Re: are antibiotics affected by ph

Hey there!!

Thank you for the question you posted on MadSci Network.
Here is what I think about it.

Yes, antibiotics do get affected by pH, for examples see below.
This information (in italics below) I accessed from the website http://www.provet.co.uk/health/diseases/antibioticsph.htm

Some antibiotics are highly influenced by pH

The pH of the local environment can influence how effective an antibiotic is, and pH is extremely important for the following antibiotics :

Erythromycin - optimal range 7.5-8
Neomycin - optimal range 5.5-6
Nitrofurantoin - optimal pH 6
Streptomycin - optimal range 7.5-8
Chlortetracycline - optimal pH 6
For the treatment of urinary tract infections - particularly in association with urolithiasis - it is also worthwhile considering the following optimal ranges of pH for antibiotic efficacy :

Ampicillin - optimal range 5.5-6.5
Chloramphenicol - optimal range 7.5-8
Kanamycin - optimal range 7.5 - 8
Methanamine maleate - optimal range 5.5-6
Penicillin G - optimal range 6-6.5
Polymyxin - optimal range 8
Oxytetracycline - optimal range 5.5-6.5
Sulphonamides are not affected by pH, but their solubility decreases in acidic urine (pH 5.5)

Here is an abstract of an article published in the journal - European Journal of Clinical Microbiology and Infectious Diseases - about the effect of pH on the antimicrobial activity of antibiotics

König C, Simmen HP, Blaser J.
Effect of pathological changes of pH, pO2 and pCO2 on the activity of antimicrobial agents in vitro.
Eur J Clin Microbiol Infect Dis. 1993 Jul;12(7):519-26.

Since standard susceptibility tests reflect the physiological rather than the pathological conditions prevailing within an infected abdomen, as recently documented, the effect of reduced pH and pO2 and increased pCO2 on the activity of antibiotics in vitro was studied. MICs were determined in vitro under standard culture conditions (MICstandard) and modified conditions (MICmodification) simulating the previously determined pathological values. Various classes of antibiotics were affected differently by the modified conditions. However, within an antibiotic class similar results were obtained for gram-negative and gram-positive pathogens. Median MICmodification/MICstandard ratios were 4 for aminoglycosides, 2 for quinolones and clindamycin, 1 for cephalosporins, and 0.5 for penicillins and vancomycin. Anaerobic conditions and a pH of 6.4 further increased the ratio of aminoglycosides to 8. Ratios were similar within an antibiotic class at inocula of 10(5) or 10(7) cfu/ml. All MICs determined in tests with imipenem against gram-negative and gram-positive bacteria and with vancomycin against gram-positive organisms were below the susceptibility breakpoint, whatever conditions and inocula were employed. In contrast, the percentage of MICs in susceptibility range using high inocula and modified conditions decreased to 78% for penicillins, 73% for cephalosporins, 22% for aminoglycosides, 11% for quinolones and 0% for clindamycin. In conclusion, routine susceptibility testing may overestimate the activity of aminoglycosides and underestimate the activity of beta-lactams under the conditions prevailing during abdominal infection.

I hope this answers your question....

Best
Shashank