Regression Line Analysis of the Synergistic Effect for the Combination of Trimethoprim/Sulphamethoxazole

Analysis of regression lines, i.e. the relationship between the zone of inhibition of bacterial growth around drug-impregnated discs and minimal inhibitory concentration for the same drug in agar, have been used to illustrate the synergistic effect of the combination trimethoprim/sulphamethoxazole in the ratio 1:20. The synergy could be well illustrated on a peptone-free blood agar medium, less well on a Mueller-Hinton agar medium containing 5% lysed horse blood but could not be illustrated on just Mueller-Hinton agar medium.