Lower respiratory tract infections (LRTIs) refer to infections that involve the respiratory tract below the level of the larynx, and includes tracheitis, bronchitis, bronchiolitis and bronchopneumonia (1,2). LRTIs are the major cause of pneumonia and bronchiolitis in hospitals worldwide (1). LRTIs cause ~3.5 million deaths, as well as 79 million cases of disabilities annually (2). LRTIs are associated with high overall morbidity and mortality in adults, with increased risk in the elderly compared to young adults (3,4). Children admitted with LRTIs were reported to have severe respiratory disease (5), in addition to suffering respiratory sequelae (6) and having a long recovery period (5). Among acute LRTIs, the major common cause for hospitalization of infants is bronchiolitis (7), whereas pneumonia in adults (mainly >65 years) and children is the most common cause of death, accounting for 3.5 million (6.1%) deaths globally and over 1.05 million (11.3%) in lower income countries (8,9) every year.

Identification of causative organisms is essential for definitive diagnosis of infectious diseases, as well as decision making with regard to appropriate management. The gold standard method is the usage of bacterial and fungal cultures; however, it is time-consuming and not suitable for identification of unculturable bacteria. PCR and antigen tests are commonly used for viral detection; however, only a defined set of microorganisms can be examined (10). Although a reliable method of identification, viral isolation is also time-consuming, and is insufficiently sensitive. Performing combinations of these procedures, did not show significantly improved pathogen identification in 34-57% of paediatric patients (10), and in 13-62% of adult patients with pneumonia (11,12). In the case of respiratory infection, diagnosis of specific microorganisms occurs when they are isolated from sterile materials such as bronchoalveolar lavage fluid (BALF) (10,13,14). BALF is more suitable than sputum for identifying pathogens of respiratory infections, as less contamination with oral bacteria is possible during collection from the locus of infection (15). BALF is usually obtained for PCR to detect certain viruses (16,17). It is to be noted that the diagnostic yield of causative pathogens from BALF in lower respiratory tract infections in immunosuppressed children was 28-68% (18). BALF is collected using a safe procedure and with a very low rate of morbidity and mortality (19). Several studies reported that the results for microbiological examination of BALF have led to alterations in the management of infections in 38.7-72.7% of patients (18-20).

LRTIs are commonly managed by treatment with antibiotics; however, clinical management of LRTIs is difficult due to antibiotic resistance caused by inappropriate use of antimicrobial agents, leading to increased hospital mortality rates, as well as excessive costs for LRTI patients (21).

As the effective selection of antibiotics is considered a considerable challenge for all physicians (22), accurate and early identification of the pathogenic bacteria is crucial for the effective selection and use of antibiotics, and may lead to improvements in the management of patients with LRTIs worldwide.

Next-generation sequencing (NGS) has been used to identify causative pathogens in various infectious diseases (23-27). It has been proven to be useful for identification of causative organisms in several diseases, such as encephalitis, hepatitis, bloodstream infection and acute myocarditis (28-31). NGS has already been used to detect pathogens from BALF samples. A recent study detected significant reads of candidate pathogens in 34% of BALF samples collected from patients with infectious as well as non-infectious diseases (32). In another study, NGS was used to detect pathogens in the BALF of patients with respiratory infection, and the results suggest that comprehensive molecular diagnostics, as well as surveillance of pathogens in BALF can be achieved using NGS (15).

In the present study, NGS technology was used to evaluate its role in accurate detection and identification of pathogens in the alveolar lavage fluid of patients with lower respiratory tract infection, and the results were compared with those of routine pathogen cultures.

In the present study, the alveolar lavage fluid of 30 patients with a pulmonary infection was tested by routine bacterial culture and NGS, and the results of pathogen detection were compared. All 30 patients showed lung consolidation in the X-ray film (Table I), and tested positive with NGS, whereas the conventional bacterial culture method provided positive results in 14 patients only (46.6%), with the remaining 16 patients (53.3%) being negative. This reflects the serious restrictions of the current diagnostic standards for infectious diseases in the clinic, where only routine bacterial culture is used to determine the infection. In 12 cases, NGS test results were in complete accordance with the bacterial culture results, indicating that in those double positive cases, NGS is compatible to the bacterial culture method. In addition, NGS detected 56 mixed infections of pathogenic microorganism, which had important guiding significance to the clinical diagnosis and treatment, compensating for the deficiency of the conventional bacteriology inspections. The routine bacterial culture method is still important as it can distinguish live bacteria from dead ones (34). However, there are a few possible reasons for the lower microbial detection rates: i) The sampling and transition conditions may cause some of the bacteria to die, thus no detection and ii) antibiotic use before sampling may kill some of the bacteria. The handling errors can be minimised through standardisation of the procedures and increased training for personnel. Antibiotic administration should be avoided before sampling, and if it has been used before sampling, then the use can be noted in the patient's medical history, so that this would be accounted for when analysing the final microbial culture results. Conversely, although NGS cannot distinguish between dead and live bacteria, it is still an important additional tool worth exploring, as it does provide additional information and thorough pathological investigation. The results from the NGS performed in the present study are consistent with the Shaanxi Province local etiology epidemic spectrum (35).

