Streptococcus anginosus Group

Molecular Subspecies

Within these three distinct streptococcal species, a number of subspecies have been recognized or proposed based primarily on molecular testing rather than phenotypic methods, the latter being highly variable. S. constellatus has three subspecies: S. constellatu s subsp. constellatus, S. constellatus subsp. pharyngis, and S. constellatus subsp. viborgensis. S. anginosus has two subspecies: S. anginosus subsp. anginosus and S. anginosus subsp. whileyi. A third genomosubspecies has tentatively been proposed for S. anginosus : S. anginosus subsp. vellorensis . Of note is that S. anginosus subsp. vellorensis harbors superantigen and extracellular DNase coding genes identical to genes found in Streptococcus pyogenes . The various species and subspecies appear to be associated with a number of different body habitats and sites and types of clinical infection. Microbiologically, members of this group are recognized by their microaerobic or anaerobic growth requirements, their formation of minute colonies, and the frequent presence of a characteristic caramel-like odor. This chapter defines the three species and subspecies currently making up the S. anginosus group and discusses their role in clinical infections.

Phenotypic Characteristics

Members of the S. anginosus group share the phenotypic characteristics of the members of the genus Streptococcus, whose classification in general is based on patterns of hemolysis, Lancefield serologic antigenic reactions, growth properties, and biochemical reactions ( Fig. 203.1 ). Like other streptococci, these organisms may be β-hemolytic, α-hemolytic, or γ-hemolytic on sheep blood agar. Members of the S. anginosus group often exhibit Lancefield antigens A, C, F, or G but can be differentiated from other Lancefield-grouped streptococci by the small size (less than 0.5 mm) of their colonies. In general, β-hemolytic strains of S. constellatus react with Lancefield serologic group F antibody. Strains containing the group F antigen may cross-react with the other grouping sera (Lancefield groupings) and therefore are of little value in identifying these organisms.

Gram staining of S. intermedius strains reveals gram-positive spherical or ovoid cells that form short chains or pairs. S. anginosus group can be differentiated from other streptococci by a combination of three rapid tests: a positive Voges-Proskauer test for acetoin production, hydrolysis of arginine, and failure to ferment sorbitol. In addition, the presence of the caramel-like odor, often attributed to the production of a diacetyl metabolite, can be helpful. These characteristics are summarized in Table 203.1 .

Taxonomy

The diversity of hemolytic and Lancefield groupings has made identification of these pathogens difficult in many laboratories. Whiley and coworkers have noted that almost all S. intermedius strains (93%) are not β-hemolytic, whereas 38% of S. constellatus and 12% of S. anginosus are β-hemolytic. Laboratories can readily differentiate the three members of the S. anginosus group by using phenotypic characteristics that correlate well with molecular taxonomic techniques. A number of commercial systems are available for the identification of viridans streptococci, and studies have shown similar performance to manual biochemical testing.

Molecular Diagnostic Methods

Nucleic acid amplification assays have been developed for the identification of clinically relevant viridans-group streptococci to the species and group level. Targets for these assays have included the 16S rRNA gene, the 16S-23S rRNA intergenic spacer region, tuf gene, rpoB gene, or groEL gene. Most assays require both amplification and sequencing of the targeted region, making the method impractical for routine use. In addition, viridans streptococci are competent—that is, they freely exchange genetic material within and between species. This phenomenon makes the taxonomic classification of viridans streptococci with DNA target sequencing and their identification to species level much more challenging. Non–sequence-based methods for identification, such as matrix-assisted laser desorption/ionization–time-of-flight (MALDI-TOF) mass spectrometry, are fast, reliable and cost-effective identification methods for S. anginosus group, but identification to the species level is challenging.

Mixed Infections Contribute to Abscess Formation

Mixed infections involving members of the S. anginosus group and other microbes (e.g., Eikenella corrodens, Enterobacteriaceae , and anaerobes) contribute to abscess formation by allowing more rapid replication of the streptococci. Murine models of pneumonia have been used to demonstrate synergy between members of the S. anginosus group and oral anaerobes. These studies have found that mortality was higher, abscesses or empyema were more frequently noted on histopathologic examination, and viable bacteria were more numerous in the lungs of mice with mixed infections caused by members of the S. intermedius group and oral anaerobes than in the lungs of mice with monomicrobial infection. In vitro studies by these and other investigators have confirmed that anaerobes enhance the growth of S. anginosus group organisms. In clinical cases in which S. anginosus group members cause acute pneumonia, pulmonary abscess, and/or thoracic empyema in humans, the predominant species recovered along with S. anginosus group isolates were anaerobic bacteria, confirming the clinical importance of anaerobic copathogens in pulmonary and thoracic infections.