[Colonization of methicillin-resistant Staphylococcus aureus in the mouse intestinal tract]

Mice were infected intravenously with rifampicin-resistant strains selected out of methicillin-resistant Staphylococcus aureus (MRSA) strains and examined for colonization of these organisms in the intestinal tract. Three of four MRSA strains colonized in the cecum of more than 80% of the mice inoculated with approximately 10(7) cfu/mouse. The extent of colonization paralleled the lethal activity, which was inversely related to the methicillin resistivity: moderately resistant strains (MIC: 12.5 micrograms/ml) tended to show more potent pathogenicity than highly resistant ones (MIC: > or = 100 micrograms/ml). Distribution and localization of the organisms in the mice infected with a moderately resistant and highly pathogenic strain S. aureus 1-6 RFPr were studied by autobacteriography. Within one day after infection, colonies of the infecting organisms were distributed all over the body and were especially dense in the liver and spleen. On day 3, the organisms in the liver and spleen disappeared, while many colonies were observed in the intestinal tract. The organisms in the intestinal tract remained for 14 days after infection. In the autobacteriograms of the mice infected with Escherichia coli KC-14 RFPr, such persistency in the intestinal tract was not demonstrated. When viable cell counts of the cecum contents of the mice infected with S. aureus 1-6 RFPr (approximately 10(7) cfu/mouse) were made, the organisms were detected as early as 3 hr after infection and then gradually increased to more than 4 logs cfu/g on day 7. With E. coli KC-14 RFPr, a few colonies were detected transiently in the cecum contents of mice at an early stage infection, but the organisms did not increase and disappeared by day 3 after infection. These results show that the colonization in the mouse intestinal tract is a specific phenomenon to the S. aureus species including MRSA strains.