Shigella sonnei strains isolated from U.S. summer students in Guadalajara, Mexico, from 1986 to 1992.

Plasmid DNA analysis and antibiotic susceptibilities were used to study strains of Shigella sonnei isolated from U.S. travelers to Guadalajara, Mexico, over a period of seven years (1986 to 1992). One hundred sixty-one isolates were analyzed. By the use of cluster analysis, eight different plasmid profiles were identified during this interval. At any point in time, three to seven different plasmid profiles were present in this population. The introduction of strains that carried a new plasmid with a molecular mass of 5.1 MDa was coincidental with an increase in isolation of S. sonnei in 1988. This new plasmid was present in 87.5% of the isolates that were resistant to chloramphenicol. Shigellosis in Guadalajara follows a pattern of hyperendemic transmission with transient peaks of high-frequency isolation of S. sonnei. This pattern results from the concurrent presence of a heterogeneous group of strains as opposed to the widespread transmission of one or a few clones.