Antibiotic Potency Testing

Antibiotic Potency Testing

Pharmaceutical products must be evaluated for efficacy as well as safety. Antibiotic formulations must go through potency testing prior to market release. To this end, a variety of validated, compendial microbial and biological assays are routinely used to confirm sufficient antibiotic efficacy. Antibiotic potency testing quantifies the degree of microbial growth inhibition by antibiotic(s) in a lot sample and thus provides a real authentication of antibiotic potency.

Our Antibiotic Potency Testing Capabilities

CPT Labs has over 40 years of experience in consumer product testing and regulatory compliance evaluations for multiple industries including, but not limited to: pharmaceuticals, OTC drugs, consumer healthcare products, cosmetic and personal care products, specialty chemical and medical device industries. Our seasoned Microbiology specialists are highly knowledgeable in validated, compendial product quality evaluation methods and current microbial safety requirements. For the highest quality testing, antibiotic potency assessments should be performed by experienced personnel with expertise in microbiology and aseptic techniques as well as analytical biology methods. CPT Labs performs all antibiotic potency testing according to validated procedures outlined in USP <81> Antibiotics—Microbial Assays. USP <81> is the primary and most commonly used reference document for biological antibiotic potency evaluations.

Sample Antibiotic Potency Testing Workflow

During antibiotic potency testing, different concentrations of the test antibiotic are selected to test against a live microorganism to determine both antibiotic efficacy against microbial growth and the concentration at which a given antibiotic inhibits microbial growth. A five-point standard curve is run concurrently to provide a point of comparison to determine antibiotic potency or “activity.” Here, growth inhibition is integrated into a linear regression curve from which antibiotic potency values may be extrapolated. Ultimate antibiotic potency depends on how specific the antibiotic is to the live microorganism included in the test. There are two main methods for determining antibiotic potency:
  1. Diffusion (Cylinder-Plate) Method

The Diffusion (Cylinder-Plate) method is a plate-based assay that operates based on diffusion of the antibiotic understudy through solid agar culture media. Test microorganisms are first plated on a plate containing solid agar culture media. Next, a vertical metal cylinder or paper disk containing the test antibiotic is placed atop the agar plate. The cylinder-plate is then incubated for hours to days, depending on the test microorganism and antibiotic(s). During this time, antibiotic is allowed to diffuse from the cylinder or disk into the agar. Effective antibiotics will yield a circular zone around the disk or cylinder where microorganism growth has been prevented by antibiotic that has diffused into the agar. This test may be performed at multiple concentrations to determine the minimum effective concentration for a given antibiotic.
  1. Turbidimetric Method

The Turbidimetric Method is a tube-based assay in which known concentrations of antibiotic are added to liquid culture media containing a live microorganism in a tube. The mixture is then incubated for several hours and solution turbidity is measured via spectrophotometry. Solution turbidity is directly proportional to microbial growth and is thus used to determine antibiotic potency. While both the Diffusion (Cylinder-Plate) Method and Turbidimetric Method measure the ability of an antibiotic to drive microbial growth inhibition, the final choice of which method to use depends on the antibiotic(s) under study. USP <81> defines the appropriate test method for a number of antibiotics as shown below:
Antibiotic Under Study Potency Assay Type Test Bacteria for Assay
Amikacin Turbidimetric Staphylococcus aureus
Amphotericin B Cylinder-Plate Saccharomyces cerevisiae
Bacitracin Zinc Cylinder-Plate Micrococcus luteus
Bleomycin Cylinder-Plate Mycobacterium smegmatis
Candicicin Turbidimetric Saccharomyces cerevisiae
Capreomycin Turbidimetric Klebsiella pneumoniae
Carbenicillin Cylinder-Plate Pseudomonas aeruginosa
Cephalothin Cylinder-Plate Staphylococcus aureus
Cephapirin Cylinder-Plate Staphylococcus aureus
Chloramphenicol Turbidimetric Escherichia coli
Chlortetracycline Turbidimetric Staphylococcus aureus
Cloxacillin Cylinder-Plate Staphylococcus aureus
Colistimethate Sodium Cylinder-Plate Bordetella bronchiseptica
Colistin Cylinder-Plate Bordetella bronchiseptica
Cycloserine Turbidimetric Staphylococcus aureus
Demeclocyline Turbidimetric Staphylococcus aureus
Dihydrostreptomycin Cylinder-Plate/Turbidimetric Bacillus subtilis/Klebsiella pneumoniae
Doxycycline Turbidimetric Staphylococcus aureus
Erythromycin Cylinder-Plate Micrococcus luteus
Gentamicin Cylinder-Plate Staphylococcus epidermidis
Gramicidin Turbidimetric Enterococcus hirae
Kanamycin Turbidimetric Staphylococcus aureus
Methacycline Turbidimetric Staphylococcus aureus
Nafcillin Cylinder-Plate Staphylococcus aureus
Neomycin Cylinder-Plate/Turbidimetric Staphylococcus epidermidis/Klebsiella pneumoniae
Netilmicin Cylinder-Plate Staphylococcus epidermidis
Novobiocin Cylinder-Plate Staphylococcus epidermidis
Nystatin Cylinder-Plate Saccharomyces cerevisiae
Oxytetracycline Turbidimetric Staphylococcus aureus
Paromomycin Cylinder-Plate Staphylococcus epidermidis
Penicillin G Cylinder-Plate Staphylococcus aureus
Polymyxin B Cylinder-Plate Bordetella bronchiseptica
Rolitetracycline Turbidimetric Staphylococcus aureus
Sisomicin Cylinder-Plate Staphylococcus epidermidis
Streptomycin Turbidimetric Escherichia coli
Tetracycline Turbidimetric Klebsiella pneumoniae
Thiostrepton Turbidimetric Enterococcus hirae
Tobramycin Turbidimetric Staphylococcus aureus
Troleandomycin Turbidimetric Klebsiella pneumoniae
Tylosin Turbidimetric Staphylococcus aureus
Vancomycin Cylinder-Plate Bacillus subtilis

Our Departments

CPT offers several state-of-the-art Analytical, Microbiological, Clinical, Photobiology and In Vitro Toxicology departments for our clients’ product testing needs. Our facilities are maintained at a high level of regulatory compliance to ensure the best quality assessments of product safety, stability and efficacy. We strongly adhere to Good Laboratory Practices (GLP), Good Manufacturing Practices (GMP) and Good Clinical Practices (GCP) standards to ensure the best quality results for our clients. Custom-tailored testing programs and flexible turn-around times are available. Contact us to learn more.
  • Clinical —Efficacy trials can be performed for safety, consumer in-use, and to support claims, such as ‘dermatologist-tested’ and ‘ophthalmologist-tested.’
  • In-Vitro Safety — Our in-vitro safety department assists clients when they are looking to perform non-animal or animal-free toxicology testing.
  • Microbiology — Microbial contamination and preservative testing is key to cosmetic products.
  • Photobiology —From SPF and Water Resistant claims to PhotoAllergy and Phototoxicity studies, CPT is the world leader in sunscreen testing.
  • Analytical — From raw materials, batch release, and stability shelf-life testing to “free-of” and low-level detection claims, our analytical department ensures that all requirements are met based on industry standards.

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