Aeroccocus urinae is an alpha-hemolytic streptococcus that is known to cause invasive infections in adults, especially the elderly. Because it is often misclassified as a viridans group Streptococcus, cultures of A. urinae from unexpected sites may be dismissed as clinically insignificant. Here we describe a case of A. urinae pyelonephritis in a child, the first report of invasive disease caused by this organism in the pediatric age group.
A 12-year-old uncircumcised boy presented to the Emergency Department at Yale-New Haven Children's Hospital (New Haven, CT) with chief complaints of fever, vomiting, and flank pain. One week before admission he developed intermittent fever to 103°F, abdominal pain, diarrhea, and nonbloody, nonbilious vomiting. The abdominal pain was initially peri-umbilical, but later evolved to include his right flank. He denied symptoms of dysuria, hematuria, and urinary urgency or frequency.
The past medical history was significant for upper pelvic juxtaglomerular obstruction and grade IV reflux of the left kidney diagnosed at 3 months of age. The obstruction was corrected with a left dismembered pyeloplasty performed at the time of diagnosis, and the patient was prescribed amoxicillin prophylaxis to prevent infection. Several follow-up ultrasound studies showed no change in dilatation of the left renal pelvis and calices, no dilatation on the contralateral side, and normal interval growth of both kidneys. The amoxicillin prophylaxis was discontinued at 4 years of age, with no history of subsequent urinary tract infections (UTIs). The child was otherwise reported to be well. His current medications included acetaminophen as needed, multivitamins, and probiotics administered by his parents.
In the emergency department, the patient's vital signs were as follows: temperature 37.4°C, heart rate 132 beats/min, respiratory rate 24/min, blood pressure 113/57 mm Hg, and a pulse oximetry reading of 100% on ambient air. On auscultation, his lung sounds were clear and the heart showed a regular rate and rhythm. His abdomen was diffusely tender without guarding or rebound. Both obturator and psoas signs were negative. He was estimated to be 3–5% dehydrated, for which he received two 20 mL/kg normal saline intravenous (IV) infusions and was admitted to the general pediatric ward.
Initial laboratory values were as follows: white blood cell count 15,300 cells/μL (73% neutrophils, 13% lymphocytes, 14% monocytes, <1% eosinophils), hemoglobin concentration 13.4 g/dL, platelet count 386,000/μL, erthyrocyte sedimentation rate 34 mm/Hr. An abdominal radiograph series showed no evidence of bowel obstruction. After admission, right costovertebral tenderness was noted, prompting culture of a clean catch urine sample, which was sterile. Because of the persistence of flank pain and fever, a second urine specimen was obtained the following day, which was notable for the presence of leukocyte esterase, many white blood cells per high powered field, and moderate bacteria in the absence of nitrites. A culture of this urine specimen grew >100,000 colony forming units (CFUs) of α-hemolytic Gram-positive cocci on 5% sheep blood agar, with no growth detected on MacConkey agar (Fig. 1, available online only). The isolate was also catalase negative, pyrrolidonyl arylamidase (PYR) negative, and rapid bile esculin negative. These findings suggested that the isolate was of the viridans Streptococcus group.
The presence of flank and abdominal pain, fever, an elevated white blood cell count, and a positive urine culture was consistent with a diagnosis of acute pyelonephritis. After obtaining a blood culture and a repeat urine culture, the patient was treated with IV ampicillin-sulbactam in a dosage of 3.0 g every 6 hours. An ultrasound examination of the abdomen and pelvis revealed bilateral hydroureteronephrosis with echogenic debris in the bladder and ureters, as well as bladder wall thickening. The ureters were normal in diameter at the ureterovesicular junction, and no ureteral jets were observed. These findings were most consistent with chronic vesicoureteral reflux without evidence of acute obstruction.
The blood culture was sterile, but the repeat urine culture again grew >100,000 CFU of Gram-positive cocci appearing in tetrads. As before, this isolate also grew on 5% sheep blood agar (but not MacConkey agar), was catalase negative, PYR negative, and rapid bile esculin negative. The isolate was subjected to biochemical analysis using the RapID STR panel (Remel, Inc.), which yielded an equivocal identification (bioscore of 1 out of 213,425) of Streptococcus mutans. The isolate was next analyzed using a Vitek 2 instrument (Biomerieux, Inc.) which yielded low discrimination identifications for Aerococcus urinae (50.54% probability) and Granulicatella adiacens. (49.46% probability) Additional analysis revealed that the isolate was hippurate positive, as are 99% of A. urinae isolates. In contrast, 99% of G. adiacens isolates are both hippurate negative and PYR positive. Further testing revealed that the isolate was capable of growing in the presence of 6.5% NaCl, was vancomycin susceptible, β-glucuronidase negative, and leucine amino peptidase positive. Together, these laboratory findings were most consistent with an identification of A. urinae.1
PCR amplification and sequencing of the first 500 base pairs of the 16S rRNA gene was performed as described previously,2 with minor modifications. Briefly 10 μL of lysed bacteria isolated from a single colony were added to 10 μL of GeneAmp 2x Fast PCR master mix (Applied Biosystems, Foster City, CA) containing oligonucleotide primers 005F and 531R.2 Thirty cycles of 2-step PCR were performed according to the manufacturer's instructions. Bi-directional sequencing of the PCR amplification product was carried out as described,2 and phylogenetic analysis and database comparisons were performed using the MicroSeq Identification Software (Applied Biosystems, Foster City, CA). The sequence was also analyzed using the blastn algorithm to identify similar sequences contained in the NCBI Genbank database.3
Comparison of the 16S rRNA nucleotide sequence obtained from the clinical isolate to the Microseq library database revealed 99.75% sequence identity with Aerococcus urinae, with the next closest match being Aerococcus christensenii (92.4% sequence identity). The Genbank phylogenetic analysis of our isolate (Genbank accession number EF619539) revealed that it was mostly closely related to Aerococcus urinae, isolate CCUG 32514 (Genbank accession number AY422716) with a 99.2% sequence identity.
