Non–O157 Shiga Toxin–ProducingEscherichia coliIs the Most Prevalent DiarrheagenicE. coliPathotype in Street‐Vended Taco Dressings in Mexico City

TO THE EDITOR—We read with great interest the article by McPherson et al [1] that addressed the risk factors for Shiga toxin–producing Escherichia coli (STEC) infection—in particular, food consumption—which have not been previously examined in Australia. Apparently, this is the first study to have examined risk factors for non-O157 STEC infections in both pediatric and adult populations. The study compared case patients infected with O157 and non-O157 STEC with control subjects, revealing that risk factors were different between these patients. Furthermore, negative associations were observed particularly with home-grown vegetables, fruits, and herbs for both O157 and non-O157 STEC infections. Although outbreaks of O157 STEC infection have been associated with consumption of raw vegetables [2, 3], little is know about the presence of non-O157 STEC [4 ] in these food items worldwide. In many developing countries, including Mexico, street-vended food is a major source of fast, ready-to-eat meals for a large proportion of the population. In Mexico, street-vended food has been found to be contaminated with diarrheagenic E. coli pathotypes [5] and has been associated with travelers' diarrhea [6]. Consequently, we analyzed 178 street-vended taco dressings (chili sauces, coriander-onion, pico de gallo, guacamole, and lettuce) collected in Mexico City (table 1). One hundred twenty (67%) of 178 samples were contaminated with E. coli, and 8 (7%) of them were positive for diarrheagenic E. coli pathotypes by polymerase chain reaction [7], of which 6 (75%) of the 8 belonged to the STEC pathotype; all were found to be serotype O157 negative by the Oxoid agglutination test (table 1). Four of the non-O157 STEC strains were resistant to ampicillin, and 1 was also resistant to chloramphenicol; all strains were susceptible to trimethoprim-sulfamethoxazole, ciprofloxacin, and cefotaxime. In Mexico City, non-O157 STEC strains have been associated with both cases of community diarrhea [8, 9] and acute diarrhea that required hospitalization in children [10]. It seems that street-vended taco dressings could be a potential vehicle of transmission for non-O157 STEC strains in Mexico City, given that all taco dressings harbored STEC levels of >1×103 organisms/g (table 1)—well above the estimated infectious dose of 50–100 CFU for O157:H7 [11]. However, to understand the epidemiology of non-O157 STEC in Mexico, studies such as the one described by McPherson et al [1] are urgently needed. Taco Dressing That Contained Escherichia coli, Diarrheagenic E. coli Pathotypes (DEPs), and Non-O157 Shiga Toxin–Producing Escherichia coli (STEC) Potential conflicts of interest. All authors: no conflicts.