The Streptococcus anginosus group (SAG), one of five groups collectively known as viridans group streptococci, consists of the species S. intermedius, S. anginosus, and S. constellatus . These organisms are commonly associated with purulent infections with abscess formation and less commonly the cause of endocarditis than other viridans streptococci [9–13]. S. intermedius has an apparent tropism for the brain and liver, whereas S. anginosus and S. constellatus have been isolated from a wider range of sites and infections [2, 14]. The three cases presented here highlight a common presentation of S. intermedius (liver and brain abscesses) and a rare complication (endocarditis).
The identification and classification of streptococci based on hemolytic and Lancefield grouping reactions is cumbersome and lacks discriminatory power. Whiley, et. al. developed phenotypic methods to better differentiate the SAG. Commercially available systems based on these phenotypic tests are widely used and provide a more rapid characterization of each species, but accuracy remains variable [15, 16]. This has led to the development of molecular techniques, such as PCR sequencing, to increase the accuracy of identification and clinical profiling [1, 7, 17, 18]. Furthermore, several species-specific genes have been targeted for the development of PCR-based assays for the identification of the SAG organisms [7, 18]. Intermedilysin (ILY) is a human-specific cytolysin that directly damages human hepatic cells and is implicated as a potential important virulence factor in causing deep-seated abscesses . The ily gene has been shown to be specific for S. intermedius. The assay can be used for rapid identification through the use of PCR with the ily gene as a species marker gene, which was amplified on all three isolates .
Pyogenic liver abscesses are an uncommon, but potentially life-threatening, infection. The first cases of SAG hepatic abscesses were reported in 1975 . Later, a study in 1981 found SAG to be the most common cause of hepatic abscesses . A prospective study by Coreedoira, et. al. compared the incidence and clinical features of SAG liver abscess to liver abscesses caused by other organisms. Members of SAG were most commonly isolated, with S. intermedius being the most frequent species. Abscesses also tended to be monomicrobial and the duration of symptoms were longer when compared to other organisms. However, there were no differences in mortality, duration of antibiotics, or complications. The majority of patients required surgical or percutaneous drainage of the abscesses for cure as was the case for our patients, whereas 20% of patients were treated with antimicrobials alone .
Members of the SAG are rare causes of endocarditis. Previous studies on infective endocarditis caused by SAG have relied on phenotypic methods for identification. Sussman, et. al. studied 36 patients with viridans streptococcal endocarditis, and identification at the species level was determined by using biochemical tests established by Facklam . Four of the cases were found to be S. intermedius. Sixteen isolates from the study were sent to five institutions to confirm identification. Not surprisingly, only 3 were characterized as the same species by all five institutions . Another retrospective study found 29 cases of endocarditis due to SAG, and only 3 were identified as S. intermedius . However, the commercial method used in this case, Rapid 32 ID Strep System (bioMérieux), has been shown to be the least accurate in identifying S. intermedius . A more recent study by Woo, et. al. applied 16S rRNA sequencing to 6 cases of SAG endocarditis, all of which were identified as S. anginosus . To our knowledge, we report the first case of S. intermedius endocarditis confirmed by 16S sequence analysis.
Brain abscesses can result via spread from a contiguous focus of infection, hematogenous spread, or as a result from head trauma or neurosurgery . Mortality and morbidity have reduced over the decades in the advent of newer antibiotic therapies and early recognition with CT scans and MRIs . Several case series have defined the clinical characteristics of patients with brain abscesses. All ages are affected with a male predominance. Headache, fever, and mental status changes are the most common initial presenting features, although only one-third of patients have the classic triad. The majority of abscesses are solitary and occur in the frontal lobe. Surgery is required in most patients, and the duration of antibiotics range from 4 to 8 weeks [26–31]. The SAG has been recently recognized as a common cause of brain abscesses, which were collectively identified with other streptococci as viridans group streptococci in prior studies [29, 31]. Several retrospective reviews and case reports have identified S. intermedius as a significant pathogen among the SAG in the development of brain abscesses [1, 2, 32, 33]. However, no study has compared S. intermedius brain abscesses with other members of the SAG or with other bacterial causes.
A unique feature of our cases was the lack of underlying medical problems in our two patients with liver abscesses. Infections caused by SAG are not common in previously healthy individuals. An underlying condition, such as diabetes, cirrhosis, or cancer, is associated with the majority of patients [12, 34, 35]. Neither of our patients with liver abscesses had chronic illnesses or evidence of immunosuppression. The third case with brain abscess, however, had chronic sinusitis, which is one of the most common predisposing factors [29, 31]. The presentation of these cases highlights the range of infection caused by S. intermedius that can be encountered in a community setting. Our findings indicate that infections with S. intermedius and the other members of the SAG may be more common that previously appreciated.