 "Salmonella enterica" serovar Typhi may result in blackening of the medium at the interface of bottom and the slant.Under anaerobic conditions (as occur toward the bottom of the tube) some bacteria use thiosulfate as an electron acceptor and reduce it to hydrogen gas.This is not very soluble and may accumulate as bubbles along the inoculation track, between the agar and the glass, or in the fluid which accumulates at the bottom of the slant.