Enterococci species are part of the healthy human gut microbiota(12) and sometimes can also colonize in the urinary tract, causing different clinical settings from asymptomatic bacteriuria (AB) to UTIs. Bacteriuria caused by enterococci has a deleterious effect on the urinary tract, which promotes innate immune suppression and increases the risk of infection by other uropathogens(13), suggesting that the treatment of AB can be successful in terms of the prevention of the recurrence of urinary tract infection. Nevertheless, paradoxically, it has been shown that antibiotic treatment of AB caused by Enterococcus, in patients with recurrent infection, increases the risk for recurrent urinary infection by other uropathogens(14). Regarding UTIs, Enterococcus spp. is the cause of cystitis, prostatitis and epididymitis(1). Experimental studies in mice have shown that enterococci can also affect the higher urinary tract, thus causing pyelonephritis(15), which is one of the most frequent pathogens involved in complicated UTIs. In a review, enterococci was the cause in 7–25% of patients with severe urinary sepsis and UTIs(3). Despite these data, pyelonephritis caused by Enterococcus spp. is not usually associated with bacteremia(2, 4)
In this sense, empiric treatment against enterococci is not included in guidelines of UTIs management or is reserved only for patients at risk (6–8). Thus, the first aim of this work was to determine the risk factors associated with UTIs caused by enterococci. Although there are several studies that have analyzed the risk factors for bacteremia, there have been few studies that have focused on urinary infections and how these risk factors affect infection. Therefore, we reviewed the records from more than one hundred in-patient cases from different hospitals in Castilla y Leon (region of northwestern Spain) with a diagnosis of UTIs caused by Enterococcus, half of whom with positive blood cultures and the rest with negative blood cultures. In our cohort of patients, the main species that was detected was E. faecalis, whereas only fifteen percent of cases were due to E. faecium. This distribution, with a predominance of E. faecalis, also have been found in both a European(15) and North-American series of complicated UTIs cases(16).
Regarding demographic characteristics, male sex has been associated with Enterococcus spp. Thus, in a cohort of 700 patients with bacteremia, which included more than 180 patients with a urinary focus, there were twice as many males as females(15). Our results also support a predominance for male sex in complicated UTI cases, with or without bacteremia. In our work, nearly half of the cases of UTI caused by Enterococcus were acquired in a hospital. In a previous series of enterococcus infections, other authors have shown similar results(15) and have supported the fact that Enterococcus spp. must be considered, similar to Pseudomonas aeruginosa, especially in cases of nosocomial UTIs. Enterococcus spp. are frequently associated with biofilms of both blood and urinary catheters. In a recent study, patients with an indwelling catheter had twice the risk for Enterococcus spp. than other patients with community-acquired UTIs(17). In our cohort, the patients frequently had bladder, nephrostomy or double J catheters. It is possible, as is the case in other works, that the presence of a nephrostomy catheter is specifically involved in Enterococcus spp infection(18).
The population in this study had a high comorbidity Charlson index. Among the different comorbidities demonstrated by the patients, the most important were urinary disorders or severe systemic diseases. Among urological diseases, urological cancer has been described as a risk factor for Enterococcus UTIs(19, 20). It is possible, as is the case in bacteremia associated with colorectal cancer(20), that lesions due to urinary epithelial colonization can be one of the most important predictive risk for urinary infections caused by Enterococcus.
Enterococcus species are considered by many authors an opportunistic pathogen(21), and factors such as immunosuppression, neutropenia or hemopathy are frequently detected as risk factors for bacteremia(16). The results of our work suggest that in this epidemiological setting, Enterococcus can be considered one of the most probable causes of UTI.
Finally, another risk factor detected in our work was previous antibiotic treatment. This finding is in accordance with other works in which patients treated with ceftriaxone were associated with an increased risk for reinfection by Enterococcus spp(22). The fact that more than one-half of our patients had been treated in the previous month with 3rd -generation cephalosporins may explain these results.
Another aim of this study was to evaluate the variables associated with bacteremia. Although there are several studies that have analyzed bacteremia caused by enterococcus and its risk factors, they included different types of infections, and the results were not focused on UTIs(16, 20). In our study, we selected only patients with a clinical setting of UTIs and an isolation of enterococcus in urine culture, and we compared the risk factors in the two groups with and without bacteremia. Our results are consistent with other findings, and we showed that neutropenia, solid organ transplant, bone medullary transplant, and immunosuppression were also risk factors for bacteremia in patients with tract urinary infection caused by Enterococcus spp.(16, 20). These are the same classic risk factors in patients with bacteremia, regardless of other origins(4). Moreover, we detected that urinary cancer was associated with bacteremia in patients with UTIs. In a recent study, urological cancer was also associated with bacteremia(20). These results support that regardless of E. faecalis and E. faecium colonizing urinary tract infections, it is difficult for these pathogens to cross the urinary epithelium when innate immunity is preserved, but when it exists, rupture of the epithelial barrier can result in bacteremia.
Another objective of this work was to study the mortality and morbidity following UTIs caused by Enterococcus. The mortality rates seen with this infection in our study were higher than those in other series of in-patients with UTIs(23, 24) and were similar to those shown by Billinton et al.(16) in a subgroup of patients with urosepsis caused by Enterococcus. In our study, mortality was not different between patients with or without bacteremia. Factors that depend on the clinical setting, such as a high SOFA score and severe comorbidity (such as malignant hemopathy and immunosuppression), and factors that depend on the species involved (E. faecium vs E. faecalis or its pattern of resistance to ampicillin and vancomycin) were the only factors associated with high mortality. Similar data have been reported by other authors regarding bacteremia(15). We did not detect a lower mortality rate for patients with correct empiric treatment for Enterococcus spp. This result could be due to the small number of patients included in the study and to the heterogenicity in the empirical treatments performed. However, because our study was retrospective, it is also possible that there was a selection bias, so patients with optimal empirical treatment according to the last antibiogram could also be the most critical.
Finally, we evaluated infectious endocarditis, a frequent morbidity detected in patients with bacteremia caused by Enterococcus spp. In a Danish cohort, 6% of patients with nosocomial infections and 25% of patients with community-acquired infections had infectious endocarditis. In our study, we found endocarditis in 5.8% of patients with UTIs and bacteremia with a urinary origin (2.8% of the total patients included in the study). These data support that in patients with bacteremia and a urinary focus, echocardiography must be performed to diagnose this severe complication(15)
We believe that our work can help to better define a group of patients with urinary infection that have a high mortality, thus requiring a differentiated antimicrobial treatment from the one included in the current guidelines. Nevertheless, we also recognize that our work has some limitations. It was a retrospective study of in-patients, with a relatively low number of patients and with selection bias, so risk factors or mortality may not be similar in other populations of patients.