Bacteria can use free sialic acids, a hydrolysate of glycoconjugates catalyzed by sialidases, as a source of carbon for their nutrition and colonization ( Figure 1A ) (Lewis et al., 2013; Agarwal et al., 2020; Agarwal and Lewis, 2021). Evidence from mouse models shows that free sialic acids released by sialidases promote the growth of group B Streptococcus and the spread of ascending vaginal tract infections (Pezzicoli et al., 2012; Gilbert et al., 2013). Bacteria lacking sialidase encoding genes can also benefit from sialoglycan in the vagina via sialidase producers such as G.vaginalis (Agarwal et al., 2020). Some bacteria, such as Fusobacterium nucleatum (Haines-Menges et al., 2015; Agarwal et al., 2020) and group B Streptococcus (Pezzicoli et al., 2012), have sialic acid transport or catabolic pathways despite being sialidase-negative themselves. Moreover, F.nucleatum can reinforce sialidase activity produced by G.vaginalis in both ex vivo and in vitro coculture studies. G.vaginalis titers exhibit a dose-dependent increase with higher inocula of F.nucleatum or increasing proportions of its cell-free supernatant in an in vitro coculture system of F.nucleatum and G.vaginalis, in which G.vaginalis could not survive itself. This suggests that F.nucleatum may secrete factors to facilitate G.vaginalis growth. Additionally, in comparison to cocultures with F.nucleatum, monocultures of G.vaginalis needed at least a 20,000-fold greater inoculum to be viable after an overnight incubation (Agarwal et al., 2020). Therefore, F.nucleatum and G.vaginalis form a mutually beneficial relationship based on their glycan cross-feeding mode, which promotes their colonization and contributes to vaginal dysbiosis.

There have also been reports of the cross-feeding between commensal bacteria in the gut. For example, Bifidobacterium breve UCC2003, which contains a functional Nan cluster for sialic consumption, can use the sialic acid produced by Bifidobacterium bifidum PRL201048 (Egan et al., 2014). Similarly, in the oral cavity, Streptococcus gordonii employs sialic acids as their only carbon source (Byers et al., 1996). During the coinfection of influenza and Streptococcus pneumoniae in the respiratory tract, sialic acids produced by influenza accelerate bacterial replication in vivo and stimulate pneumococcal proliferation (Siegel et al., 2014).

Sialidases can also promote infections by damaging the protective physical and biochemical barriers against pathogens through exposure of receptor binding sites for adhesins and toxins. In the oral cavity, adhesion of S.gordonii to oral epithelial cells is greatly increased by the presence of Streptococcus oralis in a sialidase-dependent manner through exposure of cryptic receptors binding sites (Beighton and Whiley, 1990; Wong et al., 2018).

Sialic acids are typically found at the terminal position of glycans. They can shield the underlying sugars (mostly galactose residues) from recognition, and then breakdown or adherence. Sialidases in the vagina may reveal glycan epitopes by the depletion of sialic acids and the exposure of underlying sugars to the surface ( Figure 1B ). In both N- and O-linked glycans, sialic acids cap Gal residues bound to N-acetylglucosamine (GlcNAc) or N-acetylgalactosamine (GalNAc), which is not accessible on the epithelial surface unless treated with exogenous sialidases or using cells from BV-positive specimens (Agarwal et al., 2023). Desialylation of glycoconjugates by sialidases causes loss of or reveal of new glycan epitopes, affecting microbe binding and host immunological recognition (Varki and Gagneux, 2012). Bacterial adhesion occurs when terminal sugars are exposed with the degradation of glycans, in which process carbohydrate-binding proteins like lectins, previously predicted in Gardnerella, serve as mediums (Bonnardel et al., 2021). According to genome screening, a greater repertory of carbohydrate-binding proteins is produced by vaginal bacterial species that are linked to infection and inflammation, which may allow them to bind a greater variety of glycans in the vagina. Compared with commensals like Lactobacillus crispatus, the mean number of lectins per strain is approximately 2-fold higher among those regarded as potential and confirmed pathogens (including Lactobacillus iners, G.vaginalis, Prevotella, group B Streptococcus, and Escherichia coli) (Bonnardel et al., 2021). With the deepening of research on the surface polysaccharide structure of the vagina and the bacterial carbohydrate-binding proteins, comprehensive insights into host–microbe interactions will be reached.

