Growth-inhibitory effects of sulfonamides at different pH: dissimilar susceptibility patterns of a soil bacterium and a test bacterium used for antibiotic assays

The ionic speciation of sulfonamides is pH-driven and this may be crucial for their bioavailability and sorption to soil constituents, as well as for their uptake into bacterial cells. The inhibition behaviour of a bacterial test strain (Pseudomonas aeruginosa; DSM 1117), which was grown in the presence of different concentrations of 8 sulfonamides at pH values from 5 to 8, could be predicted by models that take the speciation of sulfonamides in- and outside of bacterial cells into account. Assuming a pH of 7.5 inside the cells (pH homeostasis), the strongest inhibition was predicted for the lowest external pH and for sulfonamides with the lowest pK(a) values. Growth experiments with Ps. aeruginosa basically reflected this predicted behaviour. However, Pantoea agglomerans -- a bacterial strain isolated from arable soil -- behaved surprisingly different regarding its pH dependency: all sulfonamides showed the strongest effects at pH 7 to 8 instead of being most effective at lowest pH, although the pK(a) dependencies followed the same pattern. Experimental and modeling results could be brought into good agreement for P. agglomerans if the cell-internal pH was admitted to approximate the external pH instead of implying pH homeostasis for modeling calculations. Thus, besides the actual concentration of sulfonamides, the pH dependent mode of reaction of different bacteria to sulfonamides may additionally govern the population dynamics in soils.