Background:
With an increase in antibiotic resistance in microorganisms, increasing the effectiveness of antibiotic therapy has become a crucial task for medical and pharmaceutical sciences. One possible way to increase the effectiveness of antibiotic therapy is to monitor biologically active substances like amino acids for their capability of increasing the activity of antibiotics. This research aimed to study the effects of tryptophan and zinc aspartate on the activity of doxycycline in relation to E. coli and P. aeruginosa.

Material and methods:
The in vitro serial dilution method was used to determine the minimum inhibitory concentration (MIC) of doxycycline independently and in combination with tryptophan and zinc aspartate, for E. coli and P. aeruginosa.

Results:
Tryptophan and zinc aspartate at a concentration of 2 μg/ml significantly increased the antibiotic activity of doxycycline against E. coli and P. aeruginosa (p<0.0001).

Conclusions:
Biologically active substances, tryptophan and zinc aspartate have a modulating effect on the antibiotic sensitivity of microorganisms towards antibacterial drugs like doxycycline.