Relationship Between The Immunosuppression And The Resistance Of The Multidrug-resistant Acinetobacter Baumannii

Objective: Due to the increasing of the resistance of Acinetobacterbaumannii and the spreading of antibacterial spectrum, Acinetobacterbaumannii infection has emerged as one of the most challenge pathogens inthe nosocomial infection in recent decades. With the discovery of newphenotype of multidrug-resistant Acinetobacter baumannii and the outbreakof multidrug-resistant Acinetobacter baumannii infection, the controlling andtreatment of MDR A.baumannii are serious and urgent to be solved inrecently decades. Recently some reports declare that the rate of resistance ofAcinetobacter baumannii in immunocompromised or elderly patients issignificantly higher than that in immunocompetent patients. However, thereare no data to exclude the impact of the application of antibiotics and othertherapeutic measures. Some scholars have raised the hypothesis thatimpaired immunity may be a factor which contributes to the spread ofdrug-resistance, but there is no experimental data on the relationship betweenthe immune status and bacterial resistance. This study researched the changeof the Acinetobacter baumannii resistance and the difference of lunghistopathology, to indicate effect of the immune in infection. The resultwould explore the mechanism of immunotherapy, which improve theefficacy of anti-infection treatment and provide a theoretical basis for clinicalimmunotherapy.Methods: The32SD rats are randomly divided into two groups:①immune normal group;②immunosuppression group, there are16rats ineach group. The immunosuppressed group: Animals are rendered transientlyneutropenia by peritoneal injection of cyclophosphamide （150mg/kg,10ml/kg, QD） two days; The immune normal group: Animals are peritoneal injection of saline（10ml/kg, QD） two days. One MDR Acinetobacterbaumannii is adopted, and the isolate is identified using the VITEK-2analyzer and API20NE strips, susceptibility testing was performed by thedisk diffusion method. All bacterial isolates are stored in Mueller-Hintonbroth with15%glycerol and frozen at80℃. Culture the isolate and pick upone pure colony to prepare for bacterial suspension （1×10⁸CFU/mL）. Thebacterial suspension is inoculated intratracheal injection （200ul each onemouse） after modeled24hours （on the third day）.At the48hour after replication model,16animals in each group areanesthesia by peritoneal injected10%chloral hydrate （0.4ml/100g）, insertthe tracheal catheter by tracheotomy to keep breathing. Then pick thespecimens from the right lung tissue, fix in4%neutral formalin, embed inparaffin to get the lung pathology, observe the morphological changes inlung tissue. Collect the bronchoalveolar lavage fluid from the left lungthrough the tracheal intubation, culture and isolated Acinetobacter baumanniifrom the bronchoalveolar lavage fluid. Test the minimum inhibitoryconcentration （MIC） of biapenem, tigecycline, cefoperazone and sulbactamrespectively by broth microdilution method. Analysis and compare the MICvalues in the two groups of the different immune status. Take the weight ofthymus and spleen tissue and analysis the thymus index and spleen index.Results:1The minimum inhibitory concentration （MIC） of cefoperazone,biapenem, tigecycline and sulbactam: Immune normal group: MIC range ofcefoperazone is64-128μg/ml and MIC₉₀is128μg/ml; MIC range ofbiapenem is32-64μg/ml and MIC₉₀is64μg/ml; MIC range of tigecycline is1-2μg/ml and MIC₉₀is2μg/ml; MIC range of sulbactam is128-512μg/mland MIC₉₀is256μg/ml. Conversely, the immunosuppression group: MICrange of cefoperazone is64-128μg/ml and MIC₉₀is128μg/ml; MIC range ofbiapenem is32-256μg/ml and MIC₉₀is64μg/ml; MIC rang of tigecycline is2-4μg/ml and the MIC₉₀is4μg/ml; MIC range of sulbactam is256-512μg/mland MIC₉₀is256μg/ml. The resistance is divided into low-level and high-level: the high-level MIC is cefoperazone of MIC≥128μg/ml,biapenemof MIC≥64μg/ml, tigecycline of MIC≥4μg/ml and sulbactam ofMIC≥512μg/ml, conversely, the low-level MIC is Cefoperazone ofMIC≤64μg/ml, Biapenem of MIC≤32μg/ml, tigecycline of MIC≤2μg/ml andsulbactam of MIC≤256μg/ml. Compared with immune normal group, therate of the high-level MIC of cefoperazone and sulbactam in theimmunosuppression group is not significant different （P>0.05）, converselythe rate of biapenem and tigecycline is significant different （P<0.05）.2Compare the immune suppression group with the normal group, thebody weight, thymus index and spleen index decreased significantly （P<0.05）.3Pathology: Visible Observation: The lung tissues ofimmunosuppressed group the color is dark, a few of haemorrhage pointssubcapsular, even visible small pustules, some light yellow liquid overflow.Light microscope: The normal immune group: the structure of alveolar isclear and complete, alveolar space and pulmonary interstitial is significantlyfull with hydrops, alveolar septum is widened and neutrophil exudateseverity. Conversely, the immunosuppressed group: the tissue structure ofalveolar is damaged, the alveolar space is full of edema fluid, less neutrophilexudation, capillary congestion obvious and the epithelium of airway isnecrotic. The pathological score of immunosuppression group pathologyscore is higher than the normal immune group, but the difference is notstatistically significant （P<0.05）.Conclusions:1The Acinetobacter baumannii pneumonia model is caused by theintratracheal injection of the bacterial suspension after peritoneal injection ofcyclophosphamide, the model of immunocompromised infection issuccessfully duplicated.2The different immune status of host can effect the resistance ofmultidrug-resistant Acinetobacter baumannii to biapenem and tigecycline incertain, cefoperazone and sulbactam resistance had no significant change. 3The pathological score is similar in the immunocompetent andimmunosuppression group, but there are some differences in specificpathological performance. In the immunosuppression group there are somedamage of the lung structure and necrosis of airway epithelial mainly.However, the immune normal observes the alveolar septum broadening andvascular congestion, without obvious structural damage.