Increased efficiency of stool culture for the detection of Salmonella and Shigella

The Wampole Bactigen Salmonella-Shigella Latex Agglutination Test (SSLA) (Wampole Laboratories, Cranbury, New Jersey) was evaluated as a possible substitute for blind subculture of selenite broths from stool cultures. Recovery rates of Salmonella and Shigella from eosin-methylene blue (EMB) agar were reviewed to determine if this medium could be eliminated from primary stool culture. Salmonella was detected in 17 of 822 stools by both SSLA and culture. There were 52 false-positive SSLA for Salmonella (sensitivity 100%, specificity 93%). Of three Shigella isolated on culture, one was SSLA positive, one was SSLA negative, and one was negative by both SSLA and subculture of selenite broth. There were eight false-positive SSLA for Shigella (specificity 99%). Of 50 Salmonella and 11 Shigella isolated from 6200 stools in 1.5 years, two Shigella were isolated on EMB only. The SSLA test is a useful screening test for Salmonella. By eliminating unnecessary subcultures of selenite broth, it reduces turnaround time by 24 hr for negative stool cultures. The combination of primary culture with SSLA screening of enrichment broth should be adequate for the detection of Salmonella and Shigella from stool specimens. Our data suggest that EMB or other differential medium should be retained for primary culture to enhance detection of Shigella.