Genetic Relationships between Clinical Isolates of Streptococcus pneumoniae , Streptococcus oralis , and Streptococcus mitis : Characterization of “Atypical” Pneumococci and Organisms Allied to S. mitis Harboring S. pneumoniae Virulence Factor-Encoding Genes

Abstract

The oral streptococcal group (mitis phylogenetic group) currently consists of nine recognized species, although the group has been traditionally difficult to classify, with frequent changes in nomenclature over the years. The pneumococcus (Streptococcus pneumoniae), an important human pathogen, is traditionally distinguished from the most closely related oral streptococcal species Streptococcus mitis and Streptococcus oralis on the basis of three differentiating characteristics: optochin susceptibility, bile solubility, and agglutination with antipneumococcal polysaccharide capsule antibodies. However, there are many reports in the literature of pneumococci lacking one or more of these defining characteristics. Sometimes called "atypical" pneumococci, these isolates can be the source of considerable confusion in the clinical laboratory. Little is known to date about the genetic relationships of such organisms with classical S. pneumoniae isolates. Here we describe these relationships based on sequence analysis of housekeeping genes in comparison with previously characterized isolates of S. pneumoniae, S. mitis, and S. oralis. While most pneumococci were found to represent a closely related group these studies identified a subgroup of atypical pneumococcal isolates (bile insoluble and/or "acapsular") distinct from, though most closely related to, the "typical" pneumococcal isolates. However, a large proportion of isolates, found to be atypical on the basis of capsule reaction alone, did group with typical pneumococci, suggesting that they have either lost capsule production or represent as-yet-unrecognized capsular types. In contrast to typical S. pneumoniae, isolates phenotypically identified as S. mitis and S. oralis, which included isolates previously characterized in taxonomic studies, were genetically diverse. While most of the S. oralis isolates did fall into a well-separated group, S. mitis isolates did not cluster into a well-separated group. During the course of these studies we also identified a number of potentially important pathogenic isolates, which were frequently associated with respiratory disease, that phenotypically and genetically are most closely related to S. mitis but which harbor genes encoding the virulence determinants pneumolysin and autolysin classically associated with S. pneumoniae.Streptococcus pneumoniae is a common and important human pathogen associated with pneumonia, septicaemia, meningitis, and otitis media. A number of distinct species of naturally transformable viridans or oral streptococci, which are closely related to S. pneumoniae, have now been identified, although the taxonomy and classification of these organisms has long been considered difficult (47). The most closely related species on the basis of 16S rRNA sequence are Streptococcus oralis and Streptococcus mitis, which share over 99% sequence identity with S.

Autolysin-targeted LightCycler assay including internal process control for detection of Streptococcus pneumoniae DNA in clinical samples

“…AF345846). The primer sites chosen showed complete agreement with the 20 S. pneumoniae strain sequences that were available from GenBank, but had differences from sequences from Streptococcus mitis and Streptococcus oralis strains previously reported to contain the autolysin gene (Whatmore et al, 2000). The probe-binding sites also showed complete identity among the 20 S. pneumoniae strain sequences but significant differences from other streptococci.…”

“…The specificity panel included examples of atypical S. mitis and S. oralis strains (Whatmore et al, 2000) containing lytA and ply genes. No positive signal was obtained from these organisms in the autolysin assay, but they were positive in the pneumolysin assay.…”

Identification, prevalence and population structure of non-typable Streptococcus pneumoniae in carriage samples isolated from preschoolers attending day-care centres

“…However, although some isolates are presumptively identified as pneumococci by routine identification tests, a negative result with type-specific antisera for the 90 serotypes is obtained. These atypical isolates are designated non-typable pneumococci (NTPn), and their correct discrimination from commensal viridans streptococci is often difficult (Carvalho et al, 2003;Hanage et al, 2005;Shayegani et al, 1982;Whatmore et al, 2000). The distinction between these closely related species is clinically important, since S. pneumoniae can cause serious disease and viridans streptococci are commensal organisms.…”

Evaluación pre-analítica de dos métodos de extracción de ADN para la amplificación del gen de la pneumolisina (PLY) de Streptococcus pneumoniae, en muestras de hemocultivo

“…El gen de la pneumolisina (ply), codifica para una proteína característica de Streptococcus pneumoniae y no está presente en ninguna otra especie bacteriana 6 . Además, su secuencia es altamente conservada tanto en cepas típicas como atípicas, estas últimas responsables de diagnósticos erró-neos mediante técnicas de rutina 7 , por lo que su amplificación mediante PCR, en muestras de hemocultivo, sería de gran ayuda en el diagnóstico precoz de infecciones por S. pneumoniae.…”