Pathogens common in restaurant sauces in Mexico, study suggests

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June 25, 2002 (CIDRAP News) – Eating tabletop sauces at restaurants in Mexico may be an important risk factor for traveler's diarrhea, a new study suggests.

Investigators from Houston found that Escherichia coli was significantly more common in tabletop sauces from restaurants in Guadalajara, Mexico, than in sauces from Mexican-style restaurants in Houston. Further, E coli counts were much higher in the Guadalajara sauces, and pathogenic E coli were found only in the Guadalajara samples, according to the report in the June 18 issue of Annals of Internal Medicine.

"We've looked at sauces before, and we knew they were a source of contamination," Herbert L. DuPont, MD, senior author of the study, told CIDRAP News. "But this is the first systematic study of sauces that we've done." DuPont is medical director of the Travel Medicine Program at St. Luke's Episcopal Hospital in Houston and a clinical professor at the University of Texas Medical School, Houston, and Baylor College of Medicine.

DuPont and several colleagues tested 71 sauces from 36 independently owned Mexican-style restaurants in Guadalajara that are popular with tourists. They also analyzed 25 sauces from 12 nonchain Mexican-style restaurants in Houston. They used E coli as an indicator of fecal contamination because it is more specific than measurements of total fecal coliform bacteria, their report says. The Guadalajara sauces were all sitting on tables at room temperature when the researchers arrived, whereas in Houston all the sauces were brought to the table and were cold.

None of the sauces from either city contained non–E coli enteropathogens. But 47 of the 71 Guadalajara sauces contained E coli, versus 10 of the 25 Houston samples (P=.03). The Guadalajara samples yielded a median of 1,000 colony-forming units (CFU) of E coli per gram, as compared with 0.0 CFU/g for the Houston samples (P=.008). "The number of samples with E. coli contamination, as well as the median and total CFUs per gram of E. coli, were consistently higher in the samples from Guadalajara compared with Houston," the report says.

Guacamole was the sauce most often contaminated in both cities—3 of 3 samples in Guadalajara and 3 of 4 in Houston. In the Guadalajara samples, the sauce with the highest E coli counts was pico de gallo, which yielded a median of 10,000 CFU/g. The median counts for the other sauces included 4,000 CFU/g for guacamole, 1,000 CFU/g for green sauce, and 100 CFU/g for red sauce. In the Houston samples, the median counts for the four types of sauces ranged from 0.0 to 10 CFU/g.

Forty-three of the 47 Guadalajara samples were available to be tested for enterotoxigenic E coli, and four of these (9%) tested positive, the report states. Of 32 Guadalajara samples tested for enteroaggregative E coli, 14 (44%) were positive. All the Houston samples were tested and found to be free of pathogenic E coli.

The report says enterotoxigenic and enteroaggregative E coli are currently the major sources of diarrhea in visitors to Guadalajara. "Enteroaggregative E. coli is a newly recognized pathogen in the group of diarrheogenic E. coli," and recent studies by the Houston group have shown that this pathogen is "nearly as important as enterotoxigenic E. coli in causing traveler's diarrhea in Guadalajara," the report says. DuPont said the enteroaggregative species attach to the epithelium of the intestinal wall and cause damage that leads to inflammation and watery diarrhea.

Differences in how long the sauces were kept at room temperature may explain the higher level of contamination in the Guadalajara samples, the report says. In Guadalajara the sauces were not refrigerated before or between meals. "They just sit there on the table all day long," and successive customers may use the same bowl of sauce, DuPont said.

The findings contradict the notion that acidity prevents food contamination, because E coli was found in sauces with acidic pH levels (an average of 4.6 for Guadalajara samples and 4.9 for Houston samples), the article says. DuPont said acidity does inhibit bacteria, but if an acid food sits at room temperature for a long time, bacteria can still grow in it. Though hot sauces are less likely to be a problem than most foods, "You can't get a pH low enough if you mishandle the food," he said.

DuPont said his studies have led him to conclude that, with a few exceptions, any restaurant or street-vendor food in Mexico that is moist and sitting at room temperature is likely to be contaminated. The exceptions, he said, are foods with high sugar content, because of their osmolality, and acidic foods, provided they are not left at room temperature for a long time.

To prevent diarrheal illness when traveling in Mexico, DuPont said, "People should request that every item brought to them be steaming hot, or they should eat things that are dry, like bread, or things with high sugar content or high acid content."

Adachi JA, Mathewson JJ, Jiang ZD, et al. Enteric pathogens in Mexican sauces of popular restaurants in Guadalajara, Mexico, and Houston, Texas. Ann Intern Med 2002;136(12):884-7
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