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

Abstract

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␣. 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 ؋ 10 ؊8 and 5.52 ؋ 10 ؊8 per generation when exposed to colicins E3 and E8 and colicins E3 and E2, respectively. Genotypic characterization of colicin-resistant E. coli (EC Cr ) and S. sonnei (SS Cr ) 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 EC Cr and SS Cr 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.A llelopathy refers to the production of toxic metabolites that suppress both the growth and survival of distinct competitors in a common niche (1, 2). Colicin is one such antimicrobial biomolecule produced by certain members of the family Enterobacteriaceae, which provides a competitive edge against microorganisms that are not immune or resistant (3-6). Colicin producers express the immunity protein constitutively, which forms a complex with the colicin protein, thereby preventing "cell suicide." Resistance to killing is attributable to two factors: (i) a mutation in the receptor, which serves as a portal for entry into the target cell (true resistance), and (ii) an alteration in some component, which constitutes a part of the translocation machinery (tolerance) (7,8).In the present study, we have focused primarily on the role of colicin produced by the Shigella sonnei strains isolated from acute diarrheal patients in preventing the growth of diarrheagenic Escherichia coli (DEC) groups and spontaneous emergence of resistance to colicin due to mutation in the btuB gene, which encodes the receptor for vitamin B 12 uptake. The BtuB protein also serves to localize type A/E colicins and T5-like phages (BF23 and EPS7) on the target cell surface (9, 10). The btuB gene consists of a single open reading frame, which is translated into a 614-amino-acid polypeptide. The fir...

Introduction and establishment of fluoroquinolone-resistant Shigella sonnei into Bhutan

“…A further pivotal event in the emergence of this Central Asia clade prior to its introduction into Bhutan was the fixation of the previously described E-type colicin plasmid pSSE3 (with 100 % identity in DNA sequence). This E-type colicin has been shown to be antagonistic against a variety of diarrhoeogenic E. coli , but not against commensal E. coli ( Calcuttawala et al , 2015 ). Plasmid pSSE3, which was found in all Bhutanese isolates ( Fig.…”

Shigella Diversity and Changing Landscape: Insights for the Twenty-First Century

“…Strains that are capable of producing a colicin are better able to compete against colicin sensitive strains over ecological niches. In the case of S. sonnei , it has been reported that up to 93% of tested strains in Kolkata, India, harbor at least one colicin-encoding plasmid which were shown to target closely related susceptible E. coli strains, which as previously mentioned, were important for restricting Shigella colonization (Calcuttawala et al, 2015 ). Meanwhile, the chromosomally encoded O-antigen chain length regulator WzzB was recently reported to contribute to colicin E2 resistance in S. flexneri (Tran et al, 2014 ).…”

Metallohelices that kill Gram-negative pathogens using intracellular antimicrobial peptide pathways

“…It derives its energy for transport by interacting with the trans-periplasmic membrane protein TonB. 44 Coincidently, BtuB also acts as the receptor for the proteinaceous antibacterial A and E colicins. 45,46 We speculate that this transporter could contribute to entry of L-5b into the periplasm.…”