Type I DNA topoisomerases are ubiquitous enzymes involved in many aspects of DNA metabolism. Escherichia coli possesses two type I topoisomerase activities, DNA topoisomerase I (Topo I) and III (Topo III). The gene encoding Topo III (topB) can be deleted without affecting cell viability. Cells possessing a deletion of the gene encoding Topo I (topA) are only viable in the presence of an additional compensatory mutation. In the presence of compensatory mutations, Topo I deletion strains grow normally; however, if Topo III activity is repressed in these cells, they filament extensively and possess an abnormal nucleoid structure. These defects can be suppressed by the deletion of the recA gene, suggesting that these enzymes may be involved in RecA-mediated recombination and may specifically resolve recombination intermediates before partitioning.

These results revealed the occurrence of three distinct clusters from different wards in Siriraj Hospital. The toxigenic C. difficile of PFGE type A and related subtypes was a predominant infective clone in adult patients, whereas non-toxigenic C. difficile types B2 and D were encountered in asymptomatic infants. This information can be useful in epidemiologic investigations.

The role of volatile fatty acids (VFA) in resistance against the colonisation of the intestinal tract ofmice with two enteric pathogens, Shigella sonnei 3SR and enterotoxigenic E. coli 2SR (ETEC 2SR) was determined. The implantation dose 50 (IDSO) of S. sonnei 3SR strain for untreated mice was greater than 5.0 x lo9 viable cells and was 1.8 x lo6 viable cells for ETEC 2SR. Administration of streptomycin to the mice prior to challenge lowered the ID50 to fewer than one viable cell for both organisms. The antibiotic caused an increase in the pH of caecal contents from 6.66 in untreated animals to 6.94 in treated animals and a decrease in total VFA concentrations from 109.1 1 microequivalent per ml in untreated animals to 78.3 1 microequivalent per ml in treated animals. The discontinuation of streptomycin treatment resulted in a gradual restoration of colonisation resistance accompanied by an increase in total VFA concentration and a decrease in pH of caecal contents. These values returned to pre-treatment levels by day 7. VFA were added to nutrient broth in concentrations present in the caeca of either untreated or streptomycin treated mice and the pH levels were adjusted accordingly. S. sonnei 3SR multiplied in broth adjusted to simulate conditions present in the caeca of treated mice but failed to multiply in broth adjusted to simulate conditions present in the caeca of untreated mice. ETEC 2SR, on the other hand, multiplied in both types of broth but its total populations after 24 h incubation were significantly smaller in broth adjusted to simulate conditions present in the caeca of untreated mice.The results demonstrate that the multiplication of both pathogens is inhibited by the presence of VFA. They indicate that VFA operating at the pH level present in the intestinal tract of conventional mice represent important factors responsible for resistance against colonisation with S. sonnei and ETEC.