ABSTRACT
An infected mycotic aneurysm due to Streptococcus constellatus subsp. constellatus has not previously been reported. We report on this condition in an 87-year-old woman who had aggravating abdominal pain and a large fusiform aneurysm over the thoracic-abdominal aorta with mural thrombus. Isolates from two sets of blood cultures and the debrided tissue were identified as S. constellatus subsp. constellatus by their biochemical reaction profiles, compatible 16S rRNA gene sequencing results, and sequencing results for the partial groESL gene and the 16S-23S intergenic spacer region.
CASE REPORTS
An 87-year-old woman visited our emergency department (ED) due to persistent abdominal pain for 2 weeks. She had had a history of uncontrolled hypertension for many years and suffered from intermittent abdominal pain during the 4 months before her visit. On arrival, her consciousness was clear and oriented. Her blood pressure, heart rate, respiratory rate, and body temperature were 240/114 mm Hg, 109 beats/min, 18 breaths/min, and 36°C, respectively. Physical examination revealed rales over the bilateral lung base. Auscultation revealed grade II/IV systolic heart murmur over the left sternal border and apex.
Abdominal bruit and a palpable pulsatile mass were found from the left to the midabdominal line. Her lower extremities had no edema but had strong bilateral pulsations. Initial laboratory data were a white cell count of 6,390/μl and a hemoglobin concentration of 8.0 g/dl. Laboratory data were negative for genitourinary infection.
A computed tomography (CT) scan showed a thoracic-abdominal aneurysm (Fig. 1), with its largest abdominal fusiform dilation measuring about 6 cm in diameter with a predominant mural thrombus. Bilateral pleural effusion was also noted, probably due to extravasation of the aneurysm. Abdominal pain improved after proper control of her blood pressure and heart rate. The patient refused to undergo surgery and was discharged on the second hospital day.
Contrast-enhanced CT scan showing a ruptured thoracic-abdominal mycotic aneurysm (arrow) caused by S. constellatus subsp. constellatus.
She revisited the ED 1 week later due to the development of aggravated abdominal pain. On arrival, her temperature, pulse, blood pressure, and respiration were 36.7°C, 85 beats/min, 120/66 mm Hg, and 20 breaths/min, respectively. The physical findings were similar to those on her previous visit, but with more severe abdominal tenderness. Furthermore, the leukocyte count had increased to 21,740/μl with 93.7% neutrophils. There was no evidence of infection of the oropharyngeal region, respiratory tract, genitourinary system, or skin. Thus, a mycotic aneurysm was suspected, and ceftriaxone (1 g every 12 h) was given. Finally, the patient agreed to surgical intervention.
A graft of the thoracic-abdominal aortic aneurysm was performed on the second day of hospitalization. Pathology revealed severe atherosclerosis with fibrin-purulent inflammation. A Gram-stained preparation of the aortic aneurysm showed numerous gram-positive cocci. Group F Streptococcus was isolated from three cultures of blood obtained before the initiation of antibiotic treatment. The antibiotic treatment was then shifted to penicillin G at 2.25 × 106 U every 4 h. A culture of excised tissue also yielded the same organism. Echocardiographic examination revealed no evidence of vegetation on the heart. The postoperative course was smooth, and the patient was discharged after 6 weeks of penicillin G treatment.
The isolates grew well and had pinpoint colonies of beta-hemolysis on Trypticase soy agar supplemented with 5% sheep blood (BBL Microbiology Systems, Cockeysville, Md.) in 5% CO2 and ambient air at 35°C. They were catalase-negative and gram-positive cocci. Group F antigen reactions (Oxoid, Unipath Limited, Basingstoke, England) of the isolates were positive. The phenotypic reaction profiles of these isolates obtained with the Vitek GPI Card (bioMerieux Vitek, Inc., Hazelwood, Mo.) (biotype profile, 70134260000; identity, 98%), the Phoenix System PID (Beckon Dickinson, Sparks, Md.) (biotype profile, 420080082444; confidence value, 98%), and the API Rapid ID32 Strep system (bioMerieux, Marcy L'Etoile, France) (biotype profile, 14023001100; identity, 99.9%) were in accordance with the identification of Streptococcus constellatus; and the reactions of these isolates with the API ZYM system (bioMerieux) were identical. The isolates displayed positive reactions for α-glucosidase and hyaluronidase and negative reactions for β-N-acetylgalactosaminidase and β-N-acetylglucosaminidase. This profile is in accordance with the identification of S. constellatus (S. constellatus subsp. constellatus) (7, 12, 19).
