In human infections, the clinical course and outcome are strongly dependent on the characteristics of the infecting microorganism and patient’s conditions. In the case of bacteria, despite the indisputable role of bacterial resistance, this does not seem to be the only property influencing the outcomes; this holds true for PD-related peritonitis. Previous publications by our group showed a high arsenal of virulence factors among Staphylococcus aureus lineages, some of which are associated with worse PD-related peritonitis outcome, in spite their low resistance rate to methicillin [28,29].
NF-GNB present both, a high antimicrobial resistance rate and a high production of virulence factors, such as biofilm, as confirmed in the present series, which potentially explains the low observed resolution rate. In addition, other virulence factors are present in NF-GNB , particularly Pseudomonas species, such as: alginate, which is associated with bacterial adhesion; exoenzyme S, an inhibitor of protein synthesis; hemolytic phospholipase C, which is associated with the destruction of cell membranes and osmoprotection; exotoxin A, which is associated with tissue destruction and inhibition of the macrophage response; alkaline protease, which is associated with tissue damage and inactivation of IgG; elastase, an immunoglobulin degradation factor; and ramnolipids, which are associated with bacterial adhesion [16-19, 30-32].
These findings corroborate the aggressive character of these bacteria and explain, at least partially, the findings of this and of the two largest series, which previously described peritonitis caused by Pseudomonas species, i.e., the most frequent etiology of peritonitis among NF-GNB. Silva et al. studied 191 episodes of col peritonitis that occurred in Australian patients reported high rates of catheter removal (44%), permanent hemodialysis transfer (35%), hospitalization (96%), and change to a second antibiotic (66%). Lu et al. [12] reviewed 153 episodes of peritonitis caused by Pseudomonas species in Hong Kong, reporting overall primary response rate was 53.6% and complete cure rate was 42.4%. Interestingly, this study showed a decrease in the incidence of germs resistant to ceftazidime and gentamicin over time, which suggests that other factors, may have influenced the outcome.
We showed that concomitant ESI and resistance to amikacin were independently associated with non-resolution, in agreement to found by Krishnan et al [33]. The high rate of this infection (25%) in our sample, can at least partly explain the observed low-resolution rate. Regarding bacterial resistance, a considerable proportion of lineages was resistant to the tested antimicrobials. However, this does not occurred in episodes caused by Pseudomonas species, over 80% of which were susceptible to the frequently used antimicrobials as amikacin and ceftazidime, which reinforces the influence of other factors on the outcome. Even so, the resolution rate of these episodes was just over 30%, like observed with peritonitis by other BF-GNB.
Biofilm production is another factor reported as a determinant of the unfavorable response to Pseudomonas infections in PD [11]. We emphasize that antibiotic susceptibility is based on the MIC of the drug for planktonic cells, which are more sensitive to antimicrobials than cells wrapped in biofilms [34]. We identifies a high proportion of biofilm-producing isolates (75%); even though moderate or strong biofilm production were associated with non-resolution in in the univariate analysis, this association was not significant in the multiple regression model. However, this result does not rule out the possibility that its pathogenic action, in concert with other virulence factors, may influence the outcome.
Notably, we observed a greater chance of non-resolution in episodes initially treated with cefazolin plus ceftazidime, in contrast to other treatments. However, this association did not reach statistical significance, likely due to the small number of treated cases (nine). Such a finding, although inconsistent from a statistical point of view, agrees with the ISPD guideline [23], which recommend the use of two antipseudomonal drugs for peritonitis caused by species of Pseudomonas and Stenotrophomonas.
A significant number of NF-GNB-induced peritonitis cases involved Acinetobacter species. Of these, 57.1% were due to Acinetobacter baumannii, similar to found by Chao et al. [35], although Li et al. [13] reported higher frequencies. This series confirms that Acinetobacter baumannii is resistant to several antimicrobials, except for imipenem, and that this is a major therapeutic challenge.
Other identified germs, such as Achromobacter species, lineages of the Burkholderia cepacia complex, and Burkholderia gladioli, have rarely been described as etiologies of PD-related peritonitis [36,37]. The precise identification of NF-GNB is a challenge for conventional microbiology due to the phenotypic similarity and taxonomic complexity of these agents. Phenotypic tests based on morphology and biochemical characteristics often provide erroneous identifications of these species [38]. In our study, such limitations were minimized with the identification of the isolates by the MALDI-TOF technique, which is used in clinical microbiology to identify bacterial species based on the microorganisms’ protein profiles. This identification technique was mentioned in the most recent ISPD guideline on PD-related peritonitis, although at the time of its publication there was insufficient evidence for its recommendation [23].
Our study has several limitations, the most important being the small sample size, aggravated by the impossibility of recovering about 20% of the isolates. However, this is a study of NF-GNB-induced peritonitis as a whole, and therefore allows comparisons between peritonitis episodes due to Pseudomonas species and those due to other NF-GNB. In addition, to our knowledge, this is the first study to address the role of biofilm production in the outcomes of NF-GNB-induced PD-related peritonitis. In addition, it revealed novel information about pathogens that cause peritonitis, including those of the genus Achromobacter, which suggests that there is some benefit to using new techniques, e.g., MALDI-TOF, to identify bacteria in peritonitis.
Finally, the prevalence of PD-related NF-GNB in our center was similar to that of the Brazilian PD cohort, the largest Latin American cohort of incident PD patients, and again highlighted the severity of these infections.