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Infection and Immunity, March 2000, p. 1374-1382, Vol. 68, No. 3
Department of Biological Sciences, University
of Warwick, Coventry, CV4 7AL,1 and Respiratory
and Systemic Infection Laboratory, Central Public Health
Laboratory, London, NW9 5HT,2 United Kingdom
Received 25 August 1999/Returned for modification 17 September
1999/Accepted 10 November 1999
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.
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Copyright © 2000, American Society for Microbiology. All rights reserved.
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
*
Corresponding author. Mailing address: Department of
Biological Sciences, University of Warwick, Coventry CV4 7AL,
United Kingdom. Phone: 44-2476-528359. Fax: 44-2476-523701. E-mail:
a.m.whatmore{at}warwick.ac.uk.
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