Studying collective action with newspaper accounts of protest events, rare only 20 years ago, has become commonplace in the past decade. A critical literature has accompanied the growth of protest event analysis. The literature has focused on selection bias-particularly which subset of events are covered-and description bias-notably, the veracity of the coverage. The "hard news" of the event, if it is reported, tends to be relatively accurate. However, a newspaper's decision to cover an event at all is influenced by the type of event, the news agency, and the issue involved. In this review, we discuss approaches to detecting bias, as well as ways to factor knowledge about bias into interpretations of protest event data.

Haemophilus influenzae undergoes phase variation in expression of the phosphorylcholine (ChoP) epitope, a structure present on several invasive pathogens residing in the human respiratory tract. In this study, structural analysis comparing organisms with and without this epitope confirmed that variants differ in the presence of ChoP on the cell surface–exposed outer core of the lipopolysaccharide. During nasopharyngeal carriage in infant rats, there was a gradual selection for H. influenzae variants that express ChoP. In addition, genotypic analysis of the molecular switch that controls phase variation predicted that the ChoP+ phenotype was predominant in H. influenzae in human respiratory tract secretions. However, ChoP+ variants of nontypable H. influenzae were more sensitive to the bactericidal activity of human serum unrelated to the presence of naturally acquired antibody to ChoP. Serum bactericidal activity required the binding of C-reactive protein (CRP) with subsequent activation of complement through the classical pathway. Results of this study suggested that the ability of H. influenzae to vary expression of this unusual bacterial structure may correlate with its ability both to persist on the mucosal surface (ChoP+ phenotype) and to cause invasive infection by evading innate immunity mediated by CRP (ChoP− phenotype).

SummaryA survey of Haemophilus in¯uenzae strains indicated that around one-third of capsular strains and over two-thirds of non-typeable strains included sialic acid in their lipopolysaccharides (LPS). Mutation of the CMP-Neu5Ac synthetase gene (siaB ) resulted in a sialylation-de®cient phenotype. Isogenic pairs, wild type and siaB mutant of two non-typeable strains were used to demonstrate that sialic acid in¯uences resistance to the killing effect of normal human serum but has little effect on attachment to, or invasion of, cultured human epithelial cells or neutrophils. We determine for the ®rst time the site of attachment of sialic acid in the LPS of a non-typeable strain and report that a small proportion of glycoforms include two sialic acid residues in a disaccharide unit.

Lipopolysaccharide (LPS) is a major virulence determinant of Haemophilus influenzae. The organism is capable of expressing a heterogeneous population of LPS which exhibits extensive antigenic diversity among multiple oligosaccharide (OS) epitopes. Structural elucidation of variable and conserved OS epitopes of H. influenzae serotype b strain Eagan was determined by the application of high-field NMR techniques and MS-based methods on oligosaccharides obtained from LPS samples by a deacylation strategy. LPS extracted by the hot aqueous phenol method gave complex electrophoretic patterns consisting of at least six low-molecular mass bands. Electrospray ionization−mass spectrometry of O-deacylated LPS revealed a series of related structures differing in the number of hexose residues as well as subpopulations of glycoforms containing additional phosphoethanolamine (PEA) groups. It was demonstrated that the LPS contains a conserved PEA-substituted, heptose-containing trisaccharide inner core moiety attached via a KDO 4-phosphate unit to a lipid A component. Tandem MS experiments unambiguously established the presence of a KDO 4-pyrophosphoethanolamine unit in the subpopulation of LPS containing additional PEA groups. The occurrence of LPS containing this structural feature was found to be dependant on the isolation procedure used. Each heptose of the common inner core element l-α-d-Hepp(1→2)-l-α-d-Hepp(1→3)-l-α-d-Hepp(1→5)-α-KDO is substituted by a hexose residue with further chain elongation from the central unit. The structures of the major glycoforms containing four (three Glcs and one Gal), five (three Glcs and two Gals), and six (three Glcs and three Gals) hexoses were determined in detail. The Hex6 glycoform contains the terminal structure, α-d-Galp(1→4)-β-d-Galp(1→4)-β-d-Glc, providing, for the first time, definitive structural evidence for the expression of the Pk-blood group antigen in H. influenzae LPS. Moreover, an analogue of the Hex4 glycoform was identified in which the third heptose residue carries phosphate at O-4.

