Diagnosis of Aerococcus urinae infections: Importance of matrix-assisted laser desorption ionization time-of-flight mass spectrometry and broad-range 16S rDNA PCR

Abstract

An increasing number of clinical reports are associating Aerococcus urinae with severe infections such as endocarditis, joint infections, and osteomyelitis. A. urinae identification has been hampered for years by morphotype ambiguity with streptococci or coagulase-negative staphylococci as this is a Grampositive α-haemolytic bacteria able to grow in pairs and clusters. In our university hospital, A. urinae was rarely identified before 2009. The identification rate of A. urinae started to increase significantly after the introduction of a matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) system (Bruker Daltonics, Leipzig, Germany) in our diagnostic laboratory in May 2009 ( Fig. 1) [1,2]. It is noteworthy that five of 11 severe infections due to A. urinae that we documented (bloodstream infections, endocarditis, and joint infections) occurred in the setting of negative cultures, and were documented by the use of eubacterial 16S rDNA PCR. The recent literature, our data and the two cases that we describe herein suggest that the prevalence of A. urinae in clinical samples and its pathogenic role might have been underestimated in the pre-molecular diagnostic era.Our first case, a 78-year-old male, presented with persistent fever of 39°C that had lasted for 2 weeks, and that had not responded to 5 days of oral amoxicillin-clavulanate treatment. Cardiac auscultation revealed a systolic heart murmur with axillary irradiations. Intravenous amoxicillin-clavulanate was administered empirically after several blood cultures that remained sterile. Transoesophageal echocardiography revealed a 17-mm mitral valve vegetation and severe valvular insufficiency, which required rapid valvular replacement. The valve showed acute necrotizing inflammation, confirming the diagnosis of endocarditis. As the valve culture remained sterile, we performed a broad-range 16S rRNA gene PCR [3], which revealed the presence of A. urinae, a pathogen that has already been reported in culture-negative endocarditis [4,5]. The Aerococcus strains isolated in our diagnostic laboratory were generally susceptible to beta-lactam antibiotics (of 176 strains, five exhibited intermediate susceptibility to ceftriaxone and two were resistant to cefriaxone). A combination of amoxicillin for 6 weeks and gentamicin for 2 weeks was given, with a favourable clinical evolution.The second patient was a 60-year-old woman who was hospitalized 2 days after a high fever with signs of hip prosthetic joint infection. A one-stage exchange procedure had been performed 4 months earlier because of mechanical failure. She was receiving chronic methotrexate therapy for rheumatoid arthritis, and daily doxycycline for hidradenitis suppurativa. There was no evidence of urinary tract or cardiac infection, and the blood cultures remained sterile. A debridement with retention strategy was chosen. The culture of the sonicated mobile parts of the prosthesis remained sterile, but two of the five perioperative biopsies became positive for Gram-posit...

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