There is only a short time of the year when you can see the sun coming down into the canyon, and then only at midday. Tours at that time of the day and that time of the year sell out far in advance. You may need to book at least six months ahead of time, for a summer month tour, to take advantage of this.
The crystal violet and bile salts inhibit the growth of gram-positive organisms which allows for the selection and isolation of gram-negative bacteria. Enteric bacteria that have the ability to ferment lactose can be detected using the carbohydrate lactose, and the pH indicator neutral red.
In a crowded portion of the plate, the local acid concentration may differ from that of an isolated colony, causing a difference in the shade of the pH indicator. Among the lactose fermenters, E. coli, which produces large amounts of acid from lactose, forms colonies with a very dark, metallic sheen. Klebsiella and Enterobacter, which also ferment lactose but accumulate less acid, form colonies with a purple center and pink periphery.