CNS infectious diseases have a high morbidity and mortality . The incidence of bacterial ME and viral ME accounts for the majority of CNS infectious diseases [2,3]. However, typical pathogen identification methods have disadvantages of being time-consuming and expensive and have low sensitivity and poor specificity[7,8]; therefore, distinguishing bacterial ME and viral ME has been a clinical problem for a long time .
As a developing novel tool for the non-invasive detection of various disease states in recent years, VOC analysis has been applied in different body fluids, such as blood, urine, and saliva [12-15]. VOC analysis is important in the diagnosis of respiratory infections as an accurate and rapid method and has clinical value in guiding the use of antibiotics [19-21]. However, there are no studies to discuss the VOCs in CSF. Considering that the CSF is a type of human body fluid and is also a type of bacterial and viral microbiological culture in vivo, our study discriminated the VOCs of bacterial and viral ME samples to discover potential biomarkers for bacterial and viral ME in CSF VOCs, which may provide a detection method with greater specificity and accuracy.
Previous studies have determined that significant VOCs released by the pathogens mainly include aldehydes, alcohols, ketones, lipids and acids . In our study, a total of 361 metabolites were detected from the bacterial and viral ME group patients. There was an obvious difference in the ion peaks in the resulting chromatograms among them; however, to serve as a potential biomarker, the VIP value must be greater than 1.2 and the P value less than 0.05.
Filipiak et al.  found that the potential volatile biomarkers produced by E. coli include an aromatic compound - Benzonitrile. E. coli is a pathogen that causes bacterial ME. Raţiu et al.  found that the significant VOC of E. coli is indole, because it is generated at three time points and is also the most produced VOC. Other studies have also confirmed this finding [27-29]. Indole is an aromatic heterocyclic organic compound with the formula C8H7N. 5-Hydroxytryptamine (5-HT) is a monoamine neurotransmitter that is mainly decarboxylated by tryptophan in the human body. E. coli produces tryptophan decarboxylase and degrades tryptophan to indole and other compounds . Karami et al.  found that 2,3-pentanedione, 1-decene, 1,3-dimer, 2,5-dimethylpyrazine, ethyl butyrate and cyclohexene are specific VOCs of S. aureus in vitro. It is believed that the VOCs released by S. aureus have the ability to degrade amino acids in its growth environment . A study conducted by Ileana-Andreea et al.  described the significant VOCs of Mycobacterium tuberculosis, which were naphthalene, 1-methyl, -cyclohexane, heptane, 2,2,4,6,6-pentamethyl benzene, 1,3,5-trimethyl- and 4-methyl, but the specific mechanism was not yet clear. In the study by Chen et al , the growth of L. monocytogenes was found to be related to 3-hydroxy-2-butanone; however, the specific mechanism was not yet clear. The studies of other bacteria causing bacterial ME should be carried out in large quantities.
Studies have shown that the mechanism underlying viral pathogenicity is the induction of antiviral activity in host cells. After the virus invades the body, it can activate the innate immune cells to secrete a large number of pro-inflammatory factors, including interleukin (IL)-1, IL-6, IL-12 and tumor necrosis factor-α . These factors can induce inflammation, which is characterized by The acute phase of inflammation promotes the production of complement proteins, and the final result is the induction of apoptosis . Abd et al.  found numerous significant VOCs in bacterial and viral microbiological cultures. Dodecane (a type of alkane) was found in the virus group, which may be related to the degradation of cell membranes by lipid peroxidation and the conversion of polyunsaturated fatty acids to volatile alkanes, but the true source is unknown.
The most common pathogens of bacterial ME are L. coli, S. aureus, and Haemophilus influenzae . The major viral causes of viral ME include enterovirus (EV), cytomegalovirus (CMV), and herpes simplex virus type 1 (HSV-1) . The significant VOCs released by the pathogen provide , and every patient has different pathogens. In our study, we analyzed the VOCs of all ME patients but not those of the pathogens. The substances in microbiological cultures in vitro and in vivo are not exactly the same. In the future, we will culture the CSF of ME (especially common pathogens) and study the specificity of the significant VOCs released by different pathogens.
There are several limitations in this study. There is no specific division of the patients’ age structure, and more accurate research needs further practice. Due to the limitations of the conditions, this study did not analyze the VOCs of healthy volunteers. Last but not least, this study is a prospective pilot study, and there are few studies discussing the VOCs produced by viral cultures; therefore, a large number of studies are required to explore and verify the results of this study.