Distribution of Shigella enterotoxin genes and secreted autotransporter toxin gene among diverse species and serotypes of shigella isolated from Andaman Islands, India

We studied the prevalence and distribution of the newly described genes for Shigella enterotoxins (ShET1 and ShET2, encoded by set and sen genes) and secreted auto-transporter toxin (encoded by sat gene) in clinical isolates from the Andaman Islands, India. A total of 153 Shigella isolates obtained from hospitalized patients during 1994-2004 were analysed. These isolates included all the four species of Shigella (S. dyseteriae- 29, S. flexneri- 75, S. sonnei- 38, S. boydii- 5) that belonged to diverse serotypes (including serologically untypable - 6) and each serotype included a wide variety of genotypes. Each isolate underwent polymerase chain reaction (PCR) for detection of set, sen and sat genes employing specific primers. We found the set gene in all S. flexneri 2a and 2b isolates (41 of 41, 100%) but not outside S. flexneri serotype 2. The sen gene was well distributed among all species and serotypes but its presence was apparently low at 49.1% (75 of 153), probably because of the loss of the large plasmid that harbours the gene in 76 of the 78 (97.4%) sen negative isolates. Also, all S. flexneri 2 isolates (including 2a and 2b serotypes) had the sat gene. It was present in 96% (72 of 75) of S. flexneri, in 6.9% (2 of 29) of S. dysenteriae, in 20% (1 of 5) of S. boydii, and in 33.3% (2 of 6) of untypable Shigella, but not in (0 of 38) S. sonnei. This study provides initial data on the prevalence and distribution of of the set, sen and sat genes in a wide variety of Shigella isolated over a 10-year period. Our results suggest a greater prevalence of the set and sat genes in S. flexneri 2 isolates than previously thought and might help in future pathochip designs.