Virulence factors of Escherichia coli strains isolated from diarrheic children: Influence of storage conditions.

The frequency of isolation of enterotoxigenic (ETEC), verotoxigenic (VTEC) and necrotizing Escherichia coli (NTEC) was determined in 167 children with diarrhea from a central area of Brazil. E.coli strains producing verotoxin (VT), cytotoxic necrotizing factor (CNF) and thermolabile enterotoxin (LT) were found in 3%, 3.6% and 7.2% of the cases, respectively. The E.coli strains were also assayed for their ability to induce mannose resistant haemagglutination (MRHA) of human and bovine erythrocytes. MRHA+ strains were observed in 42% of the cases and colonization factor antigens I and II were identified in 32% of the MRHA+ cases. After long term storage at room temperature, the stability of virulence factors produced by 28 selected isolates was evaluated. Production of cytotoxins was maintained in 3 of the 5 VT+ and in 5 of the 6 CNF+ isolates. Expression of LT was maintained in 9 of the 12 LT+ isolates. Of the 23 CFA+ strains, only 7 maintained the haemagglutination pattern. Expression of ST was detected in 4 strains and, besides those, 4 further ST+ strains were recognized by colony hybridization. The LT genotype was found in 11 of the selected isolates. In conclusion, by studying strains of E. coli isolated from children's diarrhea, NTEC strains were identified for the first time in Brazil. This study also confirms the great importance of storage conditions for the detection of virulence factors of ETEC and shows that VTEC and NTEC may lose its cytotoxin producing ability after prolonged storage at room temperature.