The complement system is an important component of the innate immune response to bacterial pathogens, including Streptococcus pneumoniae. The classical complement pathway is activated by antibody-antigen complexes on the bacterial surface and has been considered predominately to be an effector of the adaptive immune response, whereas the alternative and mannose-binding lectin pathways are activated directly by bacterial cell surface components and are considered effectors of the innate immune response. Recently, a role has been suggested for the classical pathway during innate immunity that is activated by natural IgM or components of the acute-phase response bound to bacterial pathogens. However, the functional importance of the classical pathway for innate immunity to S. pneumoniae and other bacterial pathogens, and its relative contribution compared with the alternative and mannose-binding lectin pathways has not been defined. By using strains of mice with genetic deficiencies of complement components and secretory IgM we have investigated the role of each complement pathway and natural IgM for innate immunity to S. pneumoniae. Our results show that the proportion of a population of S. pneumoniae bound by C3 depends mainly on the classical pathway, whereas the intensity of C3 binding depends on the alternative pathway. Furthermore, the classical pathway, partially targeted by the binding of natural IgM to bacteria, is the dominant pathway for activation of the complement system during innate immunity to S. pneumoniae, loss of which results in rapidly progressing septicemia and impaired macrophage activation. These data demonstrate the vital role of the classical pathway for innate immunity to a bacterial pathogen.

SummaryRestricted iron availability is a major obstacle to growth and survival of pathogenic bacteria during infection. In contrast to Gram-negative pathogens, little is known about how Gram-positive pathogens obtain this essential metal. We have identified two Streptococcus pneumoniae genetic loci, pit1 and pit2, encoding homologues of ABC iron transporters that are required for iron uptake by this organism. S. pneumoniae strains containing disrupted copies of either pit1 or pit2 had decreased sensitivity to the iron-dependent antibiotic streptonigrin, and a strain containing disrupted copies of both pit1 and pit2 was unable to use haemoglobin as an iron source and had a reduced rate of iron uptake. The pit2 2 strain was moderately and the pit1 2 /pit2 2 strain strongly attenuated in virulence in mouse models of pulmonary and systemic infection, showing that the pit loci play a critical role during in vivo growth of S. pneumoniae. The pit2 locus is contained within a 27 kb region of chromosomal DNA that has several features of Gramnegative bacterial pathogenicity islands. This probable pathogenicity island (PPI-1) is the first to be described for S. pneumoniae, and its acquisition is likely to have played a significant role in the evolution of this important human pathogen.

Bacteria frequently have multiple mechanisms for acquiring iron, an essential micronutrient, from the environment. We have identified a four-gene Streptococcus pneumoniae operon, named pit, encoding proteins with similarity to components of a putative Brachyspira hyodysenteriae iron uptake ABC transporter, Bit. An S. pneumoniae strain containing a defined mutation in pit has impaired growth in medium containing the iron chelator ethylenediamine di-o-hydroxyphenylacetic acid, reduced sensitivity to the iron-dependent antibiotic streptonigrin, and impaired virulence in a mouse model of S. pneumoniae systemic infection. Furthermore, addition of a mutation in pit to a strain containing mutations in the two previously described S. pneumoniae iron uptake ABC transporters, piu and pia, resulted in a strain with impaired growth in two types of iron-deficient medium, a high degree of resistance to streptonigrin, and a reduced rate of iron uptake. Comparison of the susceptibilities to streptonigrin of the individual pit, piu, and pia mutant strains and comparison of the growth in iron-deficient medium and virulence of single and double mutant strains suggest that pia is the dominant iron transporter during in vitro and in vivo growth.

The facultative intracellular pathogen Salmonella enterica causes a variety of diseases, including gastroenteritis and typhoid fever. Inside epithelial cells, Salmonella replicates in vacuoles, which localize in the perinuclear area in close proximity to the Golgi apparatus. Among the effector proteins translocated by the Salmonella pathogenicity island 2-encoded type III secretion system, SifA and SseG have been shown necessary but not sufficient to ensure the intracellular positioning of Salmonella vacuoles. Hence, we have investigated the involvement of other secreted effector proteins in this process. Here we show that SseF interacts functionally and physically with SseG but not SifA and is also required for the perinuclear localization of Salmonella vacuoles. The observations show that the intracellular positioning of Salmonella vacuoles is a complex phenomenon resulting from the combined action of several effector proteins.

We have previously described a 27-kb pathogenicity island of Streptococcus pneumoniae, termed pneumococcal pathogenicity island 1 (PPI1), which contains iron uptake locus piaABCD, required for full virulence in mice, and a further 28 previously uncharacterized genes. We have investigated one of these, Sp1051, which encodes a protein of unknown function. Disruption of Sp1051 does not affect growth in laboratory broth, serum, or blood but impairs virulence in mouse models of infection. When S. pneumoniae capsular serotypes were analyzed by PCR and Southern hybridization, it was found that 33% did not contain Sp1051. Analysis of other genes within PPI1 demonstrated that, compared to the serotype 4 genome published by The Institute for Genome Research (TIGR), the genomes of many strains contain deletions of a variable number of genes between Sp1046 and Sp1064, conforming to one of six different patterns. Amplification by PCR of this PPI1 variable region from a capsular serotype 17 strain and comparison of the sequence to TIGR serotype 4 strain sequence showed that Sp1051 is contained within an 11.3-kb segment of DNA flanked by 7-bp direct repeats within the serotype 4 strain which is not present in the serotype 17 strain. Further comparison of the sequences of this region between the three published S. pneumoniae genomes demonstrated that serotype 19F and strain R6 contain novel complements of genes not present in the serotype 4 strain. These data indicate that there is striking variation in gene content and structure of the 3 region of PPI1 among strains and that this region includes at least one virulence determinant. Gene variation within horizontally acquired DNA such as that of PPI1 may be one factor modulating differences in virulence among strains.

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Humans are the sole reservoir for mumps virus (MuV), the causative agent of mumps. No animal model currently exists; therefore, in vivo knowledge of the virus is limited. Ferrets were assessed for their susceptibility to MuV based on their success as a model for influenza. We infected ferrets with clinical or attenuated vaccine MuVs by the nasal route and demonstrated evidence of immunogenicity in these animals with generation of a serum antibody response specific to MuV infection and cytokine production consistent with infection. However, no live virus or viral RNA was detected in nasal washes, oral swabs, urine, faeces or tissue homogenates, and no animals exhibited clinical signs. We suggest results to be obtained from ferrets are limited in fundamental in vivo MuV research and that they may not be a suitable animal model for this virus.