| Abstract [eng] |
Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, bile soluble diplococcus aerotolerant anaerobe and a member of the genus Streptococcus (phylum Firmicutes). Streptococcus pneumoniae (S.pneumoniae) is known in medical microbiology as the pneumococcus. It has a polysaccharide capsule that acts as a virulence factor for the organism; more than 90 different serotypes are known, and these types differ in virulence, prevalence, and extent of drug resistance. Streptococcus pneumoniae is a normal inhabitant of the human upper respiratory tract. The serotype distribution among nasopharyngeal carriage isolates varies by country, age-group, origin, type of cohort. Pneumococcal disease will not occur without preceding nasopharyngeal colonization with the homologous strain. In addition, pneumococcal carriage is believed to be an important source of horizontal spread this pathogen within the community. Because the highest frequency of the pneumococcal colonization and the highest crowding index are found in young children, this risk group is thought to be the most important vector for horizontal dissemination of pneumococcal strains within the community. In Europe asymptomatic nasopharyngeal carriage of pneumococcal infection is 30% -60% and in Asia or Africa it is up to 98%. Carriage rate of different serotypes in the same population changes during the time. S.pneumoniae is a common bacterial agent that causes a wide variety of infections including mucosal infections (e.g. sinusitis and otitis media), pneumonia, arthritis, pericarditis, peritonitis and severe invasive infections such as meningitis and septicemia. Morbidity and mortality due to pneumococcal infections is high, especially in developing countries. The incidence of invasive pneumococcal diseases (IPD) in young children residing in the United States and Europe is 8-75 cases/100000 population/years, whereas the incidence in young children residing in developing countries is several times higher, 100 to >500 cases/100000 population/year. Different serotypes have different ability to cause serious invasive pneumococcal diseases. Only some serotypes cause serious invasive diseases. According to a number of studies there is a significant inverse correlation between invasive disease and carriage prevalence for considered serotypes, which implies that the most invasive serotypes and serogroups are rarely carried, and that the most frequently carried serotypes and serogroups not allways cause invasive diseases. Historically pneumococci have been susceptible in vitro to penicillins, cephalosporins, macrolides (including erythromycin, clarithromycin, azithromycin), clindamycin, rifampicin, vancomycin, and trimethoprim-sulfamethoxazole. In the early 1970s multiresistant S.pneumoniae strains as well as resistant to penicillin were detected. In the early 1990s, however, pneumococcal strains resistant to penicillin and other antimicrobial agents emerged throughout the world. Resistance rates to penicillin and macrolides in various countries differ from 60% to 5%. The continuing morbidity and mortality of pneumococcal infections in the antibiotic era led to the development and licensure of a polyvalent polysaccharide vaccine in the late 1970s. The currently available pneumococcal polysaccharide vaccine, 23PS, is composed of purified capsular polysaccharide antigens of 23 serotypes that represent up to 90% of the serotypes causing invasive pneumococcal infections. But polysaccharide vaccines elicit type-specific antibody responses in most healthy adults and children of 5 years and older, the serologic response to the polysaccharide antigens is generally poor in children younger than 2 years of age. New conjugate vaccines are immunogenic for infants as young as 2 month of age, including B-lymphocyte memory cells resulting in an anamnestic response with subsequent doses, and reduce carriage of vaccine serotypes. A 7-valent type of pneumococcal conjugate vaccine was approved for infant immunization since late 1990s. Great changes in epidemiology of IPD were detected in countries where universal infant immunization started a few years ago. In Lithuania there is no universal vaccination with any of pneumococcal vaccines. S.pneumoniae is able to stimulate immune responses. In addition to ‘innate immunity’, the specific, antibody-mediated defence takes action already at the mucosal surfaces. Relatively few data exist on immune responses to the pneumococcus after natural exposure. It was demonstrated previously that contacts with Streptococcus pneumoniae induced natural salivary IgA responses against protein and polycaccharide antigens in children and adults. IgA is the predominant and relatively important in host defence immunoglobulin isotype in all mucosal secretions. In addition to IgA, pentameric IgM is likewise actively enriched in most exocrine fluids and is associated with the secretory component. IgG in mucosal secretions has traditionally been regarded as originating from serum by diffusion. In addition, IgG can be produced locally. This suggests that three isotypes: IgA, IgM and IgG are important in mucosal immunity. |
