Synergistic interaction of bactericidal drugs and a factor present in serum ultrafiltrate

Ultrafiltrate of serum of guinea-pig and man contains a factor which interacts synergistically with cephaloridine, kanamycin, and nalidixic acid against Escherichia coli. The factor on its own is not bactericidal. Gel filtration showed that the main peaks of the fractions with biological activity correspond to a MW of about 200. Chemical analysis excluded the presence of carbohydrates, proteins, ribonucleic acid at levels detectable with standard methods. Moreover the activity of the factor was not destroyed by meat nor by acid and enzymatic hydrolysis. A sequential mechanism or a reaction of the factor with drugs are considered as two possible explanations of the interaction.