Aerococcus urinae infective endocarditis

Aerococcus spp. are gram-positive bacteria, facultative anaerobes and arranged in pairs and clusters. Since this genre was defined in 1938¹ new species have been identified —Aerococcus viridans, A. sanguinicola, A. christensenii, A. urinaehominis, A. urinaeequi and A. suis—, but it wasn't until 1992 thatAerococcus urinae was defined as a new species.²Aerococcus ssp. are found ubiquitously on the ground and in the air.³ Their participation as part of the normal flora of the human urinary tract and of the human oral flora in patients undergoing treatment with cytostatics is relevant.⁴ Despite this, A. urinae is a rare cause of urinary tract infection (UTI) and invasive disease or bacteraemia are unusual (0.5-3 cases/10⁶ people-year). The overall incidence of infective endocarditis (IE) due to A. urinae is unknown, but to the best of our knowledge, no more than 50 cases.⁵

61-year-old male with a history of obesity, type 2 diabetes mellitus  , arterial hypertension and dyslipidaemia under treatment with enalapril, metformin and gemfibrozil. He is admitted for community-acquired pneumonia and requires bladder catheterisation for acute urinary retention during admission without finding any previous structural or infectious urological history. He is taking levofloxacin when he presents with dyspnoea, poor general condition, and fever following 2 days of antibiotics treatment. On physical examination he has a fever of 39 °C and systolic murmur III/VI in the aortic area. The lab tests show 19,600 leukocytes/μl with 85% neutrophils and  C-reactive protein (CRP) of 145 mg/l (normal value up to 5 mg/l). Blood cultures are taken and the growth of A. urinae is seen in two bottles. The microorganism was susceptible to the following antibiotics: penicillin (MIC = 0.008 mg/l), ampicillin (MIC = 0.015 mg/l), meropenem (MIC = 0.06 mg/l), vancomycin (MIC = 0,25 mg/l) and rifampicin (MIC = 0.015 mg/l). Ciprofloxacin was classified as resistant (MIC > 2 mg/l). Transthoracic and transesophageal echocardiograms are conducted, which reveal two large vegetations anchored to the aortic valve. With the diagnosis of IE, treatment with ceftriaxone (2 g I.V./24 h) and gentamicin (3 mg/kg/day) is started. Subsequently, the patient presents with signs and symptoms of acute heart failure, so valve replacement surgery is performed and the valve is sent to culture, showing growth ofA. urinae. The patient is treated with gentamicin (3 mg/kg/day) for 15 days and ceftriaxone (2 g I.V./24 h) for 6 weeks.

It is likely that the scarce information about A. urinae as the cause of IE, is related to incorrect identification. The morphology that is not always constant —pairs or clusters—, catalase negativity and its alpha-haemolytic potential on blood agar may have contributed to it being erroneously identified in the past as a staphylococcal or streptococcal species.⁶ However, 16S rRNA sequencing and MALDI-TOF mass spectrometry has contributed to better identification, and it is estimated that its isolation in blood culture has increased to 20 cases./10⁶ people-year.⁷ In the Sunnerhagen et al.⁸ study, patients with IE due to A. urinae had a profile characterised by being male, elderly - with an average age of 79 years old—, and urinary tract diseases, such as prostate cancer, having a urinary catheter, and presenting or having recently presented with a UTI treated with fluoroquinolones. Clinical presentation similar to IE is also due to other bacterial aetiologies. Antibiotic susceptibility varies between species, but generally Aerococcus spp. are sensitive to beta-lactams. In particular, A. urinae has intrinsic resistance to sulfamethoxazole and minimum inhibitory concentrations for fluoroquinolones.⁹

Although there are no studies evaluating optimal treatment, treatment regimens have been based on beta-lactams with or without synergistic use of aminoglycosides. However, combined use of aminoglycosides has limited clinical evidence, since antibacterial synergy is not present in all isolates.⁸ Therefore, the use of aminoglycosides should be individualised, especially due to their higher risk of adverse effects in the profile of patients with IE due to A. urinae. Duration of treatment is 4–6 weeks with beta-lactams and an average of 10 days with aminoglycosides. In the study published by Yabes et al⁵ 43 patients with endocarditis due to A. urinae were gathered and different regimens and durations of antibiotic treatment are described. In 37 of the cases a beta-lactam was used for 4–6 weeks in many of them and an IV aminoglycoside with an average duration of 10 days.

Aerococci have a morphology similar to the viridans Streptococcus group so correct identification of species was difficult in the past until the introduction of MALDI-TOF MS (matrix-assisted laser desorption ionization-time of flight mass spectrometry)).¹⁰ The European¹¹ and American guidelines¹² on the management of IE recommend monotherapy with ceftriaxone in those caused by the viridans Streptococcus group susceptible to penicillin (MIC ≤ 0.125 mg/l), which is why this therapeutic scheme could have been an option in the case presented.

From the present clinical case, as well as from previous reviews on the subject, we believe that IE should be considered in bactaeremia due to A. urinae —especially in patients with diseases of the urinary tract - and the association of aminoglycosides with a beta-lactam for its treatment be individually assessed.

Conflicts of interest

The authors declare that they have no conflicts of interest.