A biofilm is an organized community of microorganisms encased in a extracellular matrix made of proteins, polysaccharides, and nucleic acids, that attaches to a biological surface (Flemming et al., 2016; Jung et al., 2017) and contributes to the survival of bacterial infections (Del Pozo, 2018). Vaginal biofilms contribute to the persistence and recurrence of BV, as well as antibiotic resistance (Swidsinski et al., 2008; He et al., 2021). According to a recent research, 11 of the 24 G.vaginalis strains were able to form biofilms, providing themselves with advantages to evade host defense mechanisms and survive against antibiotics (Ma et al., 2022). An in vitro study suggests that most of the BVAB have a tendency to grow biofilms, and G.vaginalis has greater propensity to form a biofilm, enhancing its virulence potential through increased adhesion and cytotoxicity of epithelial cells compared to other anaerobes (Alves et al., 2014).

The lifecycle of biofilm formation is considered to include several stages: (i) adhesion to a surface, (ii) production of extracellular matrix, bacterial aggregation and biofilm accumulation until the development of a mature biofilm structure, and finally (iii) detachment (Joo and Otto, 2012). The initial adherence to vaginal epithelial cells has been acknowledged to be a necessary process to elicit BV (Swidsinski et al., 2005). As a dominant component of BV biofilm, Gardnerella spp. replaces pre-dominant L.crispatus, initiate bacterial colonization on vaginal epithelium and then serve as a scaffold for the attachment of other BVAB, including Atopobium vaginae (found in 80% of the samples and compromises 40% of the biofilm mass) and other heterogeneously mixed bacteria belonging to the Bacteroides, Corynebacterium, Lactobacillus, Staphylococcus, Streptococcus genera and so on (Castro et al., 2015; Castro et al., 2019; Swidsinski et al., 2005; Verstraelen and Swidsinski, 2013; Schwebke et al., 2014). The process is known as coaggregation ( Figure 1C ). Sialidases serve as a trigger at this initial stage of colonization. By means of its mucinase activity, the enzymes alter the characteristics of mucus discharges, catalyze them as a meal for bacteria and expose adhesion receptors on polysaccharides to promote bacterial colonization, increasing the potential for G.vaginalis to contact closely with the epithelium. Then early biofilm forms with the aggregation of other BVAB and the accumulation of extracellular matrix (Verstraelen and Swidsinski, 2013). Though sialidase A gene is not found to be associated with sialidase activity, it has been found to be associated with the presence of G.vaginalis biofilms, suggesting its possible contribution to biofilm formation (Hardy et al., 2017). There is still a lack of research comparing the expression levels of sialidase in biofilms and planktonic cells, which can provide us with deeper insights into the role of sialidase in biofilm formation. What’s more, interactions between the microorganisms within vaginal biofilms are worth investigating as sialidase activity may be affected by those sialidase-negative bacteria. Besides the finding that F.nucleatum and G.vaginalis benefit from each other, an in vitro dual-species biofilm model demonstrates that other BVAB, such as Actinomyces neuii and Enterococcus faecalis, can upregulate sialidase and vaginolysin expression in G.vaginalis to reinforce its virulence (Castro et al., 2019).

Similar findings of the involvements of sialidases in biofilm formation also presents in infections of other systems. In the early phases of pulmonary infection, Pseudomonas aeruginosa and its sialidases, existing on the highly sialylated surfaces of the upper respiratory tract, can target bacterial glycoconjugates, promote cell-cell interactions, and initiate biofilm formation (Soong, 2006). Viral sialidase inhibitors have demonstrated the ability to block the process of biofilm formation in clinical in vitro, suggesting a potential novel pharmacological target in bacterial pneumonia prevention (Soong, 2006). In Porphyromonas gingivalis, the main pathogenic bacterium in chronic periodontitis, the sialidase encoding gene shows a higher expression level than that in planktonic cells (Lo et al., 2009). Sialidase-deficient strains also demonstrates less and discontinuous biofilm formation compared with wild-type P.gingivalis strains (Xu et al., 2017).

