## 4 Discussion  
BRD has become one of the major diseases in the global cattle industry [36]. One of the main pathogens causing this disease is \(Pm\). At present, the treatment of \(Pm\) mainly depends on antibiotics. However, due to the irrational use of antibiotics in clinical treatment, \(Pm\) is prone to drug resistance. In China, fluoroquinolones are commonly used to treat BRD infected by \(Pm\)[37]. In recent years, multidrug-resistant strains of \(Pm\) have appeared, and \(Pm\) has caused huge economic losses to the cattle industry worldwide. [PERSON] et al. discovered that 93.2% of the 92 \(Pm\) strains were resistant to fluoroquinolones [36]. The mutation of the drug resistance target gene has a vital impact on the formation of drug resistance. [PERSON] et al. detected the target genes in quinolone-resistant \(Pm\) and found that the mutation of \(gyrA\) and \(parC\) genes had a significant influence on the generation of drug resistance [37]. In this study, three strains (\(PmS\), \(PmR\) and \(PmHR\)) with different resistance phenotypes in relation to fluoroquinolones were screened. We found that when the \(satP\) gene was deleted, \(\Delta PmS\), \(\Delta PmR\) and \(\Delta PmHR\) had no QRDR mutation, compared with those without deletion. The different levels of resistance between \(PmHR\) and \(PmR\) might be mediated  
Figure 5: Survival rates of the mice treated with ENR infected by \(PmS\) and \(\Delta PmS\). by other resistance mechanisms or tolerance. Relevant research has shown that antibiotic tolerance can increase the probability of antibiotic resistance [38]. With respect to the research question, we sequenced the transcriptome of three strains of bovine capsular \(Pm\) with different levels of drug resistance, treated by sub-inhibitory concentration, and counted the differential genes. On the basis of ensuring stable and accurate transcriptome data, we analyzed the functional annotation and metabolic pathways, and analyzed and screened the genes that might be related to the formation of drug resistance. The results showed that with the increase in drug resistance, the expression of some genes, namely \(satP\), \(foCA\), \(napF\), \(glgP\) and \(napD\), increased significantly. The most significant difference was the \(satP\) gene, which was up-regulated 17 times in the drug-resistant group and 20 times in the highly drug-resistant group compared with the sensitive group. \(satP\) is a representative member of the AceTr family, which is preliminarily identified as a succinate acetate/proton cotransporter transmembrane protein [39]. [PERSON] et al. found that the outer membrane protein, ompk36, is an important influencing factor of cefoxitin and carbapenem resistance [40]. [PERSON] et al. found that the membrane protein mmpl5 has an effect on the resistance of clofazimine and is also related to the tolerance of a variety of antibiotics [41]. [PERSON] et al. studied the function of membrane channel protein, \(lamB\), and found that it can produce antibiotic resistance by reducing the pressure of protein translation and DNA replication. In order to further detect antibiotic resistance, the \(lamB\) gene was knocked out and was found to increase antibiotic resistance by reducing the activity of metabolic pathways such as the tricarboxylic acid (TCA) cycle, pentose phosphate pathway and glycolysis/gluconeogenesis. By comparing the function of the wild-type strain and deletion strain after ciprofloxacin treatment, it was found that the down-regulation of the \(lamB\) gene can increase the resistance of bacteria to antibiotics [42]. Based on the research background of the \(satP\) gene, in order to further confirm the function of the \(satP\) gene, we constructed the gene deletion strain of a sensitive strain and drug-resistant strain. The results showed that the tolerance of \(satP\) decreased after the deletion of the \(satP\) gene. [PERSON] et al. have pointed out that when antibiotics act on bacteria, antibiotic tolerance first appears.