Structural elucidation of the lipopolysaccharide (LPS) of Haemophilus influenzae, strain Rd, a capsule-deficient type d strain, has been achieved by using high-field NMR techniques and electrospray ionization-mass spectrometry (ESI-MS) on delipidated LPS and core oligosaccharide samples. It was found that this organism expresses heterogeneous populations of LPS of which the oligosaccharide (OS) epitopes are subject to phase variation. ESI-MS of O-deacylated LPS revealed a series of related structures differing in the number of hexose residues linked to a conserved inner-core element,, and the degree of phosphorylation. The structures of the major LPS glycoforms containing three (two Glc and one Gal), four (two Glc and two Gal) and five (two Glc, two Gal and one GalNAc) hexoses were substituted by both phosphocholine (PCho) and phosphoethanolamine (PEtn) and were determined in detail. In the major glycoform, Hex3, a lactose unit, b-d-Galp- (134) The fully extended LPS glycoform (Hex5) has the following structure.PPEtnThe structural data provide the first definitive evidence demonstrating the expression of a globotetraose OS epitope, the P antigen, in LPS of H. influenzae. It is noteworthy that the molecular environment in which PCho units are found differs from that observed in an Rd 2 derived mutant strain (RM

The availability of the complete 1.83-megabase-pair sequence of the Haemophilus influenzae strain Rd genome has facilitated significant progress in investigating the biology of H.influenzae lipopolysaccharide (LPS), a major virulence determinant of this human pathogen. By searching the H. influenzae genomic database, with sequences of known LPS biosynthetic genes from other organisms, we identified and then cloned 25 candidate LPS genes. Construction of mutant strains and characterization of the LPS by reactivity with monoclonal antibodies, PAGE fractionation patterns and electrospray mass spectrometry comparative analysis have confirmed a potential role in LPS biosynthesis for the majority of these candidate genes. Virulence studies in the infant rat have allowed us to estimate the minimal LPS structure required for intravascular dissemination. This study is one of the first to demonstrate the rapidity, economy and completeness with which novel biological information can be accessed once the complete genome sequence of an organism is available.

Otitis media, a common and often recurrent bacterial infection of childhood, is a major reason for physician visits and the prescription of antimicrobials. Haemophilus influenzae is the cause of Ϸ20% of episodes of bacterial otitis media, but most strains lack the capsule, a factor known to play a critical role in the virulence of strains causing invasive H. influenzae disease. Here we show that in capsule-deficient (nontypeable) strains, sialic acid, a terminal residue of the core sugars of H. influenzae lipopolysaccharide (LPS), is a critical virulence factor in the pathogenesis of experimental otitis media in chinchillas. We used five epidemiologically distinct H. influenzae isolates, representative of the genetic diversity of strains causing otitis media, to inoculate the middle ear of chinchillas. All animals developed acute bacterial otitis media that persisted for up to 3 wk, whereas isogenic sialic acid-deficient mutants (disrupted sialyltransferase or CMP-acetylneuraminic acid synthetase genes) were profoundly attenuated. MS analysis indicated that WT bacteria used to inoculate animals lacked any sialylated LPS glycoforms. In contrast, LPS of ex vivo organisms recovered from chinchilla middle ear exudates was sialylated. We conclude that sialylated LPS glycoforms play a key role in pathogenicity of nontypeable H. influenzae and depend on scavenging the essential precursors from the host during the infection.ex vivo isolate ͉ phylogeny ͉ mass spectrometry C arried by up to 80% of humans, Haemophilus influenzae (Hi) is a common nasopharyngeal commensal. Capsule-deficient or nontypeable (NT) Hi can cause upper and lower respiratory tract infections, the most common being episodes of otitis media (OM) in young children. On average, children experience two or more episodes of acute OM by age 2 yr (1), making OM a major cause of physician visits (24 million per year in the U.S.) and of the profligate use of antibiotics in general practice. OM causes sequelae, including impaired hearing (Ϸ20% cases) and cognitive development (2). NTHi is also the most frequent pathogen recovered from the middle ear in children with recurrent OM (3). Although immunity against infection due to NTHi appears to develop after acute OM (4, 5), protection is strain specific, therefore permitting recurrent episodes due to distinct isolates.We have reported that all NTHi OM isolates have the potential to incorporate sialic acid [N-acetylneuraminic acid (Neu5Ac)] into their lipopolysaccharide (LPS), and that strains expressing this sugar are more resistant to the bactericidal activity of normal human serum in vitro (6-8). Although sialylation of LPS has been implicated in bacterial virulence (9), the contribution of sialylated LPS glycoforms of Hi has not been investigated in vivo.We have investigated the role of sialic acid as a virulence factor of NTHi in a well described chinchilla model of OM (10, 11). By comparing isogenic sialic acid-proficient and sialic acid-deficient strains, we show that sialylation of LPS is a major factor in...

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