Discussion and Conclusion
The novel coronavirus has been found to have membrane glycoprotein, spike protein, nucleocapsid protein, envelope protein and coagulase[5]. The spike glycoprotein on the surface of the virus plays a major role in its attachment and entry into host cells, and the infection can lead to lethal damage in the lungs, heart, kidneys, circulatory system, gastrointestinal tract and nervous system tissues [6]. Elderly people are particularly vulnerable to the virus due to their weakened immune systems, and evidence suggests that a large number of hospitalized patients during the COVID-19 pandemic had mixed bacterial infections and secondary bacterial infections, which were a major cause of higher mortality in elderly patients [7]. When the novel coronavirus invades the lungs, it causes damage to the lung cells and tissues, which attracts neutrophils and macrophages to the infected site and promotes inflammation, and eventually leads to bacterial adhesion and invasion into the cells and proliferation [8]. Therefore, it is essential to identify the bacterial pathogens of LRTIs in the elderly.
The results of this study showed that Klebsiella pneumoniae,Pseudomonas aeruginosa, Bacteroides fragilis, Escherichia coli, Stenotrophomonas maltophilia, Staphylococcus aureus, andStreptococcus pneumoniae were the main pathogenic bacteria in the sputum specimens of 163 hospitalized elderly patients in this study. This is consistent with similar studies [9], which showed that the common bacteria identified in respiratory cultures, even the sputum culture results of elderly patients with pulmonary infections, were mostly Gram-negative bacteria, mainly includingKlebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Bacteroides fragilis, etc.; the Gram-positive bacteria infected included Streptococcus pneumoniae, Staphylococcus aureus , etc.Klebsiella pneumoniae is a Gram-negative bacteria with a thick capsule, mostly distributed in the human respiratory and gastrointestinal tracts, and is a common pathogen for elderly respiratory infections. SAIDK et al. [10]conducted a screening for combined microbial infection in 301 COVID-19 patients, and the main microorganisms detected were Klebsiella pneumoniae (37%, 48/301), Pseudomonas aeruginosa (8.5%, 11/301), and Escherichia coli (18.6%, 24/301).
The opportunistic Gram-negative bacillus Stenotrophomonas maltophilia is known to infect mostly elderly patients. According to one study, individuals over the age of 60 were more prone to S. maltophilia infection, which could be attributed to immunosuppression[11]. In recent years, S. maltophilia has been relatively stable in third place among non-fermenting Gram-negative bacteria, after Pseudomonas aeruginosa and Acinetobacter baumannii, according to the CHINET monitoring service[12]. Patients with severely impaired or weakened immune functions were more susceptible to serious consequences when infected with S. maltophilia, suggesting that the respiratory tract was the main site of colonization and infection ofS. maltophilia , which may have been associated with its ability to form biofilms and colonize the respiratory tract of hospitalized patients [13]. Recently, S. maltophilia was reported as one of the secondary bacterial infections in ICU patients with COVID-19 [9], which is similar to what was found in this study: the infection rate of S. maltophilia was higher in the COVID-19 group than in the CNT group.
Acinetobacter baumannii is a Gram-negative bacterium with strong vitality, and it is a major opportunistic pathogen that can cause double infection, especially in hospitalized patients with viral respiratory infection [14]. Recent studies have shown that the infection rate of Acinetobacter baumannii in COVID-19 patients varies widely in different regions. For example, Nanshan Chen et al.[15] reported that among 99 COVID-19 patients in Wuhan, only one was cultured with Acinetobacter baumannii , with an infection rate of 10.1% (1/99); another domestic report showed that among 1495 hospitalized COVID-19 patients in Wuhan, 102 (6.8%) had bacterial mixed infection, mainly Acinetobacter baumannii(35.8%), and nearly half (49.0%, 50/102) died during hospitalization[16]; Iranian scholars SHARIFIPOUR et al.[14] reported that 17 out of 19 elderly COVID-19 patients had Acinetobacter baumannii infection, with an infection rate of 89.5% (17/19); Brazilian researchers SILVADL et al.[17] reported that the infection rate ofAcinetobacter baumannii was 32.8% (21/64); SAIDKB et al. in Saudi Arabia reported [10] that 34 of the 301 COVID-19 patients had drug-resistant Acinetobacter baumannii(26%, n=34).
The infection rate of Acinetobacter baumannii varies in different regions, and further research is needed to investigate the specific reasons. In the hospital environment, COVID-19 elderly LRTIs patients may face another threat to health, which is mixed bacterial infection and nosocomial secondary bacterial infection, especially the mixed infection and secondary infection of Staphylococcus aureus andAcinetobacter baumannii , which may seriously affect the clinical outcome of COVID-19 patients.
Whereas, This study has certain limitations, for it was solely focused on bacteria, not including fungi, mycoplasma, chlamydia, parasites and other viruses into the study of pathogens in elderly LRTIs patients; The number of patients enrolled is small, which could result in a sample size bias ,therefore large-scale clinical studies need to be conducted to investigate the incidence, prevalence, characteristics and microbiological distribution of COVID-19 complicated infections. As a result, a multicenter, prospective, longitudinal study is required to confirmed the research.
In conclusion, the current study has identified the main bacterial pathogens of elderly LRTIs patients, and has highlighted the importance of identifying and treating secondary bacterial infections in elderly patients with COVID-19. Furthermore, the findings of this study suggest that Klebsiella pneumoniae, Pseudomonas aeruginosa, Bacteroides fragilis, Escherichia coli, Stenotrophomonas maltophilia, Staphylococcus aureus, and Streptococcus pneumoniae are the main bacterial pathogens of elderly LRTIs patients, and that Stenotrophomonas maltophilia is an opportunistic pathogen that is particularly dangerous for elderly patients with weakened immune systems.