Study population
Of the 288 patients with SM positive culture in the hematology department during the study period, 55 (19.1%) with SM bacteremia were included (Figure 1). The median age was 51 (range 12-72) years old and 56.4% were male in our study population. The clinical characteristics of this cohort were shown in Table 1. The most common underlying diseases diagnosed when admitted to hospital were acute myeloid leukemia (AML) (31 cases, 56.4%), followed by acute lymphoblastic leukemia (ALL) (12 cases, 21.8%), non-Hodgkin's lymphoma (NHL) (6 cases, 10.6%), aplastic anemia (AA) (3 cases, 5.5%), hemophagocytic syndrome (HLH) (2 cases, 3.6%) and myelodysplastic syndrome (MDS)(1 case, 1.8%). Thirteen patients (23.6%) received HSCT. In addition to SM, 11 patients (20%) were also concomitant with other microbial bacteremia, included Klebsiella pneumonia in 3 cases, Escherichia coli in 3 cases, Staphylococcus haemolyticus in 2 cases, Pseudomonas cepacia in 2 cases, Staphylococcus epidermidis in 1 case, and Enterococcus faecium in 1 case. Fifty-two (94.5%) patients with SM bacteremia had thrombocytopenia and 50 (90%) patients’ platelet counts were less than 50,000/uL. Bleeding was one of the most common clinical manifestations in this study. There were 30 (54.5%) patients with hemoptysis, 11 (20.0%) patients with gastrointestinal bleeding and 5 (9.1%) patients with intracranial hemorrhage. Hemorrhagic pneumonia was confirmed in 27(49.1%) patients. However, SM was only identified in 14 (25.5%) patients from sputum culture simultaneously.
The cohort was categorized into hemorrhagic pneumonia group and non-hemorrhagic pneumonia group (27 cases and 28 cases, respectively). The comparisons of clinical characteristics between the two groups were listed in Table 1. The median age was 56 years old in the hemorrhagic pneumonia group, which was older than the 48.5 years observed in the non-hemorrhagic pneumonia group (P=0.049). There were more patients (40.7%) isolated SM from sputum in the hemorrhagic pneumonia group. The proportions of neutropenia and elevated procalcitonin were significantly higher in the hemorrhagic pneumonia group than the non-hemorrhagic pneumonia group (P=0.016 and P=0.003, respectively). There were no significant differences for patients’ gender, polymicrobial bacteremia, anemia, platelet count, coagulation function, underlying hematologic diseases, C-reactive protein level, inadequate initial antimicrobial treatment and prior courses of chemotherapy between the two groups.
Mortality
The overall mortality was 37(67.3%), and all patients died within 30 days after SM bacteremia onset. It was noteworthy that 20 (36.4%) patients died before obtained the positive blood culture reports, and all these patients had hemorrhagic pneumonia. The 30-day mortality rate was significantly higher in patients with hemorrhagic pneumonia than those without hemorrhagic pneumonia (85.2% and 50.0%, respectively, P=0.003), showed in Figure 2a. In this study, 35 (63.6%) patients received inadequate initial antimicrobial treatment. The 30-day mortality rate was 77.1% in patients who received inadequate initial antimicrobial treatment, which was significantly higher than that in patients treated with adequate initial antimicrobial (50.0%, P=0.040), showed in Figure 2b.
Previous antimicrobial therapy within 1 month before BSI onset
All patients in this study were administrated with antibiotics before SM bacteremia onset, either due to a previous other infection or as an empirical therapy for unknown origin infection (Table 2). The majority of patients received carbapenems (46 cases, 83.6%) and/or antifungal drugs (45 cases, 81.8%). But there were only a few patients who had treated with TMP-SMX (3 cases, 5.5%). It was to be observed that a total of 23 (41.8%) cases received tigecycline, and tigecycline was more frequently prescribed in the hemorrhagic pneumonia group (59.3% and 25%, respectively, P=0.010).
In vitro antimicrobial susceptibility
The antimicrobial susceptibilities of SM isolate in vitro are shown in Figure 3. All patients were tested for TMP-SMX sensitivity, with a sensitivity rate of 89.09%. Susceptibility to levofloxacin, minocycline, cefoperazone/sulbactam was 83.3% (40/48), 100% (46/46) and 77.8% (35/45), respectively. There was no significant difference in susceptibility to four antibiotics mentioned above between the hemorrhagic pneumonia group and the non-hemorrhagic pneumonia group. In the present study, only six cases performed susceptibility tests to tigecycline, and the susceptibility is 83.3% (5/6).
Risk factors associated with hemorrhagic pneumonia
For patients with SM bacteremia, once hemorrhagic pneumonia develops, the patients’ conditions always rapidly deteriorate and nearly all these patients die in a very short time. Univariate analysis showed that neutropenia, high levels of procalcitonin (>0.5ug/L) and prior tigecycline therapy within 1 month were significant risk factors for hemorrhagic pneumonia in SM bacteremia patients. In multivariate analysis showed in Table 3, independent risk factors associated with hemorrhagic pneumonia were neutropenia (HR=4.965, 95% CI [1.010; 24.403)], P=0.049), high levels of procalcitonin (HR=11.241, 95% CI [1.825; 69.241], P=0.009), prior tigecycline therapy within 1 month (HR=4.465, 95% CI [1.108; 18.000], P=0.035).