The dose at which neomycin and polymyxin B can be applied for selective decontamination of the digestive tract in mice.

Oral treatment of mice with various doses of neomycin or polymyxin B was performed in order to determine which dose caused substantial suppression of aerobic gram-negative rods. In addition the effect of the various doses on Streptococcus faecalis and on other factors of the colonization resistance (CR) of the digestive tract were studied. It was found that polymyxin B was effective in suppressing sensitive gram-negative bacteria following daily doses of 3.2 mg/mouse, and that even extremely high daily doses of 9.7 mg/mouse did not affect the CR. Neomycin was effective in suppressing Enterobacteriaceae species following oral daily doses of 5.4 mg/mouse. With this dose, however, the CR was somewhat decreased which was also evidenced by the increased concentration of beta-aspartylglycine in the faeces and the increased size (weight) of the caecum in these animals. Suppression of Str. faecalis was seen from doses of 24 mg/mouse on.