Abstract: Rapid differentiation between viable gram-positive and gram-negative bacteria is accomplished with certain polypeptide antibiotics which are used in combination with a compound which is normally reducible by the bacteria. The antibiotics selectively inhibit the reduction of the reducible compound by gram-positive bacteria but do not substantially affect the reducing capacity of the gram-negative bacteria. The particular antibiotics useful are cyclic polypeptides which affect the function of the cytoplasmic membrane of bacteria. A particular polypeptide antibiotic, polymixin B, will distinguish Proteus bacteria from other gram-negative genera.

Abstract: Rapid differentiation between viable fungi (for example, yeast) and viable bacteria is accomplished with certain polyene antibiotics which are used in combination with a compound which is normally reducible by both the fungi and the bacteria. The antibiotics selectively and substantially inhibit the reduction of the reducible compound by the fungi but do not affect the reducing capacity of the bacteria. The particular antibiotics useful are polyenes which selectively affect the function of the cytoplasmic membrane of fungi.

Abstract: Rapid differentiation between viable gram-positive and gram-negative bacteria is accomplished with polyether antibiotics which are used in combination with a compound which is normally reducible by the bacteria. The antibiotics selectively and substantially inhibit the reduction of the reducible compound by gram-positive bacteria but do not substantially affect the reducing capacity of the gram-negative bacteria.