Acinetobacter baumannii complex is a group of nosocomial pathogens that have emerged as a devastating public health threat in healthcare settings, and particularly in ICUs, where it is widely distributed and can colonize human mucosal surfaces and invade the bloodstream in critically ill patients with impaired immune function. There are few treatment options and infections caused by MDRABC and CRABC can lead to higher mortality . Several studies have suggested that respiratory tract colonization or infection is a risk factor for ABC bacteremia , but few have compared the clinical characteristics of pneumonia- and non-pneumonia-related ABC bacteremia . This 12-year retrospective single-center study examined long-term changes in incidence and antibiotic resistance among patients with ABC bacteremia in an ICU in eastern China, compared the differences between pneumonia- and non-pneumonia-related ABC bacteremia, and identify possible independent risk factors for 30-day mortality.
A nationwide prospective cohort study conducted from 2007 to 2016 in 16 teaching hospitals across China suggested that A. baumannii was one of the top four pathogens responsible for bacteremia, accounting for approximately 7.03% of bloodstream bacterial isolates . Data from the China Antimicrobial Surveillance Network (CHINET) revealed significant increases in the rates of resistance to carbapenem antibiotics, which ranged from 31% in 2005 to 79.2% in 2018 . A recent study revealed that the frequency of ABC bacteremia increased significantly in the ICUs in eastern China during 2009–2018, as did the resistance rate to carbapenem . In this study, the resistance rate to imipenem was 95.7% in the years 2017–2018, which is higher than in the CHINET data. We analyzed the clinical data of patients with ABC bacteremia during the past 12 years. Carbapenem-sensitive strains accounted for only 9.0% of the total number of cases and the proportion of CR strains was 100% in 2017, 2019, and 2020. There was a sudden drop in 2018 and the possible cause was that we performed strict rectification after a hospital sense event. However, the frequency of ABC bacteremia decreased recently (2015–2020), contrary to previous research. In our hospital in 2020, ABC had fallen out of the top 10 pathogens for bacteremia, suggesting that bacterial epidemiological studies in local hospitals are even more important.
ABC is commonly isolated from intubated patients in ICUs; in this study, lower respiratory tract infections were the most common source of ABC bacteremia acquired in the ICU. The reported mortality rate was higher in cases in which the respiratory tract was the source of bacteremia . Although many studies have reported risk factors for MDR and CR acquisition in ABC bacteremia [12, 13, 23], only Teng et al. compared pneumonia and non-pneumonia patients with ABC bacteremia . As shown in Table 1, our pneumonia-related group had a significantly higher rate of hypertension and significantly more CR strains, while the non-pneumonia-related group had a significantly higher rate of previous tigecycline use. Compared with the non-pneumonia-related group, patients with pneumonia-related ABC bacteremia had a higher 30-day mortality rate, which decreased the total hospital stay because of the high mortality. This finding differed slightly from that of Teng et al (2015). This might be because our study examined ICU patients, while most of their cases were from general wards. In addition, despite the lack of statistical significance, the patients with pneumonia-related ABC bacteremia in our study had a higher rate of septic shock and low lactate levels, implying that respiratory tract-colonized ABC can invade the blood.
Next, we analyzed patients with different prognoses. In univariate and multivariate analysis, immunosuppression and the APACHE II and SOFA scores at the time of bacteremia were independent risk factors for 30-day mortality in patients with ABC bacteremia. We also found that patients with pneumonia-related ABC bacteremia were more likely to have a poor prognosis, although it was not an independent risk factor for 30-day mortality on multivariate analysis. We reviewed the literature on ABC bacteremia in the last 10 years. In 2019, a systematic review and meta-analysis of 10 eligible studies of 923 patients with ABC bacteremia reported that risk factors for attributable mortality included neutropenia, chronic liver disease, chronic renal failure, steroid therapy, immunosuppressant use, septic shock, the severity of illness (as defined by the Pitt bacteremia score), and inappropriate empirical antimicrobial treatment . Three recent studies all found that a high Pitt bacteremia score was an independent risk factor for ABC bacteremia-related mortality [10, 12, 25]. Moreover, Zhou et al. and Park et al. showed that bacteremia occurring after the pneumonia was an independent risk factor for death, while the results of Gu et al. and our study countered this conclusion. Kim et al. and Yu et al. all showed that catheter-related infection and early colistin therapy were independent favorable prognostic factors associated with 28-day mortality in patients with CRAB bacteremia [14, 26]. Liat et al. found that, to be a protective factor, appropriate antibiotic therapy must be started within 48 hours . All of these studies had very small sample sizes, so larger studies are required to confirm our findings.
The effective management of sepsis and septic shock should focus on timely intervention, including removal of infection source, early initiation of appropriate antimicrobial therapy, fluid resuscitation, and resolution of organ dysfunction . CRAB bacteremia is resistant to the currently used antibiotics, except for tigecycline and polymyxin. Kim et al. found that early colistin therapy can reduce the mortality of septic shock patients with CRAB bacteremia . Among antibiotic strategies, Son et al. showed that colistin combined with tigecycline or other antibiotics was significantly associated with lower mortality after adjusting for confounding factors , which differed from Lee et al . For physicians who lack clinical experience, starting appropriate antimicrobial therapy at the time of bacteremia is very difficult because of bacterial resistance. We found no significant difference in appropriate treatment rates between survivors and those who died. We also found that immunosuppression and illness severity, as defined by the APACHE II and SOFA scores, were significantly associated with higher mortality. Therefore, the assessment of critically ill patients with ABC bacteremia should include host factors, particularly immune status, and the severity of disease. More studies are needed to clarify whether immunotherapy can improve patient outcomes.
Our study has some limitations. Its main limitation was the small number of ABC bacteremia patients, which decreased the power of our statistical analyses. Second, the study used a retrospective, observational, single-center design, potentially limiting the generalizability of our results to other hospitals. Further randomized controlled trials with larger sample sizes and multicenter designs are required. Third, since this study was retrospective, we could not determine whether the virulence of ABC strains changed significantly over time.