Myelopoiesis in experimentally contaminated specific-pathogen-free and germfree mice during oral administration of polymyxin.

Oral administration of polymyxin to specific-pathogen-free C3H/Law mice which with previously contaminated with gram-negative bacteria resulted in complete suppression of cecal gram-negative bacteria. Suppression of cecal gram-negative bacteria was accompanied by reduction of the cecal endotoxin concentration from 10 to 1 microgram/g of cecal content as measured with a microtechnique for the Limulus amebocyte lysate assay. Endotoxin determination by this assay appeared to be unaffected by the amount of polymyxin present in cecal preparations after oral administration of this antibiotic. In experimentally contaminated specific-pathogen-free mice, the femoral concentration of progenitor cells forming granulocyte-macrophage colonies in vitro (CFU-GM) decreased significantly (P less than 0.001) to 66% of the initial control after 4 days of polymyxin treatment. However, the femoral CFU-GM concentration in germfree mice and splenic CFU-GM concentration in experimentally contaminated specific-pathogen-free and germfree mice was not affected by polymyxin treatment. The kinetic behavior of femoral and splenic CFU-GM in experimentally contaminated specific-pathogen-free and germfree mice was expressed as the in vivo sensitivity to the S-phase-specific cytostatic drug hydroxyurea, i.e., the hydroxyurea kill. Administration of polymyxin to experimentally contaminated specific-pathogen-free mice significantly diminished the hydroxyurea kill of femoral CFU-GM from 29 to 13% (P less than 0.02) and of splenic CFU-GM from 53 to 27% (P less than 0.005). The hydroxyurea kill of femoral CFU-GM in germfree mice was not significantly affected by polymyxin treatment. On basis of these results we conclude that the effect of polymyxin treatment on myelopoiesis is most likely due to elimination of intestinal gram-negative bacteria and may indicate a significant role of intestinal gram-negative bacteria in the regulation of myelopoiesis.