Asymptomatic Bacteriuria in Kidney Transplant Recipients—A Narrative Review

Urinary tract infections (UTIs) are the most prevalent complications in kidney transplant (KTx) recipients. The most frequent finding in this group of patients is asymptomatic bacteriuria (ASB). Here, we provide an overview of the available evidence regarding ASB in KTx recipients, including its etiopathology, clinical impact and management. There is a growing body of evidence from clinical trials that screening for and treating ASB is not beneficial in most KTx recipients. However, there are insufficient data to recommend or discourage the use of a “screen-and-treat strategy” for ASB during the first 1–2 months post-transplant or in the case of an indwelling urinary catheter. Despite its frequency, ASB after KTx is still an understudied phenomenon.


Introduction
Urinary tract infections (UTIs) are the most prevalent complications in kidney transplant (KTx) recipients [1][2][3][4][5]. The clinical manifestation or phenotype of symptomatic infections may vary from cystitis to urosepsis. However, a frequent finding in this group of patients is asymptomatic bacteriuria (ASB) [6][7][8][9], which is nowadays considered to be more a form of colonization than infection. The reported prevalence of ASB varies greatly depending on the study, population enrolled, time elapsed since KTx, use of antibiotic prophylaxis and length of follow-up from 3% to 50% [8,10]. ASB is the most frequent clinical phenotype in the first month after KTx, after which symptomatic infections predominate and the relative proportion of ASB decreases [8,11].

Definitions
As stated in the guidelines of the Infectious Diseases Society of America (IDSA), ASB is defined in patients without indwelling catheters as the presence of ≥10 5 colony-forming units (CFU)/mL (≥10 8 CFU/L) in a voided urine specimen, without coinciding signs or symptoms typical for UTI [12][13][14][15]. For men, a single urine specimen is sufficient for diagnosis, while in women two consecutive specimens should be obtained to confirm the persistence of bacteriuria [16]. This is because in up to 60% of women bacteriuria is transient, and is not present on repeat screening after an initial positive specimen [17,18]. In patients with indwelling devices, the presence of ≥ 10 5 CFU/mL is still the best diagnostic criterion for bladder bacteriuria [19]. Lower quantitative counts of bacteria (≥10 2 to <10 5 CFU/mL) are likely to be a contamination of the urine specimen from the biofilm covering the device rather than true bacteriuria. The presence of from ≥10 2 to <10 5 CFU/mL of bacteria in In another work, K. pneumoniae NDM-1 strains were isolated from KTx recipients, some of them suffered due to UTI and others had ASB or were only colonized. The genetic background of bacteria was examined using WGS (whole genome sequencing). It was found that the strains had high genetic similarity and clonal origin. The profiles of virulence were similar too, regardless of whether they caused only colonization, ASB or UTI. The authors suggested that, for some patients, the innate immune response may not be strong enough to generate symptoms [34]. Asymptomatic colonization of the human bladder by ASB E. coli strain 83,972 has been shown to suppress selected disease-associated signaling pathways [35,36]. Heitmuller et al., using model insect host G. mellonella larvae infected with either uropathogenic or urocolonizing E. coli strains, showed strain-specific differences in the class and expression levels of genes encoding antimicrobial peptides, cytokines and enzymes controlling DNA methylation and histone acetylation [37]. Even though ASB strains are able to prevent the development of a systemic inflammatory reaction, they may still induce a local inflammatory response. In patients with low-grade, papillary, and non-muscle-invasive tumors and chronic ASB the number of recurrences and recurrent tumors was lower, and tumor-free survival times were longer than similarly staged uninfected patients. Therefore, it has been hypothesized that urocolonizing strains may reduce bladder tumor recurrence by activating local immune mechanisms [38]. In KTx recipients, the urine IL-8 level was elevated in both patients with symptomatic UTI and in cases of asymptomatic bacteriuria in comparison to the control group, most probably indicating an ongoing occult inflammatory process in the urinary tract [39].

