A kinetic microplate method for quantifying the antibacterial properties of biological fluids

The microplate assay for measuring antibacterial activity was adapted by incorporating a known concentration range of gentamicin as an internal standard. Staphylococcus aureus subsp. aureus strain ATCC 6538P, Escherichia coli ATCC 25922, and Lactobacillus spp. were used as target organisms, although other indicator organisms and antibiotics can be examined. Assay range and sensitivity were dependent on the species and density of indicator organism, and conditions (e.g., type, concentration, and pH of growth medium). Plotting the area under the growth curve (AUGC) versus gentamicin concentration (log transformed) yielded a linear curve that was used to quantify in units of gentamicin the antibacterial activity of a secretory fluid (SCF; pancreatic juice) and for comparisons of samples collected at different times, analysed on different days, and from different studies. This adaptation of the microtiter broth method will be useful for investigating man-made compounds, and the antibacterial activity of secretory fluids and the influences of age, diet, and health status.