The MIC values should be interpreted according to the following criteria:
|≤ 0.5||Susceptible (S)|
|≥ 8||Resistant (R)|
A report of “Susceptible” indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable. A report of “Intermediate” indicates that the result should be considered equivocal, and, 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 drug can be used. This category also provides a buffer zone which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “Resistant” 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.
Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures. Standard erythromycin powder should provide the following MIC values:
|S. aureus ATCC 25923||0.12-0.5|
|E. faecalis ATCC 29212||1-4|
Quantitative methods that require measurement of zone diameters (erythromycin ethylsuccinate) also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure2 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 15-μg erythromycin to test the susceptibility of microorganisms to erythromycin.
Reports from the laboratory providing results of the standard single-disk susceptibility test with a 15 μg erythromycin disk should be interpreted according to the following criteria:
|Zone Diameter (mm)||Interpretation|
|≥ 23||Susceptible (S)|
|≤ 13||Resistant (R)|
Interpretation should be as stated above for results using dilution techniques. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for erythromycin.
As with standardized dilution techniques, diffusion methods require the use of laboratory control microorganisms that are used to control the technical aspects of the laboratory procedures. For the diffusion technique, the 15-μg erythromycin disk should provide the following zone diameters (erythromycin ethylsuccinate) in these laboratory test quality control strains:
|Microorganism||Zone Diameter (mm)|
|S. aureus ATCC 25923||22-30|
1. National Committee for Clinical Laboratory Standards, Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically , Third Edition. Approved Standard NCCLS Document M7-A3, Vol. 13, No. 25. NCCLS, Villanova, PA, December 1993.
2. National Committee for Clinical Laboratory Standards, Performance Standards for Antimicrobial Disk Susceptibility Tests, Fifth Edition. Approved Standard NCCLS Document M2-A5, Vol. 13, No. 24. NCCLS, Villanova, PA, December 1993.
Last reviewed on RxList: 2/11/2009
This monograph has been modified to include the generic and brand name in many instances.
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