Inhibition zone diameter values from disc diffusion tests were recorded manually by the laboratory technicians in 1977 in Lund, in order to analyze zone size distributions species-wise.

The different locations of the wild-type populations for the two species, E.coli and P.mirabilis for ampicillin (blue bars) and nalidixic acid (red bars), respectively, illustrate the need for species-related interpretive breakpoints. This is even more evident when also other bacterial species are taken into account.