Species identification was further confirmed by sequence analysis of 16S rRNA gene, the 16S-23S intergenic spacer region, and the partial groESL gene. PCR amplification of the nearly complete 16S rRNA gene (1,463 bp) with two primers (primers 8FPL and 1492) and the 16S-23S intergenic spacer region (380 bp) was performed as described previously (4, 18). The partial groESL gene (487 bp) was amplified with primers Strep-ES-UP and Strep-EL-120-100, as described previously (15). The amplification products were purified and sequenced. A search was performed with the BLAST program to compare the sequence of our isolate with those in the GenBank and Ribosomal Database Project databases. While the 16S rRNA gene analysis of our isolate shared a high degree of sequence similarity with both S. intermedius (GenBank accession number AF104674 ; 99.73%) and S. constellatus (GenBank accession number AY277939 ; 99.66%), the 16S-23S intergenic spacer region sequence of the isolate showed the highest degree of similarity (100% match) with the corresponding S. constellatus sequence (GenBank accession number L36933 ) in the GenBank database. The partial groESL gene sequence also showed the best match (97%) with that of S. constellatus (GenBank accession number AF378196 ).
The susceptibilities of the isolates to penicillin were determined by the E-test (PDM Episilometer; AB Biodisk, Solna, Sweden) on Mueller-Hinton agar supplemented with 5% sheep blood (BBL Microbiology Systems). The S. constellatus isolates were susceptible to penicillin (MIC, 0.047 μg/ml).
Discussion.Organisms commonly involved in infections of an atherosclerotic aneurysm include Staphylococcus aureus, the Salmonella species, nonhemolytic streptococci, Streptococcus pneumoniae, and gram-negative rods, such as Escherichia coli (2). Although some beta-hemolytic streptococci have been documented as the causative agents of mycotic aneurysms (10), S. constellatus has not been reported as a cause of this disease entity.
S. constellatus strains, which were previously categorized as belonging exclusively to the milleri group, are currently classified as members of both the anginosus and milleri groups (6, 10, 17). Strains belonging to the anginosus group are part of the oral and genitourinary floras (1) and have been associated with endocarditis and purulent infections of the internal organs (1, 10, 17). Strains of S. constellatus are mainly beta-hemolytic and frequently possess Lancefield group F antigens, although some strains may be of group A, C, or G or are nongroupable (10, 13). They are frequently associated with suppurative and deep-seated infections, particularly at thoracic sites (8, 10, 17).
Nucleotide sequence analysis of the 16S rRNA gene yields valuable information for the identification of bacterial strains (5, 7). However, a close relationship between S. constellatus and S. intermedius has been observed by 16S rRNA sequence analysis (7). The sequences of the isolates from our patient had high levels (>99%) of similarity to the sequences of both S. constellatus and S. intermedius, which supports this observation. However, the sequencing results for the partial groESL and 16S-23S intergenic spacer sequences and the characteristic biochemical phenotypes confirmed the identification of S. constellatus (19). Although Goto et al. (4) reported on a PCR analysis with the ily gene which was positive for S. intermedius but not the different subspecies of S. constellatus, only three S. constellatus isolates were tested. The phenotypic characteristics of the isolate from our patient obtained with three biochemical identification kits (API ID32 Strep, Vitek GPI Card, and Phoenix System PID) and the API ZYM system further confirmed the identity of the isolate as S. constellatus subsp. constellatus.
An early study of the susceptibilities of group F streptococcal strains indicated that they were uniformly susceptible to penicillin G (1), although a later study showed that 9% of the group F streptococcal strains tested were highly resistant to penicillin G (MICs, >4 μg/ml) (3). Previous studies (11, 14, 16) found that the penicillin MICs at which 50 and 90% of S. constellatus isolates are inhibited were ≤0.03 to 0.06 μg/ml and 0.12 to 0.25 μg/ml, respectively; and one study (14) found that 92% of the isolates were susceptible to penicillin (MICs, ≤0.12 μg/ml). Clinically, infections caused by group F streptococci have responded well to penicillin G and cephalosporins (8, 9). Our patient was successfully treated by surgical resection of the mycotic aneurysm and by 6 weeks of treatment with penicillin G, which was demonstrated to have activity against the isolate in vitro.
In summary, this is the first documented case of an infected thoracic-abdominal aneurysm caused by S. constellatus. This organism should be included in the list of pathogens causing mycotic aneurysms.
FOOTNOTES
- Received 3 November 2003.
- Returned for modification 12 December 2003.
- Accepted 29 December 2003.
- Copyright © 2004 American Society for Microbiology