Predisposing Factors
Many pre-, peri-and post-transplant factors are believed to increase the risk of UTIs, including ASB, in KTx recipients. These include female gender, diabetes mellitus, immunosuppression and kidney allograft dysfunction or rejection, underlying urinary tract abnormalities and instrumentation of the urinary tract, including ureteral stent placement and prolonged urinary catheterization. So far, the dose or type of maintenance immunosuppression has not been found to impact the risk of UTIs. Various authors have suggested different potential ASB risk factors. The only consistent finding was the increased risk of ASB in females [8,10,40]. In a study by Coussement et al., female KTx recipients over the age of 50 were the population at highest risk of having ASB [10]. In an analysis of 2363 UTI events recorded in 2368 KTx recipients, including 1111 episodes of ASB, the 1-year incidence of ASB was also significantly higher in females [11]. Notably, an association between the Charlson Comorbidity Index (CCI) [41] and risk of ASB was suggested in one of the studies. While the authors did not find a statistically significant association between risk of ASB and any specific comorbid conditions, they showed that upon multivariate analysis CCI that takes into account a variety of medical conditions, how advanced they are, together with age, also predicting long-term mortality, was a predictor of ASB. [8].

Etiology
The etiology of ASB mirrors the etiology of symptomatic UTIs, with a predominance of Gram-negative rods typically colonizing the gastrointestinal tract such as E.coli or Klebsiella spp. It is noteworthy that strains isolated early post-KTx differ from those found in late periods after KTx. In a single center study by Gołębiewska et al., in the first month after KTx E. faecium predominated, followed by E. faecalis and E. coli. Beginning from the second month, E. coli was the most frequently isolated urinary pathogen, followed by Proteus species and Klebsiella species [8]. Brune et al., in a multicenter observational study, reported distinct pathogen profiles in ASB compared to UTIs, including urosepsis. Specifically, in ASB the proportion of Enterococcus species and coagulase-negative Staphylococci was significantly higher than in symptomatic infections. The pathogen profile in symptomatic infections included E. coli, Enterococcus species and Klebsiella species, which accounted for about 85% of all causative agents. Notably, the pathogen profiles in all UTI phenotypes remained stable over the first twelve months after KTx and were not affected by either the removal of the double J-stent or the use of prophylaxis with trimethoprimsulfametoxazole [11]. The frequency of the three most common etiological agents in ASB episodes according to various authors is presented in Figure 1.
was significantly higher than in symptomatic infections. The pathogen profile in symptomatic infections included E. coli, Enterococcus species and Klebsiella species, which accounted for about 85% of all causative agents. Notably, the pathogen profiles in all UTI phenotypes remained stable over the first twelve months after KTx and were not affected by either the removal of the double J-stent or the use of prophylaxis with trimethoprimsulfametoxazole [11]. The frequency of the three most common etiological agents in ASB episodes according to various authors is presented in Figure 1.

Clinical Impact of ASB
At least in theory avoiding UTI in KTx recipient might improve patient and kidney allograft survival. In KTx recipients microorganisms colonizing the lower urinary tract and causing ASB may constitute a hazard for upper urinary tract invasion and development of full-blown symptomatic infection, including urosepsis. In the setting of immunosuppressive therapy, the host's immune response may not offer a chance for a spontaneous recovery from even a benign infection, especially soon after KTx. The available data on the effect that UTIs have on long-term kidney allograft function are inconclusive. The true influence of various phenotypes of UTIs on both the recipient and kidney allograft outcome has so far not been established. The general belief is that asymptomatic bacteriuria (AB) is benign, in contrast to symptomatic upper UTIs, such as acute graft pyelonephritis or urosepsis. However, even if UTIs do not affect kidney allograft survival directly, they can pose a significant indirect risk by causing bacteraemia, contributing to, e.g., acute rejection.

Risk of Symptomatic UTI
All published studies analyzed the impact of ASB on the risk of symptomatic UTI development. In retrospective analyses, when all episodes of ASB were treated, ASB was a risk factor for the development of symptomatic UTI. Fiorante et al. showed that the incidence of acute graft pyelonephritis (AGPN) in KTx recipients was seven-fold higher in those with ASB compared with those without ASB. A history of recurrent ASB bacteriuria episodes (either two to five or more than five episodes) was a significant independent factor associated with AGPN [42]. The observations were similar in another retrospective [%] Percentage of isolated strains in ASB episodes Fiorante, 2010 Coussement, 2019 Ruiz-Ruigomez, 2021 Fontsere, 2021 Brune, 2022

