Here, we reported a case in which mNGS facilitated clinicians rapidly and accurately identifying E. faecalis in a patient with UTI. This patient was admitted to hospital due to high fever and chills. Traditional culture and serological testing did not determine possible infection etiology, while mNGS identified E. faecalis in the urine specimen. Combined with his clinical characteristics, his diagnosis was confirmed to be a UTI.
Enterococci are a class of bacteria typically found in the human gastrointestinal tract, mouth or vagina.
E. faecalis and
E. faecium are the two most common enterococci isolated in clinical samples [
12]. A survey indicated that
E. faecalis can be identified in about 80% of human infections [
13]. It is known to be one of the main causes of human UTI worldwide [
1]. In recent years, the incidence of UTI caused by
E. faecalis has been estimated to be five times than that of
E. faecium [
14]. In our case, the patient had coexisting kidney stones that are common in the urinary system. Theoretically, once the bacteria has invaded the urinary tract and contributed to urinary stone formation, it triggers UTI easily, and further develops chronic pyelonephritis [
15]. In addition to
E. faecalis, JC polyomavirus and human herpesvirus 6 were detected in the patient’s urine, but were not considered to have caused the patient’s UTI as both of them usually cause asymptomatic persistent or latent infection [
16,
17]. Therefore, clinicians need to have a good professional knowledge when interpreting mNGS findings. We employed mNGS to evaluate the treatment effects by comparing unique reads before and after treatment. Once the treatment was switched to Linezolid therapy, the patient recovered immediately after 3 days. In addition, the unique reads of
E. faecalis in our patient declined dramatically to 2 within 2 weeks, suggesting
E. faecalis may be the cause of the UTI, and further confirmed the treatment effects.
mNGS, as a non-biased method for rapid diagnosis of pathogens, overcomes many of the deficiencies of traditional detection methods, and directly performs DNA or RNA sequencing on samples [
11,
18,
19], which is increasingly being applied in clinical laboratories. Compared with other diagnostic methods, mNGS has many advantages, but also some limitations. A prominent advantage of mNGS is that it is a completely unbiased technology that can replace many target pathogen tests with a single mNGS assay, which targets all pathogens (bacteria, fungi, viruses, and parasites) in the specimens without the need for doctors to prejudge. Therefore, for the diagnosis and identification of some rare or unknown pathogens, mNGS has comparative advantages. In addition, mNGS is appropriate for a variety of specimen types, including peripheral blood, cerebrospinal fluid, tissue, sputum, and bronchoalveolar lavage, and could be implemented in the clinical practices of sepsis, immunosuppressive host with severe infection, severe pulmonary infection, rare or new pathogen infection, and other infectious diseases [
20,
21]. It would broaden the application of mNGS in the clinical practice and further bedside decision making. Furthermore, mNGS can greatly reduce the turnaround time for pathogen identification, and is more sensitive than the cultivation method [
19,
22]. Routine culture is a gold standard method for organism identification, but their sensitivity is often low due to prior antibiotics and antifungals exposure [
23]. Bacterial and yeast cultivation is generally time consuming, and fastidious organisms are not easy to culture. Additionally, for the identification of viruses or parasites, the role of cultivation is often limited [
24]. Therefore, multiple infections are often easily overlooked, and the detection of pathogens in unexplained fever patients is even more difficult for clinical diagnosis. mNGS has the capability to avoid the limitations of traditional culture tests allowing for quickly and effectively identifying the known and unknown pathogens in the specimen within 24–48 h [
11] and improving the clinical diagnosis rate [
20]. Many successful cases and studies have proved the great potential of mNGS in infectious disease diagnostics. Most articles are published as case reports, such as identifying
Leptospirosis [
25],
Bornavirus [
26],
Chlamydia psittaci [
27], varicella-zoster virus [
28],
Parvimonas micra [
29], and 2019-nCoV [
30] in the specimens using mNGS, compared to negative findings using traditional methods. In some multi-center or multi-sample research, mNGS has demonstrated better diagnostic performance for pathogens compared to culture or other methods for different disease types. For the diagnosis of sepsis [
31], severe pneumonia [
32], encephalitis and meningitis [
33], suspected focal infection [
22], and infection caused by immunodeficiency after transplantation [
18], mNGS can significantly improve the clinical diagnosis rate, and the sensitivity of pathogen identification is significantly higher than traditional microbial diagnostic methods. However, there are still many practical problems in the clinical application of mNGS. It is not easy to distinguish between colonizing bacteria, background bacteria and pathogenic bacteria among the various species detected [
21]. In our case, the detected microorganisms might be from the environment (
Meiothermus and
Comamonas), reagents (
Yarrowia and
Acinetobacter), consumables, or the surface of the patient’s skin (
Staphylococcus lugdunensis and
Malassezia), or elsewhere. Therefore, it is necessary to set up negative controls on the same batch of samples during the experiment and excluding background pathogens through the establishment of a large sample database in the early stage. Another disadvantage of mNGS is the amplification of host nucleic acids. More than 99% of reads generated by sample sequencing are from human hosts [
19], and microorganisms account for only a small proportion. Therefore, sequencing all nucleic acids reduces the sensitivity of pathogen identification. The host nucleic acids can be depleted by certain methods during wet experiments [
34‐
36]. Reducing the proportion of human-derived nucleic acid sequences can increase the data volume of microorganisms to a certain extent and increase sensitivity.
In conclusion, our case illustrated the potential application of mNGS in detecting pathogenic microorganisms in samples which were not detected by traditional culture and serological testing. This study suggests that mNGS could be implemented for monitoring the progress of the disease and evaluating therapy effects. It is believed that in the near future, as the cost of sequencing continues to decline, mNGS will be more and more widely used in clinics, benefiting more doctors and patients.