Bacterial membrane vesicles from Acinetobacter baumannii induced by ceftazidime are more virulent than those induced by imipenem

Abstract

Patients with Acinetobacter baumannii bacteremia treated with antipseudomonal cephalosporins showed higher 14-day mortality than patients treated with antipseudomonal carbapenems. We hypothesized that the bacterial membrane vesicles (BMVs) induced by antipseudomonal cephalosporins are more virulent than BMVs induced by antipseudomonal carbapenems.To simulate the clinical condition with inadequate antimicrobial treatment, carbapenem-resistant A. baumannii was treated with ceftazidime (an antipseudomonal cephalosporin) or imipenem (an antipseudomonal carbapenem) at 1/2 the minimum inhibitory concentration. BMVs and BMV-carried lipopolysaccharide were measured by nanoparticle tracking analysis and western blotting, respectively. Cytokine expression in RAW264.7 macrophages or mice serum induced by the BMVs was determined by ELISA, fluorescent bead-based immunoassay or western blotting. The virulence of the BMVs was assessed in mice. Liquid chromatography tandem-mass spectrometry was used to determine the protein contents of the BMVs.We found that ceftazidime induced a higher number of BMVs (CAZ-BMV), which carried more LPS, and induced higher expression levels of iNOS, IL-1β, and IL-6 in macrophages, higher expression of many cytokines in mice, more neutrophil infiltration in lung interstitium, and higher mortality in mice than imipenem-induced BMVs (IMP-BMV). When adjusted to same amount of LPS, CAZ-BMV still led to higher mortality than IMP-BMV. Proteomic analysis revealed different protein contents in CAZ-BMV and IMP-BMV. In conclusion, A. baumannii BMVs induced by ceftazidime are more virulent than BMVs induced by imipenem.

Keywords: Acinetobacter baumannii; Bacterial membrane vesicles; ceftazidime; imipenem; virulence.

Figure 1.

Cytokine expression in macrophages after treatment with bacterial membrane vesicles (BMVs) from antimicrobial-treated Acinetobacter baumannii. BMVs induced by ceftazidime (CAZ) treatment led to higher expression of IL-1β (a) and IL-6 (c) in murine RAW264.7 macrophages than BMVs induced by imipenem (IMP) treatment or BMVs from untreated A. baumannii (LB). BMVs from ceftazidime and imipenem-treated cultures induced similar levels of TNF-α (b), IL-12 (d), and IFN-ϒ (e) expression, which were similar to that induced by untreated supernatant (LB). RAW264.7 macrophages treated with LPS plus ATP were used as a positive control, and phosphate-buffered saline (PBS) were used as negative controls. * P < 0.05 between CAZ-BMV and IMP-BMV.

Figure 2.

IL-1β and TNF-α expression in macrophages after treatment with bacterial membrane vesicles (BMVs) and culture supernatants from antimicrobial-treated Acinetobacter baumannii. BMVs pretreated with polymyxin B (PMB) did not induce IL-1β (a) or TNF-α (b) expression. Supernatants from ceftazidime and imipenem-treated cultures induced similar levels of IL-1β (c) and TNF-α (d) expression, which were similar to that induced by untreated supernatant (LB). RAW264.7 macrophages treated with LPS plus ATP were used as a positive control, and IL-4 and phosphate-buffered saline (PBS) were used as negative controls. LB-BMV-PMB, CAZ-BMV-PMB, and IMP-BMV-PMB are BMVs pretreated with polymyxin B (25 μg/ml). Shown are the mean and SD of results from triplicate studies. * P < 0.05 between two compared groups.

Figure 3.

Bacterial membrane vesicles (BMVs) induced by ceftazidime (CAZ) caused higher mortality in mice than BMVs induced by imipenem (IMP) treatment. In the study, mice were inoculated with 30 μl (a), 48.3 μl (b), or 77.5 μl (c) of BMVs derived from treatment of A. baumannii with CAZ, IMP, or no antimicrobial (LB). In the second study, the mice were inoculated with BMVs that carried the same amount of LPS (270 EU) (d). * P < 0.05 vs. BMVs derived from A. baumannii grown in Luria-Bertani broth without antibiotics (LB-BMV).

Figure 4.

(a) Cytokine expression in mice serum after inoculated with bacterial membrane vesicles (BMVs) from antimicrobial-treated Acinetobacter baumannii. BMVs induced by ceftazidime (CAZ) treatment led to higher expression of many cytokines in mice than BMVs induced by imipenem (IMP) treatment or BMVs from untreated A. baumannii (LB). Phosphate-buffered saline (PBS) were used as negative controls. * P < 0.05 between CAZ-BMV and IMP-BMV. (b) Cytokine expression in mice serum after inoculated with bacterial membrane vesicles (BMVs) from antimicrobial-treated Acinetobacter baumannii. BMVs induced by ceftazidime (CAZ) treatment led to higher expression of many cytokines in mice than BMVs induced by imipenem (IMP) treatment or BMVs from untreated A. baumannii (LB). Phosphate-buffered saline (PBS) were used as negative controls. * P < 0.05 between CAZ-BMV and IMP-BMV.

Figure 4.

Continued.

Figure 5.

Lung pathology study showed that more neutrophil infiltration was found in lung interstitium of mice treated with ceftazidime-induced bacterial membrane vesicles (CAZ-BMV) than those treated with imipenem-induced bacterial membrane vesicles (IMP-BMV). Increased neutrophil infiltration occurred in the CAZ-BMV group (a), whereas no significant neutrophil recruitment in the IMP-BMV (b), LB-BMV (c), and control (d) group. Mice intraperitoneally administered 30 μl of phosphate-buffered saline (PBS) were used as control group.

Figure 6.

Treatment with 1/2 the minimal inhibitory concentration of ceftazidime (CAZ) induced the release of a higher number of bacterial membrane vesicles (BMVs) than treatment with imipenem (IMP). The number of BMVs (a), surviving colonies (b), and secreted BMVs per surviving colony (c) of A. baumannii after CAZ or IMP treatment, or without antimicrobial treatment (LB). Shown are the mean and SD of results from triplicate studies. *P < 0.05 vs. the sample derived from A. baumannii grown in Luria-Bertani broth without antimicrobials (LB).

Figure 7.

Treatment with 1/2 the minimal inhibitory concentration of ceftazidime (CAZ) increased the amount of lipopolysaccharide (LPS) carried by bacterial membrane vesicles (BMVs) but not the amount in supernatants, compared to treatment with imipenem (IMP). LPS was detected with an anti-LPS antibody by western blot analysis. Cell lysate of ATCC 17978 was used as a positive control.