Distribution of Pseudomonas Fragi

Pseudomonas fragi is of special significance to the dairy industry because of the off flavors it causes in milk, cream, butter, cottage cheese and other products (1).It is both proteolytic and lipolytic and in addition is psychrophilic so that it can bring about objectionable changes at temperatures commonly used for holding dairy products.Because of the frequency with which Ps. fragi is responsible for spoilage in dairy products its distribution was studied, and the results are reported herein.Most of the samples examined for the organism originated in Iowa, but a number were obtained in Kentucky and some in other states. ISOLATION PROCEDURE Collection of samples.Milk samples were collected in sterile, cottonstoppered test tubes and samples of such materials as butter, water, soil, etc., were collected in sterile, screw-cap, glass jars or bottles.Material for examination Was obtained from such possible sources as stanchions, milking utensils, floors, churns, vats, etc., by means of sterile moistened swabs. Isolation of cultures.In attempting to isolate cultures an enrichment procedure (1) was regularly used.Milk samples, or tubes of sterile litmus milk inoculated with other materials sampled or with swabs used on equipment, floors, etc., were kept at 5 ° to 10 ° C. for 7 to 10 days and then plated on beef extract or beef infusion agar to which nile blue sulfate and butter fat emulsion had been added (2).Dilutions used were from 1-10,000 to 10,000,000.Plates were incubated at room temperature (20 ° to 22 ° C.) for 2 to 3 days.Colonies of tipolytic organisms suggestive of Ps. fragi were picked into tubes of litmus milk.Characteristic colonies take up very little dye from the medium and appear almost chalk white when examined from the bottom of the plate with a lens.The litmus milk cultures were incubated at room temperature.Usually, after 2 to 3 days cultures of Ps. fragi developed a May apple odor and also a pink or acid ring at the surface of the milk next