Detection of Shigella in feces using DNA amplification

A rapid diagnostic method employing a polymerase chain reaction procedure (PCR) was used to identify Shigella and enteroinvasive Escherichia coli. This procedure amplified a region of the invasive-associated locus (ial) from a crude DNA extract of feces. A synthetic 21-base oligonucleotide corresponding to the ial gene sequence was shown to specifically hybridize only with enteroinvasive E. coli (EIEC) strains and Shigella species. Upon PCR amplification, a 320-base pair fragment was generated in DNA extracted from feces reconstituted with EIEC or Shigella flexneri but not in DNA from 70 normal stools lacking these organisms and could be readily detected by the ial probe. For identifying Shigella and EIEC, the PCR assay was 10(5)- and 10(2)-fold more sensitive than standard biochemical tests and the macrocolony hybridization assay, respectively. These findings demonstrate a novel methodology for rapid, sensitive, and culture-independent diagnosis of diarrhea caused by these pathogens and underscores the utility of PCR in the diagnostic laboratory.

Multi-gene amplification: simultaneous detection of three virulence genes in diarrhoeal stool

Enterotoxigenic Escherichia coli (ETEC) and Shigella account for a substantial proportion of acute diarrhoeal illnesses among Third-World children. Rapid detection of these infectious agents in faeces followed by the prompt implementation of public health measures could help reduce their spread during the early phase of epidemics. Towards this end, three pairs of synthetic oligonucleotide primers were prepared and shown to hybridize specifically to the genes encoding the heat-stable (ST) and the heat-labile (LT) enterotoxins of ETEC and to invasion-associated loci (ial) of the large Shigella virulence plasmid. When the three primer pairs were used together in the polymerase chain reaction (PCR), the three corresponding genetic loci could be simultaneously amplified using DNA extracted directly from stool; the amplified products were readily detected by ST-, LT- and ial-specific, alkaline phosphatase-labelled oligonucleotide probes (AP probes). The performance of this system was evaluated in a Mayan community in southeastern Mexico, where diarrhoeal illnesses are a common cause of childhood morbidity and mortality. Using only simple and inexpensive laboratory equipment, multigene amplification with these primers and probes led to the identification of ETEC and/or Shigella in the stools of 20 out of 71 children with diarrhoea; the procedure could be completed in seven hours and was more sensitive than conventional diagnostic tests or DNA probes used without amplification.

High serum levels interfere with the normal differentiated state of avian tendon cells by altering translational regulation

In low serum (0.2%) medium, ascorbate stimulates primary avian tendon cells to increase procollagen synthesis from 12 to 50% of total protein synthesis. This is reversibly blocked by an increase of serum levels from 0.2 to 3%. Ascorbate in low serum medium has been shown previously to stimulate the procollagen pathway by sequentially increasing by sixfold the secretion rate constant, then translation rates, and finally mRNA levels. We now show that addition of ascorbate to cultures containing 3% serum induces a sixfold increase in the secretion rate constant but translation rates and mRNA levels remain unchanged. In fully induced cells, an increase in serum levels causes a down-regulation of procollagen synthesis. In this case, the translational products of the induced cell are rapidly altered (less than 1 h), with noncollagen protein synthesis being stimulated preferentially over procollagen synthesis. This change is not reflected in procollagen mRNA levels since they remain constant for at least 6 h following addition of high serum. After 48 h in high serum, the induction of procollagen synthesis by ascorbate is reversed and the level of procollagen mRNA drops to that of uninduced cells. The data are consistent with the model that serum acts primarily at the translational level. High serum levels break the coupling in the ascorbate induction process that ties the stimulation of procollagen secretion rates to the increase in procollagen translation rates, and this prevents the maintenance of the induced state.