English: This photograph depicts the colonies of Gram-negative, Proteus mirabilis bacteria, grown on a xylose-lysine-deoxycholate (XLD) agar plate. Xylose Lyseine (XL) agar is used when trying to culture and isolate Gram-negative enteric bacilli. When XL agar is supplemented with sodium thiosulfate, ferric ammonium citrate, and sodium deoxycholate, it is then termed XLD agar, and is then an even more selective medium than XL alone. The presence of any black color indicates the deposition of hydrogen sulfide, (H2S) under alkaline conditions.