Pneumococci possess multiple virulence factors [57], including its polysaccharide capsule, cell surface proteins, the cell wall, and pneumolysin, a pore-forming toxin [58]. Of these, the most important is the polysaccharide capsule, of which there are at least 98 different serotypes, each capable of shielding the organism from the immune system [59]. Individual serotypes vary in their capacity to colonize, cause local or invasive disease, and express antibiotic resistance genes [57, 60]. Serotypes also vary geographically and change over time, perhaps in response to local ecological competitive pressures from other organisms co-habiting the nasopharyngeal space, as well as selection pressures from antibiotics and PCVs [61]. Indeed, most pneumococcal serotypes associated with PNP (Tables 1 and 2) were not included in the 7-valent PCV, notably serotypes 3, 5, 7F, and 19A, reflecting possible temporal shifts in circulating serotypes and/or vaccine-induced strain replacement disease. Of these, serotypes 3 and 19A were most closely associated with PNP. Serotype 3 has a very thick capsule, which strongly resists opsonophagocytosis and induces a marked inflammatory response, including an intense neutrophilic infiltration with suppurative necrosis [1, 62]. In contrast, serotype 19A strains have greater invasive potential, may have a growth advantage over other pneumococcal serotypes in normally sterile sites, and are often resistant to multiple antibiotics [34, 63].
Engrailed-1 (En-1) is a homeodomain protein expressed specifically at the mid-hindbrain junction of the early somite stage embryo, as well as in a number of sites later in embryogenesis. Figure 4 shows an experiment in which R26R-YFP mice were crossed with a strain of mice in which Cre was knocked into the En-1 locus [26], resulting in the expression of YFP specifically at the mid-hindbrain junction in an E8.5 embryo. For comparison, we also crossed the En-1/Cre mice with the original R26R lacZ allele [4], resulting in lacZ expression in the same region at E8.5 (Fig 4C).
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Functionally, at the colonization sites, OMVs are implicated in inter-microbial communications through exchanges of genetic materials, chemical compounds, and signaling molecules [33, 34] (Fig. 3B). The communications can be conducted between the same and different bacterial strains [32]. OMVs may also play direct roles in bacterial survival [35, 36], since OMVs can increase the resistance to antibiotics and bacteriophages by serving as decoy targets of these molecules [33]. Furthermore, OMVs can be used for the delivery of bacterial virulence factors, such as toxins, into host epithelial cells at the colonization sites. On the other hand, OMVs can activate both innate (macrophage/dendritic cell/microglia) and adaptive (T cell/B cell) immune cells in the remote organs once OMVs are delivered, since the components of OMVs, such as LPS, are known as activators of various immune response pathways [34]. 2ff7e9595c
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