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Published on: November 16, 2016
Effects of environmental factors on the in vitro potency of azithromycin
J A Retsema1, L A Brennan, A E Girard
1Central Research Division, Pfizer Inc., Groton, CT 06340.
Abstract:
The effects of media, pH, cations, serum, CO2 or anaerobic atmosphere, inoculum size and time of incubation on the in vitro potency of azithromycin were determined. The potency of azithromycin against all genera was particularly sensitive to changes in pH. The MIC for Staphylococcus aureus strains ranged from 50 micrograms/ml at pH 6 to less than or equal to 0.025 micrograms/ml at pH 8; for erythromycin the MIC change was less (1.6 to 0.05 micrograms/ml). Incubation for 18 h in 5% CO2 or an anaerobic atmosphere (10% CO2, 10% H2, 80% N2) lowered the pH by approximately 0.8 units with gram-negative organisms and 0.4 units with gram-positive organisms. This resulted in an MIC eight times greater than the aerobic MIC. In addition, the MIC100 for azithromycin and erythromycin against Bacteroides strains growing in Wilkins-Chalgren broth fell from 3.1 micrograms/ml in the anaerobic atmosphere to 0.2 and 0.4 micrograms/ml, respectively, when using the Oxyrase enzyme system to remove oxygen. With the Oxyrase system, the pH of the medium at the MIC remained at 7.2, while it fell to 6.7 in the anaerobic gas mixture. An increase in potency for both agents was also observed with other anaerobic species when using the Oxyrase system. The addition of serum produced an increase in potency of azithromycin and erythromycin that correlated with an increase in pH during incubation, despite the use of buffered media. Adding cations to Mueller-Hinton broth resulted in increased MICs for gram-negative organisms; the highest increases observed were four-fold for Escherichia coli.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Azithromycin potency is highly sensitive to pH, with lower pH significantly reducing its effectiveness against bacteria. Optimizing conditions like pH and atmosphere enhances antibiotic activity.
Area of Science:
- Microbiology
- Pharmacology
- Infectious Diseases
Background:
- Azithromycin is a widely used antibiotic.
- Understanding factors affecting antibiotic potency is crucial for effective treatment.
- Previous studies have indicated environmental factors can influence antibiotic efficacy.
Purpose of the Study:
- To investigate the impact of various in vitro conditions on azithromycin potency.
- To compare the effects of pH, atmosphere, and media components on azithromycin and erythromycin activity.
- To identify optimal conditions for azithromycin activity against a range of bacterial genera.
Main Methods:
- Determined minimum inhibitory concentrations (MICs) of azithromycin and erythromycin.
- Varied parameters including pH, cations, serum, CO2/anaerobic atmosphere, inoculum size, and incubation time.
- Utilized the Oxyrase enzyme system to manipulate oxygen levels and observe effects on anaerobic bacteria.
Main Results:
- Azithromycin potency was most sensitive to pH changes, with MICs varying significantly between pH 6 and 8.
- Acidic conditions (lower pH) due to CO2 or anaerobic atmospheres increased MICs (reduced potency) for both gram-positive and gram-negative organisms.
- The Oxyrase system improved azithromycin and erythromycin potency against anaerobic bacteria by maintaining a neutral pH.
- Serum addition increased potency, correlating with increased pH, while cations generally increased MICs for gram-negative bacteria.
Conclusions:
- pH is a critical determinant of azithromycin in vitro potency.
- Anaerobic conditions and associated pH shifts reduce azithromycin efficacy.
- The Oxyrase system shows promise for enhancing the study of anaerobic bacteria and antibiotic activity.
- Careful consideration of in vitro conditions is necessary for accurate assessment of antibiotic potency.
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