Novel Experimental Mouse Model to Study the Pathogenesis and Therapy of Actinobacillus pleuropneumoniae Infection

Abstract

Actinobacillus pleuropneumoniae (APP) is a major cause of lung infections in pigs. An experimental mouse has the edge over pigs pertaining to the ease of experimental operation, disease study and therapy, abundance of genetic resources, and cost. However, it is a challenge to introduce APP into a mouse lung due to the small respiratory tract of mice and bacterial host tropism. In this study, an effective airborne transmission of APP serovar 1 (APP1) was developed in mice for lung infection. Consequently, APP1 infected BALB/c mice and caused 60% death within three days of infection at the indicated condition. APP1 seemed to enter the lung and, in turn, spread to other organs of the mice over the first 5 days after infection. Accordingly, APP1 damaged the lung as evidenced by its morphological and histological examinations. Furthermore, ampicillin fully protected mice against APP1 as shown by their survival, clinical symptoms, body weight loss, APP1 count, and lung damages. Finally, the virulence of two extra APP strains, APP2 and APP5, in the model was compared based on the survival rate of mice. Collectively, this study successfully established a fast and reliable mouse model of APP which can benefit APP research and therapy. Such a model is a potentially useful model for airway bacterial infections.

Keywords: Actinobacillus pleuropneumoniae; clinical score; inflammation; lung; mouse model; pleuropneumonia; pulmonary bacterial infection.

Figure 1

An apparatus for aerosol-mediated lung infection in mice. Aerosol system controller (a) and air-tight chamber with nebulizer head connected with the nebulizer control unit (b) are assembled into an aerosol generation and infection device (c).

Figure 2

The survival rate, clinical scores, and body weight loss of the mice were measured. (a–c) Three groups of mice, 5 animals per group, were randomly assigned. One group of mice (NC) was exposed to PBS aerosols, the second group (APP1) was exposed to APP1 aerosols (5 × 10¹⁰ CFU), and the third group (PC) was exposed to the same amount of APP1 aerosols, followed by an oral gavage of ampicillin (Amp, 20 mg/kg) at 2 h post-infection. Each group of mice was monitored daily for survival rate (a), clinical score (b) and percentage of body weight (c). p * < 0.05.

Figure 3

Bacterial count of different organs of the mice over 5 days following APP1 infection. The mice that received the same treatments as those in Figure 2 were sacrificed, three mice per group, at one to five days post-infection (DPI). The colony-forming unit (CFU) per gram of the lung (a), spleen (b), heart (c), liver (d), and kidney (e) from the mice was counted. p * < 0.05.

Figure 4

Comparison of the survival rate of BALB/c mice infected with APP5 and APP2. Three groups of mice, 5 animals per group, were randomly assigned. One group of mice (NC) was exposed to PBS aerosols, the second group (APP2) was exposed to APP2 aerosols (5 × 10¹⁰ CFU), and the third group (APP5) was exposed to the same amount of APP5 aerosols. Each group of mice was monitored daily for survival rate. The mouse number is shown in the parenthesis. p * < 0.05.