Clinical Impact of ASB
At least in theory avoiding UTI in KTx recipient might improve patient and kidney allograft survival. In KTx recipients microorganisms colonizing the lower urinary tract and causing ASB may constitute a hazard for upper urinary tract invasion and development of full-blown symptomatic infection, including urosepsis. In the setting of immunosuppressive therapy, the host's immune response may not offer a chance for a spontaneous recovery from even a benign infection, especially soon after KTx. The available data on the effect that UTIs have on long-term kidney allograft function are inconclusive. The true influence of various phenotypes of UTIs on both the recipient and kidney allograft outcome has so far not been established. The general belief is that asymptomatic bacteriuria (AB) is benign, in contrast to symptomatic upper UTIs, such as acute graft pyelonephritis or urosepsis. However, even if UTIs do not affect kidney allograft survival directly, they can pose a significant indirect risk by causing bacteraemia, contributing to, e.g., acute rejection.

Risk of Symptomatic UTI
All published studies analyzed the impact of ASB on the risk of symptomatic UTI development. In retrospective analyses, when all episodes of ASB were treated, ASB was a risk factor for the development of symptomatic UTI. Fiorante et al. showed that the incidence of acute graft pyelonephritis (AGPN) in KTx recipients was seven-fold higher in those with ASB compared with those without ASB. A history of recurrent ASB bacteriuria episodes (either two to five or more than five episodes) was a significant independent factor associated with AGPN [42]. The observations were similar in another retrospective study that included ASB as an independent risk factor for symptomatic UTIs. However, the authors indicated that only a small proportion of symptomatic UTIs were preceded by ASB with the same pathogen and concluded that recurrent ASB episodes may be considered a risk factor, a marker of increased susceptibility to symptomatic infections or a result of repeated and prolonged exposure to antibiotics [8]. When a comparison was made between cases of treated and untreated ASB, it clearly indicated that the risk of developing a symptomatic UTI or a symptomatic UTI requiring hospital admission increased only in those who received antibiotic therapy [43,44]. In pediatric KTx recipients, most untreated ASB episodes resolved without sequelae and on the other hand, most cases of both lower and upper symptomatic UTIs were de novo infections, which developed without preceding ASB [45].
Studies that assessed the prevalence, outcomes and management of asymptomatic bacteriuria (ASB) in KTx recipients are outlined in Table 1.

Impact on Kidney Allograft Function
A number of studies evaluated the impact of ASB on kidney allograft function. In a study on 189 KTx recipients who were followed for 36 months, which included 96 KTx recipients who developed 298 episodes of ASB, no differences were found in serum creatinine, creatinine clearance or proteinuria between patients with and without ASB [42]. In another retrospective analysis, the evolution of kidney allograft function also did not differ significantly between recipients with and without different UTI phenotypes, including ASB. However, patients suffering from symptomatic UTIs had significantly worse kidney allograft function measured by eGFR from the baseline in comparison with those without UTIs. The eGFR of recipients with a history of only ASB did not differ significantly when compared to both patients with symptomatic infections and patients with no infections [8].
Other studies have confirmed that both one-year patient and graft survival were comparable in recipients with or without ASB [11,44,46]. Furthermore, the antibiotic treatment of ASB did not affect the serum creatinine concentrations at baseline or the end of the study, when compared with the no treatment group [43,[46][47][48][49].

