Comparison of penicillin-G susceptibility testing methods of staphylococci isolated from bovine mastitis.

Penicillin-G susceptibility was analyzed on forty staphylococcal strains isolated from mastitic bovine quarters (29 coagulase positive and 11 coagulase negative) by the standard Kirby-Bauer agar diffusion method, the epsilon-Test, the Vetmic, turbidimetric MIC analysis in Iso-Sensitest Broth (ISB) and whey. Penicillinase production was tested. Parallel susceptibility tests were carried out in whole milk, whey and ISB using resazurin and triphenyltetrazolium as the indicators of bacterial activity.

Relevance of sensitivity testings (MIC) of S. aureus to predict the antibacterial action in milk.

Bacterial susceptibility testings were carried out in parallel Iso-sensitest broth (ISB) and bovine milk cultures using 16 antibacterials and 4 sensitive strains of mastitic isolates of Staphylococcus aureus. Bacterial activities were analyzed by continuous turbidity monitoring (broth cultures), continuous fluorometric monitoring of the resazurin-reducing redox activity, and by analyzing the triphenyltetrazolium (TTC)-reducing capacity at the end of the incubation period. To obtain an equipotent bacteria-suppressing activity, milk cultures required in general several times more antibiotic than the respective ISB cultures.

Automation of the resazurin reduction test using fluorometry of microtitration trays.

Microtitration plate technique and -fluorometry was applied to automate the resazurin reduction test for monitoring bacterial numbers in broth cultures and milk. The effect of resazurin and resorufin concentration, bacterial species, growth medium, pH, and redox potential on the fluorescence response was studied. The timing of the appearance of maximum fluorescence was directly related to the logarithm of the number of colony-forming units (log CFU).