title: Avirulence of rough mutants of Shigella flexneri: requirement of O antigen for correct unipolar localization of IcsA in the bacterial outer membrane.
abstract: Mutations in the lipopolysaccharide (LPS) of Shigella spp. result in attenuation of the bacteria in both in vitro and in vivo models of virulence, although the precise block in pathogenesis is not known. We isolated defined mutations in two genes, galU and rfe, which directly affect synthesis of the LPS of S. flexneri 2a, in order to determine more precisely the step in virulence at which LPS mutants are blocked. The galU and rfe mutants invaded HeLa cells but failed to generate the membrane protrusions (fireworks) characteristic of intracellular motility displayed by wild-type shigellae. Furthermore, the galU mutant was unable to form plaques on a confluent monolayer of eucaryotic cells and the rfe mutant generated only tiny plaques. These observations indicated that the mutants were blocked in their ability to spread from cell to cell. Western immunoblot analysis of expression of IcsA, the protein essential for intracellular motility and intercellular spread, demonstrated that both mutants synthesized IcsA, although they secreted less of the protein to the extracellular medium than did the wild-type parent. More strikingly, the LPS mutants showed aberrant surface localization of IcsA. Unlike the unipolar localization of IcsA seen in the wild-type parent, the galU mutant expressed the protein in a circumferential fashion. The rfe mutant had an intermediate phenotype in that it displayed some localization of IcsA at one pole while also showing diffuse localization around the bacterium. Given the known structures of the LPS of wild-type S. flexneri 2a, the rfe mutant, and the galU mutant, we hypothesize that the core and O-antigen components of LPS are critical elements in the correct unipolar localization of IcsA. These observations indicate a more precise role for LPS in Shigella pathogenesis.
authors.affiliation: Department of Microbiology and Immunology, F. Edward Hébert School of Medicine, Uniformed Services University of the Health Sciences, Bethesda, Maryland 20814-4799.
authors.forename: R C
authors.initials: RC
authors.lastname: Sandlin
authors.forename: K A
authors.initials: KA
authors.lastname: Lampel
authors.forename: S P
authors.initials: SP
authors.lastname: Keasler
authors.forename: M B
authors.initials: MB
authors.lastname: Goldberg
authors.forename: A L
authors.initials: AL
authors.lastname: Stolzer
authors.forename: A T
authors.initials: AT
authors.lastname: Maurelli
mesh_terms: D000199:Actins / Q000378:metabolism; D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D001483:Base Sequence; D002240:Carbohydrate Sequence; D002462:Cell Membrane / Q000378:metabolism*; D016764:Cell Polarity / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000378:metabolism*; D029968:Escherichia coli Proteins; D006367:HeLa Cells; D006801:Humans; D008070:Lipopolysaccharides / Q000096:biosynthesis; D008954:Models, Biological; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D019081:O Antigens; D011135:Polysaccharides, Bacterial / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D012763:Shigella flexneri / Q000166:cytology / Q000235:genetics / Q000472:pathogenicity*; D014157:Transcription Factors / Q000378:metabolism*; D017855:Transferases (Other Substituted Phosphate Groups) / Q000235:genetics; D005957:UTP-Glucose-1-Phosphate Uridylyltransferase / Q000235:genetics; D014774:Virulence / Q000235:genetics
chemical_list: D000199:Actins; D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; D008070:Lipopolysaccharides; D019081:O Antigens; D011135:Polysaccharides, Bacterial; D014157:Transcription Factors; C058483:virG protein, Shigella flexneri; D005957:UTP-Glucose-1-Phosphate Uridylyltransferase; D017855:Transferases (Other Substituted Phosphate Groups); C083617:wecA protein, E coli