A new colorimetric nucleic acid hybridization assay for Listeria in foods

Non-isotopic colorimetric detection has been applied in a rapid nucleic acid dipstick hybridization assay for detection of Listeria spp. in food and environmental samples. The assay takes approximately 2.5-3 h following a two day broth and plate enrichment. Hybridization occurs between fluorescein labeled detector probes, poly(deoxyadenosine)-tailed capture probes, and Listeria-specific regions of 16 S ribosomal RNA. These target:probe complexes are captured on poly(deoxythymidine) coated plastic dipsticks. Detection is based on binding of horseradish peroxidase conjugated anti-fluorescein antibody to the hybridization complex, and enzyme-mediated color development. The colorimetric endpoint is read on a photometer at 450 nm. In these initial studies, 306 inoculated dairy, meat and seafood samples, and 200 environmental samples were tested. When compared with total culture results the hybridization assay had unconfirmed positive and false-negative rates of approximately 1.4-2.9% and 0.8-4.7%, respectively.

Some features of base pair mismatch and heterology repair in Escherichia coli

We have used artificially constructed heteroallelic heteroduplex molecules of bacteriophage lambda DNA to transfect Escherichia coli, and E. coli mutants deficient in various functions involved in the adenine methylation-directed mismatch repair system, MutL, MutS, MutH, and UvrD (MutU). Analysis of the allele content of single infective centers shows that this repair system often acts on several mismatches, separated by as many as 2000 bp, on one of the strands of a heteroduplex molecule. When the methyl-directed mismatch repair system is disabled by mutH or uvrD mutations, localized mismatch repair becomes prominent. This prominent localized repair that can result in separation of very closely linked markers requires the functions MutL and MutS, is independent of adenine methylation, and appears to reflect another mechanism of mismatch repair. Heterology-containing heteroduplex molecules with a deletion in one strand often escape processing. However, when the heterology includes the stem and loop structure of a transposon, Tn10, the transposon is lost.