Studies on transformation in Shigella

Transformation of pBR322 DNA into Shigella occurred at a low frequency. The efficiency of transformation was highest in S. dysenteriae 1 and lowest in S. flexneri. Treatment of cells with CaCl2 for a prolonged period (24h) increased the efficiency of transformation in all strains, except in S. flexneri, where transformation efficiency could not be improved by a variety of manipulations. Transformation efficiency did not increase in any of the strains when transformation was carried out with plasmid DNA obtained from a transformant (homologous transformation), suggesting the absence of a strong restriction-modification system. Extracellular deoxyribonuclease (DNase) levels were low in all the strains tested, but the levels of endogenous DNAse, released after CaCl2 treatment or sonication of the cells, were high. Washing the cells with a solution of CaCl2 did not enhance transformation, suggesting that endogenous DNase could be a significant factor affecting transformation efficiency in species of Shigella.