OTHER MICROORGANISMS:

Actinomyces species
Bacillus anthracis
Balantidium coli
Borrelia recurrentis
Chlamydia psittaci
Chlamydia trachomatis
Clostridium species
Entamoeba species
Fusobacterium fusiforme
Mycoplasma pneumoniae
Rickettsiae
Propionibacterium acnes
Treponema pallidum
Treponema pertenue
Ureaplasma urealyticum

Susceptibility Testing

Diffusion Techniques

Quantitative methods that require measurement of zone diameters give the most precise estimate of the susceptibility of bacteria to antimicrobial agents.

One such standard procedure¹ that has been recommended for use with disks to test susceptibility of microorganisms to tetracycline uses the 30-mcg tetracycline disk. Interpretation involves the correlation of the zone diameters obtained in the disk test with the minimum inhibitory concentration (MIC) for tetracycline.

Reports from the laboratory giving results of the standard single- disk susceptibility test with a 30-mcg tetracycline disk should be interpreted according to the following criteria:

A report of “Susceptible” indicates that the pathogen is likely to be inhibited by generally achievable blood levels. A report of “Intermediate” suggests that the organism would be susceptible if high dosage is used or if the infection is confined to tissues or fluids in which high antibiotic (or antimicrobial) levels are attained. A report of “Resistant” indicates that achievable concentrations are unlikely to be inhibitory and other therapy should be selected.

Standardized procedures require the use of laboratory control organisms. The 30-mcg tetracycline disk should give the following zone diameters:

Dilution Techniques

Use a standardized dilution method² (broth, agar, microdilution) or equivalent with tetracycline powder. The MIC values obtained should be interpreted according to the following criteria:

As with standard diffusion techniques, dilution methods require the use of laboratory control organisms. Standard tetracycline pow- der should provide the following MIC values:

Animal Pharmacology and Animal Toxicology

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