Resistotyping of Shigella Sonnei

Resistotyping consists of testing bacterial strains against a set of arbitrarily chosen chemicals that show selective toxicity at a critical concentration. The pattern of resistances, which defines the resistotype, is believed to describe isolates for epidemiological purposes.

Such a system was devised for Shigella sonnei and 247 strains obtained from various outbreaks were tested. Thirty-four outbreaks in families, schools and localities were originally defined by their timing and geographical incidence, together with the colicine type and antibiotic resistance pattern of the strains isolated.

In general, strains from an outbreak belonged to the same resistotype. This was not altered in those strains that showed a change in antibiogram or colicine type. In some cases, strains from different sources but of the same colicine type could be differentiated into separate resistotypes, which corresponded to geographical outbreaks. In other instances, however, strains of the same resistotype were subdivided by their colicine type into different outbreaks.

In-vivo findings and in-vitro transfer of plasmids indicated that the resistotype markers were independent of the colicine type and antibiotic resistance markers. Thus, although the resistotype markers may also be episomally located, they are apparently not on the same episomes as the determinants of colicine type and antibiotic resistance.

All typing systems ultimately depend on the genetic stability of their markers, and it is theoretically impossible to devise a perfect system for epidemiology. This statement applies equally to phage and colicine type and to antibiogram and resistotype. In the case of S. sonnei, the method of resistotyping appears to be a useful adjunct for detailed epidemiology.