Background

Bacterial infective endocarditis (IE) is associated with increased morbidity and mortality and has become one of the leading life-threatening infection syndromes [1]. IE caused by Proteus mirabilis is exceedingly rare, and only a limited number of endocarditis due to Proteus spp. have been documented in the literature [2,3,4]. Due to this scarcity, there is a lack of definitive therapeutic guidelines of IE due to P. mirabilis [1, 2]. Documented treatment of Proteus endocarditis thus far has involved a prolonged course of antibiotics with up to half of patients requiring surgical intervention [5, 6]. Of the existing literature on IE due to Proteus spp., immunologic phenomena have not been frequently reported or well characterized [2]. Therefore, we present a unique case of mitral valve endocarditis caused by PM that highlights the potential role of immunosuppression in this uncommon presentation of IE.

Case presentation

A 65-year-old female with a history of rheumatoid arthritis on chronic prednisone and scheduled Tofacitinib ER 11 mg q daily, factor V Leiden hypercoagulability, prior saddle pulmonary embolism, and no known valvular heart disease presented to the emergency department following a mechanical fall. On admission, she had a complaint of generalized fatigue. She had a temperature of 36.7 °C, heart rate (HR) of 97 beats per minute and a blood pressure (BP) of 155/86 with a normal lactate level. Her lungs were clear and there was no appreciable heart murmur noted. The remainder of the physical exam was normal.

Her white blood cell count was 11,370/µL. Within 1 day of admission, patient had temperature of 40 °C, HR 104 bpm, BP dropping to 103/67 mmHg and white blood cell count increased to 18,480/µL. An abdominal/pelvic CT scan showed evidence of acute/subacute splenic emboli, with wedge-shaped zones of hypoattenuation in the inferior and superior aspects of the spleen (Fig. 1) and a 4 mm ureteral stone without hydronephrosis. Her urinalysis had bacteria, nitrite, and leukocyte esterase with only 5 WBCs. She was admitted to the general medical floor with a diagnosis of cUTI and was started on piperacillin/tazobactam 3.375 g every 8 h as an extended infusion. A transthoracic echocardiography revealed a 1.2 × 0.5 cm mobile echogenic density on the anterior leaflet of the MV (Fig. 2) with moderate mitral regurgitation (Fig. 3). Urine and blood cultures collected on the day of admission grew a pan sensitive strain of P. mirabilis (Table 1). Antibiotics were changed to ceftriaxone (2 g IV every 24 h) and gentamicin 5 mg/kg/day divided q 8 h. On hospital day 3, the patient developed respiratory distress and hypoxemia due to acute pulmonary edema on CXR (Fig. 4) requiring non-invasive positive pressure ventilation and was transferred to the intensive care unit. She became hypotensive and required intravenous nor-epinephrine. She developed new onset atrial fibrillation with a rapid ventricular response. Follow-up blood cultures continued to grow out P. mirabilis. Her white blood cell count peaked at 41,000 on hospital day 4. She subsequently required a second vasoactive agent, neosynephrine. Repeat echocardiography revealed an enlarging non-mobile vegetation 1.1 × 0.8 cm attached to the atrial side of the anterior MV leaflet. The mitral regurgitation was now severe with flow reversal into the pulmonary veins.

Fig. 1
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Abdominal/pelvic computed tomography scan showed evidence of acute/subacute splenic emboli

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Fig. 2
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Transthoracic echocardiography revealed a 1.2 × 0.5 cm mobile echogenic density on the anterior leaflet of the mitral valve

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Fig. 3
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Transthoracic echocardiography revealed at least moderate mitral regurgitation

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Table 1 Antimicrobial susceptibility test of the isolated Proteus mirabilis
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Fig. 4
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Chest X-ray revealed acute pulmonary edema

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The patient underwent cystoscopy for left ureteral stent placement for the noted stone. She continued to have mixed cardiogenic and septic shock secondary to P. mirabilis. bacteremia and MV endocarditis, now requiring continuous infusions of inotropic and two different vaso-active agents. A pulmonary artery catheter was placed to guide management (Fig. 5). On hospital day 7, she underwent surgical MV replacement with a 29 mm magnaease valve. Intra-op transesophageal echo revealed a well-seeded bioprosthetic mitral valve with no mitral regurgitation.

Fig. 5
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A pulmonary artery catheter was placed to guide management

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Repeat blood cultures negative for P. mirabilis. Continuous vasoactive agents and inotropes were tapered to off. Cardiac valve pathology confirmed acute MV endocarditis and intraoperative (Fig. 6). Ceftriaxone (2 g IV every 24 h) was given for 6 weeks post procedure and gentamicin was discontinued. The patient was discharged home post op day 5.

Fig. 6
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Pathologic specimen confirming acute mitral valve endocarditis

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Discussion and conclusions

Proteus mirabilis is Gram negative rod-shaped bacterium, member of the order Enterobacteriales, family Enterobacteriaceae, remains to be one of the common causes for complicated urinary tract infections consisting of flagellae and swarm cell differentiation contributing to its ability to cause ascending UTI and bacteremia. P. mirabilis is known to be producing urease, making urine alkaline from conversion of urea to ammonia and subsequently ammonium by consuming free hydrogen ion. Alkaline urine, in turn, facilitates struvite stones formation consisting of phosphate, carbonate and magnesium and subsequent vicious cycle of leukocytes, struvite, proteinaceous matrix & bacteria forms nidus for infection as infected staghorn calculus [7,8,9,10,11].

Cases of P. mirabilis endocarditis has rarely been reported in the current literature, with only 16 reports of IE caused by any Proteus species presented with high mortality of 43.8% [2]. In a large study of IE, only 3 out of 2761 (0.1%) definite cases were due to Proteus species [12]. While few cases have been documented, most patients with Proteus endocarditis presented with notably severe disease with high mortality [13,14,15,16]. The most recent incidence of native valve Proteus endocarditis was documented in 2016, in which the patient presented with P. mirabilis endocarditis of the aortic valve and was successfully treated with a dual antibiotic regimen of ceftriaxone and gentamicin [3]. Previously, two cases of Proteus endocarditis of native mitral valves were successfully treated without surgical intervention [17, 18].

Similar to factors found to be associated with non-HACEK Gram-negative bacillus (GNB) endocarditis, endocarditis due to enteric bacilli other than Salmonellae, and other previously reported cases of Proteus endocarditis, our patient presented with urinary tract infection (UTI) [3, 13, 19]. In a recent systematic review of Proteus endocarditis, 43.8% of patients had cocontaminant UTI [2]. Therefore, in retrospect, Proteus urosepsis was likely due to a 4 mm ureteral stone in our patient.

Interestingly, while embolic phenomena were frequently reported in cases of Proteus IE, only one other case reported splenic infarctions similarly seen in our patient [2, 18, 20]. Additionally, only 2 out of 14 (14.3%) documented cases of endocarditis due to any Proteus species reported immunologic involvement [2]. We hypothesize that our patient’s immunocompromised status following steroid and Janus Kinase inhibitor usage for rheumatoid arthritis contributed to Gram-negative bacteremia following Proteus urinary tract infection, ultimately seeding the native MV. Additional studies with larger numbers of Proteus endocarditis cases are needed to investigate an association between immunosuppression and P. mirabilis endocarditis. Overall, we summarize a case of P. mirabilis cUTI accentuating into persistent bacteremia and IE due to immunocompromised status of the host and yet being successfully managed by early surgical intervention and extended single antibiotic regimen.