The host-mucosa-sialidase can be regarded as a whole because sialidase functions on the mucosa. Sialidase is central to the suppression and overwhelm of host immune response. Meanwhile, it is also supported by other hydrolytic enzymes, vaginolysin, and immunomodulatory metabolites ( Figure 1D ) (Amabebe and Anumba, 2022). Sialylation of glycoconjugates, such as mucins, immunoglobulins (especially secretory immunoglobulin A, sIgA), and cytokines, cleave the molecules’ terminal sialic acids and uncover their carbohydrate residues to all kinds of glycosidases, thus making them more susceptible to proteolytic degradation and hampering immune response against bacteria (Cauci et al., 2003; Cauci, 2004). For example, during the incubation of sIgA and BV vaginal specimens, the release of products with lower molecular weight into the extracellular environment are observed and the phenomenon can be reproduced by adding three exogenous enzymes: sialidase, β-galactosidase and hexosaminidase, which suggests the deglycosylation and proteolysis of sIgA in BV (Lewis et al., 2012).

In BV-positive women with a specific IgA immune response against G.vaginalis hemolysin (Gvh, vaginolysin), increased cleavage of IgA and a 5-fold higher sialidase activity is observed compared to those with a weaker IgA response (Cauci et al., 1998). Later, another study reconfirmed that elevated sialidase and prolidase levels reduce this mucosal adaptive immune response. Vaginolysin, another virulence factor of G.vaginalis, is a cholesterol-dependent cytolysin (CDC) which forms pores on cell membranes, free host intracellular contents and disrupts genital epithelial cells (Morrill et al., 2023). The immunosuppression allows vaginolysin to fully carry out its cytolytic action, which results in the detachment and destruction of the vaginal epithelial cells that eventually produce clue cells (Castro et al., 2019).

High sialidases and prolidases levels are also associated with elevated vaginal IL-1β, leading to tissue damage and increased susceptibility to sexually transmitted infections (STIs) (Cauci et al., 2008). Despite that IL-1β stimulates IL-8 secretion, sialidase level is also inversely correlated to vaginal IL-8 and neutrophils, which inhibits neutrophil infiltration and the proinflammatory cascade (Cauci et al., 2008). According to in vivo research, BVAB can evade the immune response by either secreting molecules that aid in the breakdown of IL-8 or by suppressing the generation and stability of IL-8 (Santos et al., 2018). These findings suggest that in BV-positive women, sialidases contribute to the suppression of innate mucosal immunity.

However, BVAB induced the secretion of IL-6, IL-8, G-CSF, IP-10, MIP-1β, RANTES, and Gro-α, while lactobacilli did not in another study that used a coculture model to characterize the response of vaginal epithelial cells to a series of vaginal bacteria, including commensal lactobacilli and BVAB such as G.vaginalis, A.vaginae, Mobiluncis curtisii, and P.bivia (Eade et al., 2012). The results is consistent with that A.vaginae induces a robust proinflammatory response by elevating transcript levels of IL-6, IL-8, and antimicrobial peptide β-defensin 4 (Libby et al., 2008). It seems that BVAB trigger mucosal innate immune response, increasing production of cytokines and defensins to eliminate pathogens. But excessive inflammatory response might lead to a disturbance of the vaginal immunological barrier and increasing susceptibility to STIs (Doerflinger et al., 2014).

Furthermore, bacterial surface sialylation may serve as an immunological mask (Ram et al., 2017; Vimr and Lichtensteiger, 2002). It has been proposed that bacteria might be passed for host cells and evade the host’s immune system by incorporating the cleaved sialic acids into their cell surface structures (Varki and Gagneux, 2012). Differentiation between self-sialic-acids and close mimics is achieved through intrinsic lectins such as sialic acid-binding immunoglobulin-like lectins (Siglecs) anchored on most immune cells (Duan and Paulson, 2020). By engaging inhibitory Siglec‐5 and Siglec‐9, group B Streptococcus can escape from host immune responses (Carlin et al., 2007). Neisseria gonorrhoeae transfers sialic acid residues to its surface lipooligosaccharide (LOS) to achieve molecular simulation, which contributes to its serum resistance and complement resistance in all three pathways (classical, lectin, and alternative) (Ram et al., 2017). A study reports that vaginolysin is able to release the contents of cervical epithelial cells, promote gonococcal LOS acquisition of sialic acids, and evade complement attack through increased binding of the regulatory protein factor H (Morrill et al., 2023), suggesting that sialidases and vaginolysin are both crucial in the regulation of the LOS sialylation level and its pathogenic ability. Meanwhile, another study reports that desialylation of gonococcal LOS by sialidases in women promotes increased transmission of infection to men (Ketterer et al., 2016). These findings suggest that sialylation and desialylation may to have unique functions during the invasion of pathogens.

