Establishment And Experimental Study Of SKH-1 Hairless Mouse Model Induced By Methicillin-resistant Staphylococcus Aureus (MRSA) Under Different Infection Conditions

ObjectiveMethicillin-resistant Staphylococcus aureus (MRSA) is a multiple drug-resistant pathogen, which is widely prevalent around the world. As the high morbidity and mortality of MRSA infection, it has been a problem in the global medical community. Burn patients are vulnerable to MRSA, for they are in an immunocompromised state. Once burn patients were infected with MRSA, the therapy will became more problem with high mortality. Therefore, we explored and established mouse models of scald with MRSA infection by different ways. For the first time we established the scald with MRSA infection SKH-1 hairless mice models, which are comparatively consistent with clinical infection. It will be of great importance in developing the clinical researches of scald with MRSA infection and the application range of SKH-1 hairless mice.MethodsPart I:Model I:Fifteen specific pathogen free SKH-1 hairless mice were randomly divided into the control group, A group, a group, B group and b group. Mice from A group, a group, B group and b group were thermally injured by pre-heated brass block(100℃), and mice from A group and a group were thermally injured for 40s;mice from B group and b group were thermally injured for 60s. Then mice from A group and B group were topically inoculated with MRSA of 109CFU/mouse, mice from a group and b group were topically inoculated with MRSA of 108CFU/mouse. Model II:Forty-eight specific pathogen free SKH-1 hairless mice were randomly divided into the PBS control group, infection group and drug-treated group. Mice from infection group and drug-treated group were thermally injured by pre-heated brass block(100℃) for 40s, after the eschar were excided, mice were inoculated with MRSA of 108CFU/mouse, and mice from drug-treated group were treated with teicoplanin intravenously. Model III:Forty-eight specific pathogen free SKH-1 hairless mice were randomly divided into the control group, scald with MRSA infection group(D group), single infection group(E group) and single scald group(F group). Mice from scald with MRSA infection group were thermally injured by pre-heated gauze (100℃,3×5cm) for 30s, then challenged with aerosolized MRSA(ST-239). Three kinds of models above were comprehensively evaluated from body weight, bacteremia rate, blood routine examination, skin and organs infection measuring and pathological examinations.Part Ⅱ:Forty-eight specific pathogen free SKH-1 hairless mice were randomly divided into the control group, single infection group and scald with MRSA infection group. Mice from scald with MRSA infection group were thermally injured by pre-heated gauze (100℃,3×4cm) for 30s, then challenged with aerosolized MRSA(ST-239). The models were evaluated by body weight, blood tests, bacteraemia rate, skin and organs infection measuring, histopathological observations and multi-PCR for the nuc and mecA genes detection.ResultsPart Ⅰ:Model I:Compared with the control group, Mice from A group, a group, B group and b group had evident weight loss on the third day; 100% bacteraemia rate; more than 108CFU/mL MRSA in the skin; inflammatory lesions of the infected tissues were obvious under the light microscope. Model II:Compared with the control group, Mice from the infection group and drug-treated group had very evident weight loss(p<0.01); the survival rate of the infection group was 50%, while the drug-treated group was 60%; both 100% bacteraemia rate; different degree of MRSA dissemination in organs; ulcers and abscesses were observed in the skin of mice from the infection group, obvious focal abscess and inflammatory cells were observed. Model Ⅲ:Compared with the control group, mice from D group had evident weight loss(p<0.05); 100% bacteraemia rate; remarkable increase of WBC and NEU count(p<0.01); The number of viable S. aureus in the skin and the lungs was 108CFU/g and 107～106 CFU/organ respectively; inflammatory lesions in the skin and lungs were obvious under the light microscope.Part Ⅱ:Compared with the other two groups, the infection with scald group had evident weight loss on the third day, remarkable increase of WBC count and a bacteraemia rate of 100%. The number of viable S. aureus in the skin and lungs were 108CFU/g and 106CFU/g respectively; The epidermis fell off, coagulative necrosis and a large number of inflammatory cells infiltration in the dermis, connective tissue hyperplasia was seen at the late state of infection. Typical bronchial pneumonia and septicemia spleen were also found under the light microscope; The results of immunohistochemistry showed that CD138 expressed positively in the skin, and positive results were also found with both nuc and mecA genes in the infected tissues of the infection with scald group.