Pseudomonas mendocina native valve infective endocarditis: a case report

The genus Pseudomonas comprises at least 140 species [4]. Among them, Pseudomonas aeruginosa possesses an abundance of virulence factors and is regarded as the most important pathogen responsible for serious life-threatening infections in humans [4]. P. mendocina, a non-aeruginosa Pseudomonas species, rarely causes human infection.

P. mendocina, like other Pseudomonas species, is commonly found in soil, water, plants, and animals. A comprehensive review of the literature was performed by searching PubMed® and Google Scholar® using the terms “Pseudomonas mendocina infection” and “Pseudomonas mendocina endocarditis”. As shown in Table 2, the case reported here is the ninth reported case of human P. mendocina infection, but the first reported case in the USA [3, 5–11]. One case report was published in French and is included in this review [8]. All eight published cases are outside North America: three cases from Asia (Taiwan, Singapore), two cases from Europe (Denmark, France), two from the Middle East (Turkey, Israel), and one from Argentina. Of the eight reported cases, five were bacteremic [3, 5, 7–9]. The remaining three cases were non-bacteremic, and associated with bone and soft tissue infections [6, 10, 11]. Among the five patients with P. mendocina bacteremia, four cases (80 %) involved infective endocarditis [3, 5, 7, 8]. The affected cardiac valves were native in all four cases of infective endocarditis. The affected heart valves were predominantly left-sided. Valvular repair or replacement was required in two cases. The mechanism of predilection of valvular endocarditis by this pathogen is obscure. No mortality associated with the P. mendocina infection was reported.

The source of P. mendocina infection was not identified in most reported cases. Nseir and colleague’s speculation on the source of infection is fascinating [9]. They found that the patient had a peculiar habit of allowing a bird to drink water from his mouth. P. mendocina was isolated both from the tap water and the bird’s drinking water. In our case, our patient had already received intravenous antibiotics for 3 days when his leg wound cultures were obtained. His skin tissue cultures failed to grow P. mendocina because of the inhibitory effect of antibiotics on the bacterial growth. Thus, we postulated that the leg wound ulcers were the likely nidus of the infection.

Because P. mendocina is not a common human pathogen, little knowledge exists about the antimicrobial resistance patterns of this organism. This pathogen was reportedly susceptible to ampicillin in some reported cases [5, 7]. However, ampicillin or ampicillin-sulbactam is not tested in the antibiotic susceptibility of non-aeruginosa Pseudomonas species by the US Clinical and Laboratory Standards Institute (CLSI) [12]. The third-generation cephalosporins (cefotaxime, ceftriaxone, cefoperazone, ceftizoxime) are included in the susceptibility testing and recommended as a treatment of choice if susceptible [4, 12]. Other traditional “anti-Pseudomonas aeruginosa” antibiotics, including ceftazidime, cefepime, piperacillin-tazobactam, aminoglycosides, carbapenems, and ciprofloxacin, show an excellent activity against most strains of P. mendocina [4, 12]. In one case report, Mert et al. reported that their isolate of P. mendocina was susceptible to all tested antibiotics (ampicillin, ceftazidime, cefepime, piperacillin-tazobactam, imipenem, and gentamicin) [7]. Aragone et al. found ampicillin and cephalothin resistance in their isolate [3]. Resistance to ampicillin and ampicillin-sulbactam was demonstrated in the isolates from a foot wound by Chiu and Wang [11]. In a case of spondylodiscitis, the isolated P. mendocina pathogen from deep spinal tissue was only resistant to trimethoprim-sulfamethoxazole [6]. Nseir et al. identified ceftriaxone and aztreonam resistance in their isolate [9]. The other cases in Table 2 failed to report antibiotic resistances of their P. mendocina isolate. The present isolate in our patient was resistant to ampicillin-sulbactam and first-generation cephalosporins. While these isolates indicate a low level of antibiotic resistance in this organism, the minimal numbers of cases preclude an in-depth analysis of antibiotic resistance trends in this organism.