Risk of Acute Rejection
A series of retrospective studies published in the literature have looked at the risk of acute rejection (AR) in KTx patients with ASB [8,40,42,44,46,50]. Fiorante et al. found that multiple ASB episodes were associated with rejection. However, the authors highlighted that not all the episodes of rejection were preceded by ASB episodes, so it was impossible to determine the causality of this relationship [42]. No other study has confirmed this finding regarding the risk for AR occurrence. Moreover, the antibiotic treatment of ASB had no effect on the risk of AR when compared to having no treatment [8,40,44,46,50].  I,  23 Type II, 9 Type III and 91 Type IV). Negative control cultures tended to be more frequent in treated Type I (p = 0.09) and in untreated Type II episodes (p = 0.08). A single-center retrospective study N/A TMP-SMX as PJP prophylaxis was not associated with reduced prevalence of ASB (HR = 1.52, 95% CI = 0.79-2.94, p = 0.213), reduced incidence of cystitis (HR = 2.21, 95% CI = 0.76-6.39, p = 0.144) or AGPN (HR = 1.12, 95% CI = 0.57-2.21, p = 0.751). A two-center retrospective study The decision whether to treat ASB episode was at the discretion of the treating physician Female gender was a risk factor for infection (p = 0.002), and the use of double-J ureteral stent was a risk factor for both ASB and symptomatic UTIs (p = 0.003). The likelihood of symptomatic UTI in the same organism was significantly higher (p = 0.002) after untreated ASB (n = 185) than in cases of treated ASB (n = 139).
Bonneric, 2019 [45] France 37 pediatric KTx recipients with 171 ASB episodes. Primary outcome was the cumulative incidence of AGPN or LUTI occurring between 2 and 24 months post-KTx. The secondary outcomes were AGPN or LUTI due to MDR bacteria, long-term GFR evolution and graft loss. The mean age of participants was 11.2 ± 5.2 years and 59% were male.
A single-center retrospective study No information provided on the treatment of ASB in the treated cases.
One hundred and sixty-four ASB episodes were untreated (95.9%) and 150 episodes (91.5%) were not followed by a symptomatic UTI. Ten episodes (6.1%) led to AGPN and 4 (2.4%) led to LUTI. There were 53 episodes of AGPN: 10 (18.9%) after untreated ASB and 43 (81.1%) de novo. There were 11 episodes of LUTI: 4 (36.4%) after untreated ASB and 7 (63.6%) de novo. There was no graft loss as a result of UTI. A single-center prospective observational study The decision to treat the ASB and the choice of the antimicrobial therapy was at the discretion of the treating physician.
The treatment of ASB increased the rates of microbiologic relapses and reinfections; treated ASB patients showed a trend towards developing symptomatic urinary tract infections in the following 6 months.

Management
The process of screening for and treating ASB is a common practice in many European kidney transplantation centers [56,57]. Seventy-two percent of European physicians involved in the care of KTx recipients always screen for ASB in their patients, while only 6% never treat ASB [57]. We fear that infections can rapidly progress in immunosuppressed patients and that it may be particularly difficult to distinguish between ASB and a true UTI in the setting of subtle symptoms mitigated by immunosuppression. Even in the general population, where guidelines are clear-cut, antimicrobials are often unnecessarily used to treat cases of ASB, especially in patients with poor functional status and men [58].
On the other hand, the concern for unnecessary treatment of ASB originates from the rapidly growing bacterial resistance and the perception of antibiotic stewardship as a public health priority [59]. UTIs caused by multidrug resistant Gram-negative bacteria are more and more common, especially in the population of solid organ transplant recipients, and are a growing concern due to limited treatment options [60,61].
On an individual level, the unnecessary exposure to antibiotics may be associated with potential side effects, negatively impacting one's microflora, and, therefore, leading to undesirable consequences such as increasing the risk of colonization or infection by multidrug-resistant bacteria, such as Clostridioides difficile infection, etc.
Whether antibiotic treatment of ASB is in fact helpful or harmful in preventing symptomatic infections in KTx recipients has not been fully elucidated so far. In a study by Cai et al. of a non-transplant population of young women, the treatment of ASB in patients affected by recurrent UTIs resulted in a higher rate of symptomatic UTIs [62]. There is a growing body of evidence from various types of studies showing that screening for and treating ASB is also not beneficial in most KTx recipients. Earlier retrospective studies showed that the incidence of symptomatic UTIs, including AGPN, was higher in KTx recipients with multiple, recurring episodes of ASB, either despite or because of the implemented antibiotic treatment [8,32,44]. At the same time, very few symptomatic UTIs were preceded by ASB with the same causative agent [8]. In a study by Green et al., the use of antibiotics in cases of ASB increased the risk of both symptomatic UTI and a hospital stay [43]. Interestingly, the chance of bacterial clearance was higher in KTx recipients with ASB who received no treatment when compared to those who were administered antibiotics [52]. The findings were similar in pediatric KTx recipients-91.5% of untreated cases of ASB were not followed by a symptomatic UTI and over 80% of cases of AGPN were not preceded by ASB [45]. Only a single study of 185 KTx recipients showed that untreated ASB was significantly more likely to be followed by a subsequent episode of symptomatic UTI in the same organism in comparison to cases of treated ASB [40]. In a recently published study by Ruiz-Ruigomez, which assessed the rate of microbiological eradication after treatment of ASB episodes with fosfomycin, the treatment failure rate exceeded 40% [55]. Moreover, the routine use of trimethoprim-sulfametoxazole prophylaxis did not reduce the prevalence of ASB [53].
To date, the results of four randomized controlled trials investigating the use of antibiotics for the treatment of ASB in KTx recipients have been published [47][48][49]54]. The results were consistent and showed no apparent benefit of systematic screening and treatment of ASB in the populations of patients beyond 2 months post-transplant. The use of antibiotics did not decrease the likelihood of developing a symptomatic UTI, but promoted the emergence of resistant organisms.
Interestingly, in a group of 420 KTx recipients, the isolates from preoperative urine, preoperative urethral swab and postoperative urinary catheter tip cultures also showed poor concordance with the causal organism during an early symptomatic UTI. Thus, perioperative microbiological screening had very limited or no value [63].
As most studies have failed to demonstrate any benefit of the "screen and treat strategy" in terms of incidence of symptomatic UTIs or long-term kidney allograft function, a paradigm shift toward a more conservative approach has occurred in the management of post-transplant ASB. Indeed, most of the more recent guidelines issued by, e.g., the Spanish Society of Clinical Microbiology and Infectious Diseases (SEIMC), the Infectious Diseases Society of America (IDSA), the American Society of Transplantation Infectious Diseases Community of Practice and the European Association of Urology have clearly recommended against screening for or treating ASB beyond the first month post-transplant (Table 2) [13,[64][65][66]. Unfortunately, a survey conducted among European transplant nephrologists involved in the care of adult KTx recipients indicated a huge discrepancy between existing guidelines and everyday clinical practice, with most physicians screening for and treating ASB in their patients, even beyond the second month post-KTx [57]. The proportion of different clinical manifestations of UTIs differs between early and late periods after KTx, and ASB episodes predominate in the first post-transplant month [8]. However, evidence concerning ASB occurring during the first post-transplant month or in KTx recipients with indwelling urinary catheters is missing, since these episodes were excluded from all published randomized clinical trials [47][48][49]54]. The net state of immunosuppression is highest during the first post-transplant months and decreases thereafter. The presence of indwelling urinary catheters and double-J stents and the highest rate of urologic interventions during this period may increase the risk of progression from ASB to symptomatic UTI, especially given the fact that symptoms of urinary tract infection may be impeded by graft denervation and inflammatory signs may be altered by immunosuppressive medications. Given the abovementioned differences, the recommendation to not screen for and treat ASB may not be applicable during months 1-2 post-transplant. Further studies are needed to evaluate the efficacy and safety of the "screen and treat strategy" in the early post-KTx period.

