Streptococcus pseudopneumoniae
Introduction
Streptococcus pseudopneumoniae was first described by Arbique et al (1) in 2004 as a novel species belonging to the viridians group streptococci (VGS). Among the VGS species, S. pseudopneumoniae is phenotypically and genetically similar to Streptococcus pneumoniae, Streptococcus mitis, and Streptococcus oralis (1, 2). Bacteria of this group have a propensity for horizontal gene transfer (HGT), and as a consequence, they share many physiological and molecular traits, which makes accurate species differentiation challenging. The pathogenic significance of S. pseudopneumoniae remains unclear. However, its antimicrobial susceptibility profile suggests it is a more resistant organism than other VGS species (3, 4, 5, 6).
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Evolution of Streptococcus pseudopneumoniae
The members of the S. mitis group can contain genetic elements for the binding, uptake, and integration of extracellular DNA by a mechanism known as HGT. HGT can result in the development of phenotypic changes that create transitional species, such as S. pseudopneumoniae, that phylogenetically sit between S. pneumoniae and S. mitis (Fig. 1) (7). DNA-DNA reassociation studies indicated that S. pseudopneumoniae is distinct from other VGS (<70% homology) but is sufficiently different genetically
Laboratory Identification of Streptococcus pseudopneumoniae
When described by Arbique et al. in 2004 (1), S. pseudopneumoniae was identified on the basis of bile solubility and optochin tests. S. pseudopneumoniae is bile insoluble and, when incubated in an atmosphere supplemented with 5% CO2, demonstrates resistant to intermediate optochin susceptibility, but it becomes susceptible when incubated in an ambient atmosphere, as shown in Fig. 2 (1). In terms of colonial morphology, S. pseudopneumoniae has been described as small, shiny, smooth, grayish
Clinical Significance of Streptococcus pseudopneumoniae
The pathogenic significance of S. pseudopneumoniae remains unclear. It appears to be an organism that treads the fine line between pathogen and commensal and has the necessary capabilities to facilitate both roles in a complex ecological niche (7). Its pathogenicity has been clearly demonstrated in a murine model in which intraperitoneal injections of S. pseudopneumoniae resulted in 100% mortality (39). Its identification as a hybrid species between S. pneumoniae and S. mitis indicates its
Antimicrobial Susceptibility Profile of Streptococcus pseudopneumoniae
Antimicrobial susceptibility data on S. pseudopneumoniae is relatively limited, and interpretation is complicated by the choice of the antibiotic breakpoints used. As S. pseudopneumoniae is in a unique phylogenetic position between S. pneumoniae and S. mitis, there is confusion over whether VGS or pneumococcus antibiotic breakpoints should be used for treatment. One study (4) has demonstrated an increase in the rates of reduced penicillin susceptibility if pneumococcal breakpoints were used
Conclusion
S. pseudopneumoniae is likely to have originated from S. pneumoniae following a complex evolution of recombination events, influenced heavily by the ability of VGS to undergo HGT. This also explains the difficulty in establishing accurate species identification. Whole-genome sequencing of an S. pseudopneumoniae isolate has provided insight into its virulence and commensalism dynamics. The classic phenotypic findings initially described for S. pseudopneumoniae are not always present, and
Acknowledgments
We thank David Beckingham for the images included in this article and Elaine Keith for sharing her knowledge and for critical reading of the manuscript.
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