Determination localized (subcutaneous) resistance to Salmonella infection in mice (capillary-tube insertion method)

Abstract

This investigation was designed to elucidate possible variation in the degree of natural resistance of mice which might arise as a consequence of experimental Salmonella infection, and to predetermine, if possible, the susceptibility of an individual mouse to Salmonella.The capillary-tube insertion method which was developed in this investigation was carried out in the following manner.One-tenth volume of broth culture (18∼24 hours) of a definite strain of bacteria was mixed with 0.8% nutrient agar previously melted and kept at 47°C. A capillary tube 0.75mm in inside diameter and 4mm in length was filled with this mixture, and then inserted into the subcutis of the abdominal wall of a mouse for 48 hours. After that the number of surviving bacteria in the tube was counted by the plate method.Actually, during the insertion period of 2 days, humoral factors diffused well, and phagocytes infiltrated moderately in the tube. The number of viable bacteria seemed to represent the sum of humoral and cellular resistance. It is conceived that this method has many advantages for studies on the natural resistance of experimental animals.When three capillary tubes containing S. enteritidis 1891, Escherichia coli B, and Staphylococcus aureus 209 P, respectively, were inserted into individual mice, the number of surviving bacteria of each strain in the same mouse was inversely proportional to the degree of the virulence of such strain. There existed a linear relationship between the number of viable organisms and the degree of natural resistance of each mouse. In other words, a mouse resistant to S. enteritidis was also resistant to E. coli B and Staph. aureus. Reversely, a mouse susceptible to the former was susceptible to the latter. A parallel relationship was demonstrated between the survival number in the tube inserted into the subcutis and the abdominal cavity.Accordingly, it was assumed that the susceptibility of each mouse to S. enteritidis 1891 might be predetermined from the viable number of Staph. aureus in a tube inserted in the subcutis.