Conclusions
ASB is a common finding in KTx recipients, especially females. Its etiology mirrors the etiology of symptomatic UTIs, with a predominance of Gram-negative rods typically colonizing the gastrointestinal tract. It seems that ASB strains are able to prevent the development of the host's systemic inflammatory reaction, although they may still induce a local inflammatory response of unrecognized significance. Given that there is no rigorous molecular definition to differentiate between uropathogenic or merely urocolonizing strains, and that KTx recipients are immunocompromised and suffer from numerous urological malformations (vesicoureteral reflux is a permanent symptom after KTx, and KTx recipients are exposed to invasive diagnostic and therapeutic procedures involving the urinary tract, predisposing them to the rapid development of symptomatic, potentially severe infections), it is common practice to screen for and treat ASB episodes. However, there is a growing body of evidence from clinical trials that this strategy is not beneficial in patients more than 2 months post-transplant in terms of symptomatic UTI prevention. In addition, antibiotic treatment of ASB increases antibiotic use and, therefore, promotes the emergence of more resistant strains. As antibiotics for ASB are typically prescribed as a consequence of the positive result of a urine culture, efforts should be made to stop obtaining urine cultures in KTx recipients who have no signs or symptoms of UTI. There is also no clear-cut repercussion of ASB on kidney allograft function prognosis; thus, there is no rationale to support antibiotic use in ASB episodes in patients more than 2 months post-transplant.
Despite its frequency, ASB after KTx is still an understudied phenomenon, especially in recipients who are 1-2 months post-transplant or who have an indwelling urinary catheter. Further randomized studies are required to provide evidence on how a screen-and-treat strategy for ASB would affect clinical outcomes in the early period post-KTx.

Conflicts of Interest:
The authors declare no conflict of interest.