These results were in line with the previous literature, confirming that NGS detection can improve the positive rate of pathogen detection and provide guidance for treatment in clinical practice. Li et al (36) performed NGS detection on BALF samples from 32 patients, and the positive detection rate of NGS reached 100%. Compared with the culture method, the diagnostic sensitivity and specificity of NGS were 88.89 and 74.07%, respectively. Cummings et al (37) indicated that for complex microbial specimens, bacterial culture may not be able to identify specific microorganisms, but NGS could detect a wider array of organisms than any other single method alone. A retrospective study aiming to evaluate metagenomic NGS (mNGS) for the clinical diagnosis and prognosis of acute respiratory distress syndrome (ARDS) caused by severe pneumonia, reported that the total positive rate detected by NGS method (91.1%) was significantly higher than that detected by the culture method (62.2%). The researchers concluded that, mNGS is a valuable tool to determine the etiological value of ARDS caused by severe pneumonia, providing more diagnostic evidence that leads to improving diagnostic accuracy and prognosis for this disease (38). An additional advantage of using NGS is that it identifies anaerobic bacteria that are not usually searched for by clinical laboratories (39).

Early usage of NGS has proven to positively impact the mortality rate as proven by Zhang et al (38). After adjusting the treatment regimen based on the microbiological test results, the 28-day mortality rate in the NGS group (composed of patients tested using NGS and conventional microbiological tests) was significantly lower than that in the non-NGS group (composed of patients given conventional microbiological tests only). NGS significantly reduced the length of stay in the ICU of immunosuppressed patients, shortened the duration of ventilation and reduced the cost of ICU hospitalization (P<0.05) (38).

In a recent study, comparing NGS with traditional pathogen detection methods in the diagnosis of peripheral pulmonary infectious lesions, NGS could detect and identify a large variety of pathogens, with identification of at least one microbial species in 89% of the patients with a pulmonary infection. The accuracy and sensitivity of NGS surpassed those of traditional pathogen detection methods. NGS also detected microbes related to human diseases in 94.5% of samples from patients with pulmonary infection who tested negative when assessed by traditional pathogenic detection methods (40).

As NGS is an accurate and complete method for detection of all the DNA information in a sample, it can be applied to the detection of various pathogenic microorganisms, including bacteria, fungi and viruses (41,42). It can quickly diagnose Acinetobacter baumannii infection (43), directly detect and confirm pneumocystis from clinical specimens (44) and has the ability to detect a variety of fungi (45). Chen et al (46) applied NGS technology to diagnose 9 cases of severe parrot fever pneumonia, and the patients were treated in a timely manner. The results of a prospective multicenter study showed that among 329 adults with severe community-acquired pneumonia, the detection rate of NGS was 90.3% (47). Cumming et al (37) indicated that for complex microbial specimens, bacterial culture may not be able to identify specific microorganisms, but NGS could detect a greater variety of organisms than any single method alone. This has also been confirmed in other studies that have shown that NGS is considerably more sensitive in the diagnosis of mixed pulmonary infection than conventional methods (97.2% vs. 13.9%) (40,48,49). Furthermore, NGS has been proposed as an important auxiliary diagnostic method for rare pathogens (50). Currently, the new corona pneumonia is also diagnosed by NGS, which can complement and improve clinical pathogenic diagnosis (51). Therefore, NGS is a good pathogenic monitoring method and is worthy of extended application in clinical settings.

For pneumonia patients who are not suitable for bronchoalveolar lavage, the NGS test of tracheal secretions can also provide ideal results (52). A recent study suggested that for bacteria and fungi detection, BALF NGS is more sensitive than blood NGS, whereas blood NGS can identify the pathogens of pneumonia, especially for some viruses (53). It is worth mentioning that NGS enabled complete sequencing of the long SARS-CoV-2 genome (~30,000 nucleotides). NGS has also provided insight into strain origin and viral evolution (54). The extensive application of NGS will provide powerful support in the fight against future public health emergencies. The application of these sequencing techniques could further benefit SARS-CoV-2 diagnostic development and novel therapeutic target discovery, as well as blocking interspecies transmission by identifying intermediate SARS-CoV-2 novel hosts (53).

The present study has local significance, providing new information regarding the pathogens present in patients with a lung infection in the Xi'an region. NGS has been used in several studies in Xi'an region for detection of pathogens in various infections. Hu et al (55) used NGS to detect SARS-CoV-2 in eye swabs. Lin et al (56) used NGS and a metagenomic approach to detect and characterize respiratory bacteria in an unexplained pneumonia case in an infant. Yang et al (57) used NGS to accurately diagnose granulomatous amoebic encephalitis caused by Balamuthia mandrillaris in a child. Li et al (58) used NGS along with electromagnetic navigation bronchoscopy to diagnose Mycobacterium chelonae. Xu et al (59) used NGS to characterize lncRNAs in 293T cells after human foamy virus infection. No comprehensive study has been performed to compare the detection of bacteria using NGS and routine culture. To the best of our knowledge, the present study is the first in the region, and may provide valuable initial data for future comparative and/or epidemiological studies.

In conclusion, early detection of pathogens can optimize antibiotic selection, and consequently improve survival rates in pulmonary infection. NGS is a new technology that uses high-throughput sequencing to sequence all the DNA in human samples. Subsequent quantitative and bioinformatics analyses of microbial species in the samples can provide a reliable basis for clinical diagnosis and treatment. The results of the present study show the prevalence of respiratory pathogens in Xi'an region and supports the implementation of NGS in clinical routine diagnosis for pathogen identification, especially when the results of routine bacterial cultures are negative. Timely sequencing in addition to routine bacterial culture testing will further assist in improving the accuracy of diagnosis and in administration of a more accurate antibiotic regimen.