Two cases of acute gastroenteritis occurred in 5-month-old infants hospitalized in a mother-and-child hospital in Queretaro, Mexico, on 24 January 2010. C. sakazakii was recovered from the powdered infant formula (PIF), rehydrated PIF (R-PIF) fed to infants, and their fecal samples. The microorganism was present at levels of 0.33 most probable number (MPN)/g and 24 MPN/ml in PIF and R-PIF, respectively. The total ingested dose for the day before the onset of the diarrheic syndrome ranged between 2,160 and 3,600 MPN/ml. All strains of C. sakazakii recovered from the three sources (R-PIF, PIF, and fecal matter) showed identical biotypes, adhesion and invasiveness factors, and pulsed-field gel electrophoresis profiles. No deaths were observed. Salmonella, Shigella, and enterotoxigenic Escherichia coli were not found in food or fecal samples.

IntroductionCertain strains of Cronobacter sakazakii can cause serious invasive infections in children, mainly those <2 months old and fed with powdered infant formula (PIF). The infectious dose of C. sakazakii is unknown but evidence suggests that it is approximately 1000 colony forming units (CFU). PIF is currently considered safe if its end-product C. sakazakii level is <1 CFU/g. In this study, we determined the lag time, generation time (GT), and growth rate of five pooled C. sakazakii isolates to evaluate the factors affecting contamination levels in reconstituted PIF.Methods1.71 log CFU/ml of C. sakazakii were inoculated into 100 and 3000 ml of reconstituted PIF and incubated at 22 and 35°C. Growth was evaluated over a 24-h period. ComBase was used for modeling.ResultsIn 3000 ml, the growth rate was 0.45 ± 0.02 log CFU/h with a lag phase of 3 ± 0.05 h and GT of 0.67 h at 22°C, while the growth rate was 0.73 ± 0.01 log CFU/h with a lag phase of 0.45 ± 0.03 h and GT of 0.41 h at 35° C.ConclusionCronobacter sakazakii grows rapidly in reconstituted PIF, especially at 35° C.

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The potential ability of Listeria monocytogenes to grow or survive in avocado pulp (AP) and processed guacamole (PG) stored at 22, 4 to 7, and -18 degrees C was studied. Both products were obtained from a factory in Michoacan, Mexico. PG consisted of AP mixed with dehydrated vegetables, antioxidants, and preservatives. Populations of L monocytogenes in AP stored at 22 degrees C increased from 2 to 6 and 9 log CFU/g after 24 and 48 h, respectively. At 4 to 7 degrees C, the growth rate of L monocytogenes in AP was greatly decreased; generation time was 8.2 h, in contrast with 1.35 h observed at 22 degrees C. L. monocytogenes populations did not increase in PG either at 22 degrees C for 48 h or at 4 to 7 degrees C for 15 days. The bacteriostatic effect in PG may have resulted from the presence of added substances, especially citric acid and disodium dihydrogen pyrophosphate. Aerobic plate counts and coliforms increased in AP and PG stored at ambient temperature and under refrigeration. However, these increments did not affect the growth of the pathogen. L. monocytogenes (50,000 most probable number [MPN]/g) survived at least 58 weeks in both products stored frozen at -18 degrees C; the final population was 335 MPN/g in AP and 23 MPN/g in PG. Although the composition of avocado fruit differs significantly (high content of lipids and scarcity of simple carbohydrates) from that typical of most fruits, these results underline AP as a potential vehicle of human listeriosis and indicate that freezing should not be used as the sole mechanism to control this pathogen.

The presence of some indicator microorganisms and pathogenic bacteria in guacamole sampled from restaurants and street vendors, and the behavior of Salmonella spp., Staphylococcus aureus, and Escherichia coli O157:H7 were studied in avocado pulp. Coliform, yeast and mold populations showed a wide dispersion, in agreement with the diversity of sanitary conditions observed among places sampled. The frequency of Salmonella spp., Listeria monocytogenes, and E. coli were 1.3, 16.0, and 60.0 %, respectively; with higher numbers among street vendors. Populations of E. coli ranged from 29 to 3800 NMP/g and S. aureus from 2.95 to 5.35 log CFU/g. Thirteen out of 16 hemolytic L. monocytogenes strains were pathogenic for mice. In avocado pulp Salmonella spp. and E. coli O157:H7 showed a lag phase close to 3 h, and a generation time of 54 min and 1.23 h, respectively. No growth of pathogens was observed in avocado pulp stored at 4‐7C.