IDENTIFICATION AND DETERMINATION OF GROWT H-INHIBITING AGENTS - MEAN CONCENTRATIONS AND INHIBITION ZONE WIDTHS

At the Tierhygienisches Institut Freiburg (Veterinary State Institute) , sample material of 39 animals which in a preceeding inhibitor test h ad shown positive or uncertain results was further examined by high pe rformance liquid chromatography (HPLC) and radioimmuno assay (RIA) as to identify and quantify the corresponding growth inhibiting agents. T hese attempts were carried out successfully in 50 % of all cases. On t he basis of ''relative susceptibilities'' of the test system (bacillus subtilis) against several antibiotics ''activity equivalents'' have b een derived from the concentrations of these substances, the logarithm s of which show a surprising grade of correlation with the widths of t he corresponding inhibition zones. By means of Linear regression, ''me an concentrations'' were estimated for the antibiotics which can be ex pected at different inhibition zone widths. The results presented sugg est that violations of given regulatory limits cannot always be recogn ized by inhibitor test findings alone. The idea of further examination of sample material with positive or uncertain inhibitor test findings by means of suitable quantitativ methods is therefore strongly suppor ted.