Urosepsis is defined as sepsis caused by a urogenital tract infection (UTI), which accounts for approximately 25% of all sepsis cases in adults, and most cases are due to complicated UTIs [1]. The bacterial spectrum in urosepsis is composed of 61% Escherichia coli, 16% other Enterobacteriaceae, 8% Staphylococcus aureus and 6% enterococci [2]. However, if the host immune system is suppressed, less virulent organisms, such as enterococci, coagulase- negative staphylococci or Pseudomonas aeruginosa, can cause urosepsis.

Aerococcus urinae is known to colonize the human urinary tract and may cause symptomatic UTI [3], infective endocarditis [4] and bacteremia [5]. However, sepsis due to UTI by A. urinae is not commonly recognized in clinical settings and inadequate treatment of this infection has been linked to fatal outcomes and severe complications [6]. Here, we report a case of urosepsis due to A. urinae identified through matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS).

An 86-year-old man was admitted to the hospital with both feet edema and generalized weakness of 15 days with elevated creatinine, suggesting the acute aggravation of chronic renal failure. He also had diabetes mellitus for more than 10 years and history of treated prostate cancer. On day 2 of hospitalization his fever spiked to 38.1°C and blood and urine cultures were drawn. The growth was detected in one anaerobic blood culture bottle (BacT/ALERT, bioMérieux, Marcy-l'Etoile, France) out of three total pairs of blood cultures. Grayish pinpoint-sized alpha-hemolytic colonies grew on blood agar and Gram-positive cocci in clusters were observed on microscopy. His urine culture grew, >10⁵ CFU/mL of a similar organism with >10⁵ CFU/mL of E. coli. The isolates from blood and urine culture were identified as A. urinae by the VITEK 2 system GP (Gram-positive) II card (bioMérieux).

16S rRNA gene sequencing and MALDI-TOF-MS (Bruker Daltonics, Bremen, Germany) further confirmed the identification of the organisms. The 16S rRNA gene sequences of the two isolates were identical to each other (783 bp) and were 99.2% similar to A. urinae (GenBank accession no. M778191), differed from Aerococcus christensenii (no. Y17005) with 94.1% similarity, and Aerococcus sanguinicola (no. AJ276512) with 94.0% similarity in EzTaxon e-database (http://www.ezbiocloud.net/eztaxon). Since the separation between different species was greater than 0.8%, A. urinae was considered as acceptable identification [7]. MALDI-TOF-MS analysis yielded isolates from blood and urine by scores of 2.216 and 2.141, respectively on Biotyper 3.1 (Bruker Daltonik GmbH), greater than 2.00, which can be considered an excellent probability for identification. The mass spectrum and peak heights were identical to each other and all m/z difference between two isolates were less than 500 ppm (difference range, 0.063-474.608), which is the limit of mass tolerance (Fig. 1A, B).

The antimicrobial susceptibility of the two isolates was tested according to the disk diffusion interpretive criteria of Streptococcus spp. viridans group provided by the Clinical and Laboratory Standards Institute. Both isolates were susceptible to penicillin, ceftriaxone, cefepime, clindamycin, erythromycin, teicoplanin and vancomycin with identical antibiogram.

The patient underwent antimicrobial therapy with ceftriaxone for five days then changed to meropenem after growing of E. coli on urine culture. Two days later, teicoplanin was added after growth of A. urinae was reported from blood and urine cultures. In the subsequent cultures, A. urinae was not detected, he discharged after 19 days of antimicrobial treatment.

A. urinae is facultatively anaerobic, catalase-negative and alpha-hemolytic Gram-positive cocci that forms tetrads and clusters. For its features of both Staphylococcus and Streptococcus, the secure identification of aerococci has relied on 16S rRNA gene sequencing, however, correct and fast identification is allowed with MALDI-TOF-MS with high sensitivity and specificity [8].

To determine the relatedness of isolates that cause simultaneous infection, genetic typing methods with high discriminatory power are conventionally used, but are time-consuming and cost-intensive. The peptide mass fingerprinting of MALDI-TOF-MS facilitates the identification of clonality rapidly with accuracy and reproducibility, and therefore might be used as a definitive tool to track course of infection.

To the best of our knowledge, this report adds a first patient confirmed by MALDI-TOF-MS to the published reports of urosepsis caused by A. urinae which were determined by antibiogram [9] or 16S rRNA sequencing [10]. There have been several studies examining the clinical significance of A. urinae, but these have not been conclusive, and it is still known as a non-virulent organism which might be able to be treated without antimicrobial treatment [3] or an organism which can cause invasive infection with mortality [6]. However, with the aid of peptide mass fingerprinting by MALDI-TOF-MS, we suggest that A. urinae acts as an invasive pathogen, which infected the bloodstream of a patient with a complicated UTI.