Improved detection of nontyphoid and typhoid Salmonellae with balanced agar formulations

A strategically balanced medium was developed for the improved detection of nontyphoid and typhoid salmonellae. Its balanced sugar (cellobiose, lactose, mannitol, and trehalose) and protein (beef extract and polypeptone peptone) formulation provided Salmonella with a selective growth advantage over non-Salmonella enteric organisms. The formulations promoted the production and detection of H2S production levels that otherwise might be missed with traditional agar formulations. In combination, these advantages contributed to increased sensitivity without the loss of specificity. In comparative studies using 86 samples of meat products (beef, pork, and chicken), the new media, Miller-Mallinson (MM) agar and xylose lysine tergitol (Niaproof) 4 agar, possessed significantly higher sensitivity (P < 0.001) and an improved specificity over bismuth sulfite, hektoen enteric, and xylose lysine desoxycholate agars. However, these samples did not contain nontyphoid salmonellae with weak to ultraweak H2S production characteristics. Modified formulations of MM agar were generally similar to bismuth sulfite and hektoen enteric agars in the identification of four of seven globally diverse strains of Salmonella serotype Typhi. Two of these seven strains were found to produce more readily identifiable black (H2S-positive) colonies on MM agar, whereas one of the seven was not readily detected by any of the media. The improved detection of nontyphoid and typhoid salmonellae attests to the sensitivity of MM agar and to its potentially broad utility in both clinical and food quality laboratories.