Antibiotic susceptibilities of the A. urinae isolate cultured from the patient were determined using the E-test (AP Biodisk Solna, Sweden) following the manufacturer's instructions for minimum inhibitory concentration (MIC) determination. The isolate had the following MICs: vancomycin 0.50 μg/mL; amoxicillin 0.047 μg/mL; tetracycline 1.0 μg/mL; trimethoprim-sulfamethoxazole >32 μg/mL.
After 48 hours, the ampicillin/sulbactam therapy was changed to IV cefazolin, 1.0 g every 8 hours. The patient's fever, flank and abdominal pain resolved by hospital day 5, and a repeat urine culture obtained during antimicrobial therapy was negative. The patient was discharged home on the seventh hospital day to complete an additional week of therapy with oral cephalexin, followed by nitrofurantoin prophylaxis. At the time of evaluation on the last day of his antibiotic therapy, the patient was asymptomatic.
The incidence of UTI in school age males is approximately 0.03–1.2%,4 and 25–50% of children with a documented UTI have a genitourinary anomaly.5 Gram-negative enterics, such as Escherichia coli, are the most commonly isolated pathogens in children with UTI.5,6 Two independent studies of the bacterial causes of UTI reported that the presence of a genitourinary anomaly increases the likelihood of isolating organisms other than E. coli.5,6 Gram-positive organisms are cultured from the urine in up to 10% of symptomatic children, with Enterococcus species (8%) and coagulase-negative Staphylococcus most commonly isolated.5,6 Marcus et al5 identified 3 risk factors for UTI caused by organisms other than E. coli, which included a history of preceding antibiotic therapy, male gender, and the presence of an underlying genitourinary abnormality. The patient described in this report had the latter 2 risk factors.
Aerococcus urinae, first described in 1989, are Gram-positive cocci that comprise 0.2–0.5% of positive urine cultures.7,8 The organism forms α-hemolytic, catalase negative colonies with a Gram-stain that shows pairs and tetrads (not typical of streptococci), which can be misclassified as a viridans Streptococcus species.9 The Gram-stain morphology and catalase reaction should alert the microbiologist to the possibility of A. urinae, but the Gram-stain is not always performed on direct urine specimens. A combination of biochemical tests can distinguish A. urinae from other Aerococcus species including PYR, LAP, bile esculin and esculin hydrolysis, and β-glucuronidase.1 Other less commonly identified catalase-negative Gram-positive cocci can occasionally be misidentified as Aerococcus sp. such as Abiotrophia defectiva, Lactococcus sp., Leuconostoc sp., and Pediococus sp. These can often be distinguished from A. urinae based on morphology, the ability to grow on nonsupplemented sheep blood agar, and vancomycin susceptibility.1A. urinae can be further differentiated from Viridans Streptococci by a panel of maltose, lactose, and ribose utilization tests. However, the biochemical assays used to discriminate A. urinae from other bacteria, particularly Streptococcus can be nondiscriminatory. In 1 series of 21 α-hemolytic, Gram-positive, catalase negative, PYR negative and LAP positive isolates, only 13 were confirmed as A. urinae by definitive analysis of the 16s rRNA sequence.7 In the case reported here, the preliminary identification of the urinary isolate was viridans Streptococcus group.
Although expensive and available primarily at reference laboratories, this further demonstrates the value of 16s rRNA gene sequencing, compared with traditional biochemical tests, in identifying Gram-positive urinary pathogens. It is likely that as 16sRNA is used with increasing frequency, infectious disease specialists will be asked to determine the clinical significance of rare or unusual organisms isolated from patient samples.
Aerococcus urinae are typically susceptible to ampicillin, but resistant to trimethoprim-sulfamethoxazole and gentamicin.9 Therefore, the proper discrimination of A. urinae from other Streptococci is essential for choosing appropriate antibiotic therapy for patients with UTI. In 1 study, in vitro susceptibility assays of 56 isolates found the majority to have low MICs to penicillin and its derivatives, as well as vancomycin.10 MICs above the range of antibiotic susceptibility were reported for gentamicin (>4 mg/mL) and amikacin (>16 mg/mL).10 For ciprofloxacin, a commonly prescribed antibiotic for adults with UTIs, the majority of A. urinae isolates had an MIC of less than or equal to 1.
The clinical spectrum of infectious diseases caused by A. urinae include urosepsis, infective endocarditis, and spondylodiscitis.11–15 Most cases of A. urinae UTI described in the literature have occurred in elderly patients with underlying illnesses, such as diabetes, heart disease, or urologic conditions.8,9 Sierra-Hoffman et al8 conducted a retrospective study of all urinary isolates of A. urinae recovered during a 1-year period. None of the 54 patients progressed to invasive disease, and 11 improved without antibiotic therapy. These data suggest that A. urinae is a low-grade pathogen that in the presence of genitourinary anomalies has the potential to cause more severe disease in adults. Based on this report, the same would also appear to be true for children.
We would like to acknowledge Dr. Robert S. Baltimore for his critical review of this manuscript.
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