Comparative assessment of the mouse immune responses to colistin-resistant and colistin-sensitive isolates of Acinetobacter baumannii.

Acinetobacter baumannii is known as the most frequent species in clinical samples that is responsible for a large number of nosocomial infection outbreaks. The colistin-resistance of this bacterium has been found to be increasing. This study aimed to evaluate the immune response to colistin-susceptible and colistin-resistant A. baumannii isolates in a mouse model. Samples were prepared from the wounds of patients suspected of A. baumannii admitted to the intensive care unit of Namazi Hospital in Shiraz. Antibiotic susceptibility was studied by disk diffusion and broth microdilution according to CLSI and EUCAST criteria. Among the isolates, one colistin-sensitive isolate and one colistin-resistant isolate were injected intraperitoneally to BALB/C mice. Blood samples were collected after 4 h and cytokines (IL1-β, IL-12, IFN γ, and IL-10) and surface markers (CD4, CD8, CD3, and CD45) were determined by ELISA and flow cytometry. Then, hematologic and histopathological factors were analyzed, and colony count in lung, liver, kidney, and spleen tissues were also performed. The results showed that levels of cytokines (IL1-β, IL-12, IFN γ, and IL-10) and markers (CD4, CD8, CD3 and CD45) were higher in mice receiving colistin-resistant A. baumannii isolates than in mice receiving colistin-sensitive A. baumannii isolates, indicating that colistin-resistant isolates 4 h following the intraperitoneal injection stimulated host innate immune system better and produced a stronger immune response. On the other hand, histopathological findings showed inflammatory cell infiltration, hyperemia, and tissue damage. In addition, the bacterial load and tissue damage in the lung was higher than other tissues. The results of this study can have promising potential for the development of a prevention and treatment strategy based on cellular immune response for infection caused by colistin-sensitive and colistin-resistant A. baumannii.