Elevated sialidase activity has been observed in BV CVF, suggesting that sialidases could be used as a promising biomarker for BV (Briselden et al., 1992). The presence of sialidase A gene was detected in all 24 G.vaginalis samples in a recent study (Ma et al., 2022), while another study reports an association between sialidase activity in molecular-BV (community state type IV, CST IV) and changes in the bacterial components of the local microbiome, assessed by using V3–V4 16S rRNA sequencing (Ferreira et al., 2022). Gardnerella, Atopobium, and Prevotella were among BV-associated the genera that were more prevalent in women with high sialidase activity (Ng et al., 2021). Increased sialidase may be attributed to the higher abundance of some BVAB that can produce sialidases by themselves, such as Prevotella (Briselden et al., 1992). At the same time, sialidases can impair the vaginal mucosal immune system, which creates a beneficial environment for the overgrowth of BVAB over the Lactobacillus spp. and increases bacterial diversity (Lewis et al., 2013).

As a biomarker for BV, sialidases could be used to develop new diagnostic tests as cheaper and quicker alternatives to the current standard clinical diagnostic tools. Current clinical diagnosis of BV is often based on the Nugent scoring system (Nugent et al., 1991) or the Amsel criteria (Amsel et al., 1983), both of which require microscopy and trained professionals. On the contrary, enzyme-based simple assays may be cheaper and quicker (Robinson et al., 2019; Wu et al., 2019; Cortés-Sarabia et al., 2020; Rodríguez-Nava et al., 2021; Avila-Huerta et al., 2023; Liu et al., 2023). Several new tests have been developed to detect sialidases. A comparison of their clinical diagnostic performance is shown in Table 1 . The most widely used is BVBlue test, a microscopy-independent bedside test that detects sialidase activity using ≥7.8 U as the cut-off value for diagnosis of BV (Myziuk et al., 2003; Bradshaw et al., 2005; Permsak et al., 2005; Madhivanan et al., 2014; Foessleitner et al., 2021). OSOM^® BVBLUE^® Test is a commercial chromogenic test that can rapidly detect elevated vaginal fluid sialidase activity, with excellent sensitivity and specificity compared to Gram Stain, and it is widely used in many parts of the world. Similarly, a sensitive colorimetric bioactive paper that changes its color from white to dark purple in the presence of sialidases demonstrates a quick reaction time and strong storage stability (Zhang and Rochefort, 2013), though its clinical performance in BV diagnosis was not evaluated. Although sialidase activity tests are performed clinically, the results are currently only used as references and not as a diagnostic criterion.

With the understanding of the molecular mechanism of sialidases and its association with the pathogenesis of BV, sialidases can be used as not only a promising diagnostic marker but also a pharmaceutical target through activity blockage using inhibitors (Keil et al., 2022). Sialidase inhibitors include transition-state analogue inhibitors, mechanism-based inhibitors, suicide substrate inhibitors, product analogue inhibitors, and natural product inhibitors (Keil et al., 2022), which can act on virus, bacteria, human and protozoa sialidases. Numerous natural compounds have been identified and examined for their ability to inhibit sialidases from human, bacteria and influenza viruses. As for bacteria sialidases, three novel compounds as potent inhibitors are isolated from Lespedeza bicolor and effect in a dose-dependent manner, among them the best inhibitor has an IC₅₀ (represents the compound concentration that causes 50% enzyme activity loss) of 0.09 μM (Woo et al., 2011). A recent discovery is a curcumin analogue against S.pneumoniae Nan A, whose IC₅₀ value is 0.2 ± 0.1 μM, exhibiting a 3-fold increase in inhibitory efficacy compared to curcumin (Kim et al., 2018). Natural products provide us with an alternate source for creating novel bacterial sialidases inhibitors and treating sepsis caused by bacteria infections, which are worth exploring for BV treatment. In our discussion of potential treatment options for BV, with G.vaginalis being the major pathogen, we will be focusing on bacterial sialidase inhibitors (Keil et al., 2022) and salic acid analogs (Agarwal and Lewis, 2021).