Assay of the antibiotic activity of serum.

One of the drawbacks of the "tube dilution" method for the assay of antibiotics in human serum has been illustrated by utilizing serum-sensitive and serum-resistant strains of Escherichia coli. In the case of serum-sensitive strains, it was found that fresh serum alone may account for the same degree of inhibition and thus yield minimal inhibitory concentrations identical to those obtained with serum combined with antibiotics, that is, "simulated" serum assay specimens. This fallacy of the method is discussed with regard to those instances in which laboratories were merely to utilize the patient's own coliform organism as the test organism, or with respect to the assay of, for example, polymyxins, in which inadvertently a R(ough) and therefore, serum-sensitive strain of E. coli were to be used as the indicator organism. It is recommended that serum-resistant laboratory strains of Staphylococcus aureus or E. coli of known antibiotic susceptibility be employed as the test organisms proper in order to circumvent the inherent bactericidal activity of serum.