The influence of the intestinal microflora on the relative thymus weight

Oral treatment of mice with a combination of non-absorbable broad spectrum antibiotics at a dose level high enough to sterilize the digestive tract within five days of treatment, has been found to have a significant negative effect on the relative weight of the thymus. From the age of 12 weeks onwards, the mean relative weight of the thymus of conventional mice appeared to differ significantly from the mean relative thymus weight in germ-free mice of the same strain. In the course of two to three weeks of continuous oral antibiotic treatment the mean relative weight of the thymus decreased from conventional values to values seen in germ-free mice of the same strain and age. It is concluded that the intestinal microflora apparently exerts an influence on the thymus.