Improved XLT4 Agar: Small Addition of Peptone to Promote Stronger Production of Hydrogen-Sulfide by Salmonellae

Xylose lysine tergitol₄ agar (XLT4) is a highly selective plating medium used for isolating salmonellae. Studies have shown that XLT4 increases the recovery of salmonellae found in food, environmental and clinical samples. Further testing demonstrated that the addition of low concentrations of proteose peptone No. 3 (pp3) to XLT4 produced blacker Salmonella colonies in shorter incubation times (increased hydrogen sulfide production), while still maintaining strong inhibition of competing bacteria. The increased black colony formation facilitates prompt recognition of the weaker hydrogen sulfide-producing Salmonella strains. Test concentrations of pp3 at 0.5, 1.2 and 1.8 g/L were added to XLT4 and compared with plain XLT4 using pure bacterial cultures. In addition, these four plating media, plus xylose lysine desoxycholate agar (XLD) were evaluated using nonspiked chicken liver and pork sausage samples. The concentration of 1.2 g/L of pp3 in XLT4 gave the best overall results. In virtually all cases, the Salmonella colonies were larger and more black than on plain XLT4 without pp3. The improved XLT4 is recommended for more reliable detection of salmonellae from food, environmental and clinical samples.