Activity Spectrum of Colicins Produced by Shigella sonnei and Genetic Mechanism of Colicin Resistance in Conspecific S-sonnei Strains and Escherichia coli

Colicin-mediated killing is an example of allelopathy, which has been found among several bacteria. Screening of 42 strains of Shigella sonnei isolated from diarrheal patients revealed that 39 (93%) S. sonnei strains were positive for colicin production against Escherichia coli DH5 alpha. In the PCR-based detection of the colicin types, 36 (92.3%) were identified as E3, 2 (5.1%) as E3 and E8, and 1 (2.6%) as E3 and E2. Representative S. sonnei strains producing heterologous colicins exhibited antagonism against diarrheagenic Escherichia coli (DEC) groups. Although it is known that mutation in the colicin receptor renders the host resistant to colicin, there is a dearth of information on the genetic characterization of such mutants. In the fluctuation test, colicin-resistant E. coli mutants were found to occur spontaneously at the rates of 2.51 x 10(-8) and 5.52 x 10(-8) per generation when exposed to colicins E3 and E8 and colicins E3 and E2, respectively. Genotypic characterization of colicin-resistant E. coli (ECCr) and S. sonnei (SSCr) strains displayed mutations in the btuB gene, which encodes the receptor for vitamin B-12 uptake. This gene was interrupted by various insertion sequences, such as IS1, IS2, and IS911. Complementation of ECCr and SSCr with plasmid-borne btuB (pbtuB) accomplished restoration of the colicin-susceptible phenotype. The vitamin B-12 uptake assay gave an insight into the physiological relevance of the btuB mutation. Our studies provide insights into the latent influence of S. sonnei colicins in governing the existence of some of the shigellae and all of the DEC and the genetic mechanism underlying the emergence of resistance.