Acinetobacter baumannii is a nosocomial pathogen with a high prevalence of multiple-drug-resistant strains, causing pneumonia and sepsis. The current studies further develop a systemic mouse model of this infection and characterize selected innate immune responses to the organism. Five clinical isolates, with various degrees of antibiotic resistance, were assessed for virulence in two mouse strains, and between male and female mice, using intraperitoneal infection. A nearly 1,000-fold difference in virulence was found between bacterial strains, but no significant differences between sexes or mouse strains were observed. It was found that microbes disseminated rapidly from the peritoneal cavity to the lung and spleen, where they replicated. A persistent septic state was observed. The infection progressed rapidly, with mortality between 36 and 48 h. Depletion of neutrophils with antibody to Ly-6G decreased mean time to death and increased mortality. Interleukin-17 (IL-17) promotes the response of neutrophils by inducing production of the chemokine keratinocyte-derived chemoattractant (KC/CXCL1), the mouse homolog of human IL-8. Acinetobacter infection resulted in biphasic increases in both IL-17 and KC/CXCL1. Depletion of neither IL-17 nor KC/CXCL1, using specific antibodies, resulted in a difference in bacterial burdens in organs of infected mice at 10 h postinfection. Comparison of bacterial burdens between IL-17a؊/؊ and wild-type mice confirmed that the absence of this cytokine did not sensitize mice to Acinetobacter infection. These studies definitely demonstrate the importance of neutrophils in resistance to systemic Acinetobacter infection. However, neither IL-17 nor KC/CXCL1 alone is required for effective host defense to systemic infection with this organism.

Acinetobacter baumannii is an important nosocomial pathogen in civilian intensive care units. Recently the incidence has increased in wounded military personnel. Morphine is documented in numerous animal studies to be immunosuppressive and to sensitize to infection. The hypotheses were tested that morphine, administered for analgesia in the battlefield, predisposes to Acinetobacter infection, and that the opioid may have an additive or synergistic effect with trauma. To test these hypotheses, an intraperitoneal infection model was established in mice using several Acinetobacter strains. Morphine administered for 48 hr by implantation of a slow-release morphine pellet increased mortality compared to animals receiving a placebo pellet, an effect that was blocked by the mu-opioid receptor antagonist, naltrexone. Acinetobacter burdens in the blood, spleens, livers, and lungs of morphine-treated mice, were significantly higher than those in placebo-treated animals, confirming that mortality was due to potentiated growth of the bacteria. There were also elevated levels of pro-inflammatory cytokines in morphine-treated versus placebo-treated mice. Morphine caused a reduction in the total number of cells in the peritoneal cavity, a decrease in the percentage and total numbers of neutrophils, and a decrease in the total number of macrophages. Morphine treatment also suppressed levels of the neutrophil-inducing molecules, IL-17A and KC/CXCL1. However, IL-17A−/− mice given morphine were not sensitized to Acintobacter infection to a greater degree than similarly treated wild-type mice. Trauma alone did not sensitize to Acinetobacter infection, and there was no additive effect between morphine and trauma. These results support the hypothesis that morphine potentiates Acinetobacter infection.

Previous studies from our laboratory demonstrated that mice treated with morphine pellets are sensitized to Salmonella enterica, serovar Typhimurium infection. However, the opioid receptor antagonist, naltrexone, only partially blocked the effect of morphine, raising the possibility that the opioid might have some of its effects through a nonopioid receptor. To further clarify whether sensitization to infection is an opioid receptor-dependent phenomenon, μ-opioid receptor knock-out (MORKO) mice were used in the present study. Wild type (WT) and MORKO mice were treated with morphine and their sensitivity to oral Salmonella infection assessed by mortality, bacterial burdens in gut associated lymphoid tissue and in blood and peritoneal fluid, and by levels of pro-inflammatory cytokines in plasma. MORKO animals treated with morphine were refractory to a sublethal dose of Salmonella, while similar treatment of WT animals resulted in 100% mortality. WT animals treated with morphine had high bacterial loads in all organs tested, while morphine-treated MORKO animals had no culturable Salmonella in any organs. Pro-inflammatory cytokine levels were elevated in morphine treated WT but not MORKO mice infected with Salmonella. These results provide definitive evidence that the morphine-mediated enhancement of oral Salmonella infection is dependent on the μ-opioid receptor.

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