O-antigen dependent colicin insensitivity of uropathogenic Escherichia coli.
Sharp C., Boinett C., Cain A., Housden N., Kumar S., Turner K., Parkhill J., Kleanthous C.
The outer membrane of Gram-negative bacteria presents a significant barrier for molecules entering the cell. Nevertheless, colicins, which are antimicrobial proteins secreted by E. coli, can target other E. coli cells by binding to cell surface receptor proteins and activating their import resulting in cell death. Previous studies have documented high rates of non-specific resistance (insensitivity) of various E. coli strains towards colicins that is independent of colicin-specific immunity, and is instead associated with lipopolysaccharide (LPS) in the outer membrane. This observation poses a contradiction: Why do E. coli strains have colicin-expressing plasmids, which are energetically costly to retain, if cells around them are likely to be naturally insensitive to the colicin they produce? Here, using a combination of transposon sequencing and phenotypic microarrays we show that colicin insensitivity of uropathogenic E. coli ST131 is dependent on production of its O-antigen but that minor changes in growth conditions render the organism sensitive towards colicins. Reintroduction of O-antigen into E. coli K-12 demonstrates that it is the density of O-antigen that is the dominant factor in governing colicin insensitivity. We also show, by microscopy using fluorescently labelled colicins, that growth conditions affect the degree of occlusion by O-antigen of outer membrane receptors, but not the clustered organization of receptors. Our study demonstrates that environmental conditions play a critical role in sensitizing E. coli towards colicins and that O-antigen in LPS is central to this role.IMPORTANCEE. coli infections can be a major health burden especially with the organism becoming increasingly resistant to 'last-resort' antibiotics such as carbapenems. Although colicins are potent narrow-spectrum antimicrobials, with potential as future antibiotics, high levels of naturally occurring colicin insensitivity has been documented which could limit their efficacy. We identify O-antigen dependent colicin insensitivity in a clinically relevant uropathpogenic E. coli strain and show that this insensitivity can be circumvented by minor changes to growth conditions. Our study suggests that colicin insensitivity amongst E. coli has been greatly over-estimated and as a consequence, colicins could in fact be effective, species-specific antimicrobials targeting pathogenic E. coli such as UPECs.