Identification and epidemiological description of enterohemorrhagic Escherichia coli O157 strains producing low amounts of Shiga toxin 2 in Aichi Prefecture, Japan

Out of 68 Esherichia coli O157 field isolates tested in vitro for Shiga toxin (Six) 2 production, 12 (17.6%) produced no or a limited amount of Stx2 (Stx 2 non- or low-producing strain; TNLP) even though all 68 possessed the stx₂ gene. The remaining 56 were Stx2 high-producing strains. The 12 TNLPs carried the q21 gene allele, which encodes a transcription antiterminator Q protein and is highly homologous to that of Φ21 phage. They also carried nucleotide substitutions and insertions in the promoter region of the stx₂ gene compared with that of O157 EDL933, producing a considerable amount of Stx2. In contrast, the Stx2 high-producing strains carried the q933 gene allele, which was first reported on an stx₂ phage (933W), but not the q21 gene allele, and did not have mutations in the promoter region of the stx₂ gene. These 2 genetic characteristics, i.e., replacement of the q gene and mutation in the promoter region of the stx₂ gene, seemed to determine the amount of Stx2 produced by each strain. The TNLPs were more frequently isolated from healthy carriers than from patients (P < 0.05), suggesting that TNLPs are less virulent than those with normal Stx2 production.