EFFECT OF DIETARY PROTEINS AND AMINO ACIDS ON THE SUSCEPTIBILITY OF MICE TO BACTERIAL INFECTIONS

Abstract

Young mice were maintained for 1 to 6 weeks on experimental diets containing all known growth factors, but different amounts of proteins and amino acids. All diets were supplemented with L-cystine. An infective dose of one of 4 pathogens (Mycobacterium tuberculosis var. bovis. Mycobacterium fortuitum, Staphylococcus aureus, Klebsiella pneumoniae, type C) was administered by the intravenous, intraperitoneal, or air-borne (aerosol) route. Whether the animals were maintained in groups of 5 throughout the tests or housed in individual cages did not affect the results detectably. Mice fed diets containing 5 or 8% casein plus a cystine supplement proved more susceptible to the experimental diseases than did mice fed diets containing 15 or 20% of casein. Susceptibility to infection developed when wheat gluten or soybean [alpha]-protein, was substituted for casein, even in 15 or 20% concentrations. In one experiment, mice were fed a diet containing a mixture of soybean and rice flour, to provide a protein concentration of 15%, with an amino acid pattern similar to that of casein. These animals gained weight at the same rate as those fed a diet containing 15% casein and exhibited satisfactory resistance to bacterial infection. The infection-enhancing effect of low casein concentration (5 and 8%) could be corrected by supplementing the diet with the proper mixture of amino acids, either in the synthetic or natural form. Susceptibility to infection developed when low casein diets were supplemented with unbalanced mixtures of amino acids. The infection-enhancing effect of gluten diets could not be corrected by supplementing them with lysine, even though such supplementation markedly improved weight gains in uninfected animals. The relative proportion of the various amino acids in the diet is apparently as important as their total amount in conditioning resistance to bacterial Infections. This has been observed with 2 strains of mice differing markedly in their natural resistance to bacterial infection.