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

Abstract

ABSTRACTColicin-mediated killing is an example of allelopathy, which has been found among several bacteria. Screening of 42 strains ofShigella sonneiisolated from diarrheal patients revealed that 39 (93%)S. sonneistrains were positive for colicin production againstEscherichia coliDH5α. 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. RepresentativeS. sonneistrains producing heterologous colicins exhibited antagonism against diarrheagenicEscherichia 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-resistantE. colimutants were found to occur spontaneously at the rates of 2.51 × 10−8and 5.52 × 10−8per generation when exposed to colicins E3 and E8 and colicins E3 and E2, respectively. Genotypic characterization of colicin-resistantE. coli(ECCr) andS. sonnei(SSCr) strains displayed mutations in thebtuBgene, which encodes the receptor for vitamin B12uptake. This gene was interrupted by various insertion sequences, such as IS1, IS2, and IS911. Complementation of ECCrand SSCrwith plasmid-bornebtuB(pbtuB) accomplished restoration of the colicin-susceptible phenotype. The vitamin B12uptake assay gave an insight into the physiological relevance of thebtuBmutation. Our studies provide insights into the latent influence ofS. sonneicolicins in governing the existence of some of the shigellae and all of the DEC and the genetic mechanism underlying the emergence of resistance.