Streptococcus pseudopneumoniae identification by pherotype: a method to assist understanding of a potentially emerging or overlooked pathogen

Abstract

The recent identification of Streptococcus pseudopneumoniae (pseudopneumococcus) has complicated classification schemes within members of the "mitis" streptococcal group. Accurate differentiation of this species is necessary for understanding its disease potential and identification in clinical settings. This work described the use of the competence-stimulatory peptide ComC sequence for identification of S. pseudopneumoniae. ComC sequences from clinical sources were determined for 17 strains of S. pseudopneumoniae, Streptococcus pneumoniae, and Streptococcus oralis. An additional 58 ComC sequences from a range of sources were included to understand the diversity and suitability of this protein as a diagnostic marker for species identification. We identified three pherotypes for this species, delineated CSP6.1 (10/14, 79%), CSP6.3 (3/14, 21%), and SK674 (1/14, 7%). Pseudopneumococcal ComC sequences formed a discrete cluster within those of other oral streptococci. This suggests that the comC sequence could be used to identify S. pseudopneumoniae, thus simplifying the study of the pathogenic potential of this organism. To avoid confusion between pneumococcal and pseudopneumococcal pherotypes, we have renamed the competence pherotype CSP6.1, formerly reported as an "atypical" pneumococcus, CSPps1 to reflect its occurrence in S. pseudopneumoniae.

Figures

Fig 1

Pherotypes of S. pseudopneumoniae CSP6.1, CSP6.3, and SK674. The mature region of the peptide is in boldface after the double glycine. CSP6.1 and CSP6.3 differ by a single amino acid at position 12 (alanine in CSP6.1 and serine in CSP6.3; underlined). SK674 is a presumptive pseudopneumococcus (25), with an extended ComC compared to CSP6.1 and CSP6.3, and shares higher identity to S. infantis SK350. A total of three pseudopneumococcal pherotypes were characterized.

Fig 2

Neighbor-joining phylogenetic tree of ComC amino acid sequences of streptococcal species of the mitis group. Two clusters of ComC based on amino acid sequence similarities (groups 1 and 2) are separated by a dotted line. Each pherotype is labeled according to the classified species: filled circle, S. pseudopneumoniae; open square, S. pneumoniae; filled triangle, S. mitis; filled square, S. oralis; open circle, S. gordonii; open upright triangle, S. peroris; open diamond, S. infantis, open inverted triangle, S. cristatus. Asterisks indicate multiple strains of the same pherotype that have been compressed for clarity. Numbers of strains with the same pherotype are indicated in parentheses. Pseudopneumococcal strains of these groups are indicated in Table 2. The pneumococcal CSP1 pherotype includes strains VA1, RFH324, RFH410, RFH577, RFH815, RFH864, and RFH904. The pneumococcal CSP2.1 pherotype includes strains 41G and CSP2.1b. CSP6.2 pherotypes include SK671 (S. mitis), Col19 (S. oralis), RFH623 (S. oralis), and RFH831 (S. oralis). The phylogenetic tree was built with 1,000 bootstrap repetitions, with support over 80 indicated. The ruler indicates amino acid substitutions per site. The tree was constructed with MEGA 5.05.