Our surveillance data showed that K. pneumoniae, P. aeruginosa and A. baumannii were the most common pathogens of LRIs. Our data was different from the results of a global multi center report from the SENTRY Antimicrobial Surveillance Program (1997–2016), which showed in Latin America, the most common pathogens were P. aeruginosa, S. aureus and Acinetobacter, while in Europe, Asia Pacific and North America, the most common pathogens were S. aureus, P. aeruginosa and K. pneumoniae. S. aureus ranked fourth in our area. Our pathogenic spectrum was consistent with the data reported by CHINET (China antimicrobial surveillance network) of China in 2018.  In the developed world, the United States a study indicated S. pneumoniae, S. aureus and Legionella pneumophila were the most common pathogens of community-acquired pneumonia in adults (≥ 18 years old) from 2010–2012.  However, the most common pathogens of adult community-acquired pneumonia in Malawi, a developing country in Africa were S. pneumoniae, Mycobacterium tuberculosis and non-Mycobacterium tuberculosis.  We found that in different regions of the world, the pathogenic bacteria of LRIs were significantly different. Therefore, it was of great significance to analyze the distribution of local pathogens for epidemiological research and clinical experience.
This study found that the main population of LRIs was 40–70 years old, for the eight most common pathogens K. pneumoniae, P. aeruginosa, A. baumannii, S. aureus, H. influenzae, S. maltophilia, E. coli and S. pneumoniae. For E. coli and S. pneumoniae, the age range of 0–5 years was also a major distribution. According to data from 195 countries around the world, S. pneumoniae and H. influenzae type b were the main pathogens of LRIs in children under 5 years old. With the global promotion of S. pneumoniae and H. influenzae b vaccine, their incidence had decreased significantly.  Among all infectious diseases, vaccines were the most effective means of control. Our study found that the age distribution of pathogens in LRIs was different from that in bloodstream infection. Studies from Malawi, Africa, have shown that K. pneumoniae, which caused bloodstream infection, mainly came from the ages of 0–5 and 75–79. To understand the age distribution of the main pathogens was helpful for clinicians to choose the appropriate antibiotics when they took experiential treatment. For example, in the treatment of children, fluoroquinolones should be avoided because of their influence on bone development. For the elderly patients with poor liver and kidney function, the liver and kidney toxicity of antibiotics should be considered in the empirical treatment.
A systematic analysis on global patterns in monthly activity of influenza virus, respiratory syncytial virus, parainfluenza virus, and metapneumovirus found influenza virus had clear seasonal epidemics in winter months in most temperate sites but timing of epidemics was more variable and less seasonal with decreasing distance from the equator and other viruses had obvious epidemic seasons. In our data, S. aureus had the highest isolation rate in spring, A. baumannii, H. influenzae and S. pneumoniae had the highest isolation rate in summer, K. pneumoniae, P. aeruginosa and S. maltophilia had the highest isolation rate in autumn, while E. coli had the highest isolation rate in winter. It is of great significance to understand the seasonal prevalence of pathogens in LRIs for disease prevention and control as well as vaccination. 
This study showed that K. pneumoniae was the main pathogen of LRIs. Cephalosporin was often used in the treatment of LRIs because of its small side effects. This study showed that the sensitivity rates of K. pneumoniae to ceftazidime and cefotaxime were 56.5% and 52.6% respectively, and the sensitivity rate to cefepime was 54.9%. The main resistance mechanism of K. pneumoniae to cephalosporin was the expression of extended spectrum β - lactamase. (ESBLs).  When cephalosporins were resistant, carbapenems were often considered in clinical experience. Our data showed that the resistance rates of K. pneumoniae to imipenem and meropenem were 30.6% and 30.8%, respectively. Large data analysis from China's multi centers shows that the antimicrobial resistance rate of K. pneumoniae to imipenem and meropenem increased year by year from 2005 to 2018, and the resistance rates of meropenem and imipenem increased from 2.9–26.3% and 3–25%, respectively. However, the situation in Germany was different from that in China. Monitoring data from the German Antimicrobial Resistance Surveillance (ARS) show that the isolation rate of K. pneumoniae, which was not sensitive to carbapenems, in Germany in 2011–2016 was only 0.63%.  In addition, K. pneumoniae, which was not sensitive to carbapenems, was often resistant to other antibiotics, such as gentamicin, Sulfanilamide and tegacyclin.  Some studies had shown that carbapenem-resistant hypervirulent K. pneumoniae may have a clonal distribution in the hospital. The increase of resistance rate of carbapenems may be related to the unreasonable use of carbapenems. Therefore, the control of carbapenem resistant K. pneumoniae in our hospital was the top priority for infection control.
This study found that in addition to K. pneumoniae, another strain with high resistance rate was A. baumannii. In addition to tegacyclin and minocycline, the resistant rates to other commonly used antibiotics of A. baumannii were more than 80%. Data from CHINET in 2018 shows that A. baumannii isolates from blood, cerebrospinal fluid and lower respiratory tract had a very high resistance rate to commonly used antibiotics (almost all more than 50%, even more than 80%). At present, the antimicrobial resistance of A. baumannii in China had reached a very serious level. The high resistance of A. baumannii may be related to the expression of carbapenemase ox-23, ox-24 and ox-51.  The formation of biofilm was beneficial to the long-term existence of A. baumannii in the environment and it was found that there was homology between the infection strains of patients and the colonization strains in the environment, indicating that the highly resistant strains were cross transmitted between patients and the environment.  Therefore, the control of A. baumannii must rely on the strengthening of hospital infection control measures.
There are several limitations in this study. First of all, the types of LRIs such as community acquired, hospital acquired and ventilator related infection types were not distinguished in this study. Secondly, the molecular epidemiology of carbapenem resistant K. pneumoniae and A. baumannii were not analyzed.