# Enhancing Disease Diagnosis: Biomedical Applications of Surface-Enhanced Raman Scattering
## 5 Applications of SERS in Disease Diagnostics
### Microbial Infections-Pathogen Detection
The differential characteristic SERS frequencies of chemical bonds are used to identify, discriminate and define phenotypes of infectious microbes at species and strain levels in just a few minutes, as demonstrated previously [18; 113]. Of the successful applications of SERS for microbial diagnostics, the report by [PERSON] and [PERSON] was the first to study urinary tract infection (UTI) [72]. In this study, 21 clinical isolates responsible for UTI, including _Escherichia coli_, _Enterococcus_ spp., _Klebsiella pneumoniae_ and _Proteus mirabilis_, were identified and classified accurately at species and strain level without recourse to DNA methods. To solve reproducibility problems linked to the simple mixing method applied by [PERSON] and other authors [72; 114; 115], [PERSON] et al. developed robust clinical grade fabricated solid Ag nanorods to classify molecular signatures of respiratory syncytial virus (RSV) associated with bronchiolitis [116], whilst [PERSON] et al. proposed aggregated Au/AgNP covered SiO\({}_{2}\) solid substrates to detect phenotypes of resistant etiological agents of sexually transmitted infections (STIs) in just 10 s [117]. In a further analysis, SERS was extended to diagnosis of aggressive fungal diseases, in order to discriminate among dermatophyte fungi strains responsible for mycotic infections using reproducible commercial Ag-coated NPs [118]. Recently, novel SERS employing in situ synthesis of NPs, wherein microbial cells act as templates for nucleation of NP, has shown much improved reproducibility of the detected microbial SERS signal. In this method when active microbial cells are soaked in an oxidant (e.g., AgNO\({}_{3}\)), metal cations are attracted to anions within cellular biomolecules to act as coordination centres. A reductant solution (e.g., NaBH\({}_{4}\)) is then introduced to reduce coordinated metal cations to form cell wall-bound monodispersed NPs [119; 120]. Intracellular deposition of NPs can also be achieved when oxidants and reductants are added in a reverse order [120]. This approach enhances clinical applicability of SERS as demonstrated for accurate identification and classification of clinical isolates of _E. coli_, _Bacillus_ spp. [120; 121], opportunistic _Staphylococcus epidermidis_[17], _Apergillus fumigatus_ and _Rhizomucor pusillus_[122], and to probe microbial cell functionality [119]. Intriguingly, an in situ SERS method proved to be very sensitive and effective for susceptibility assessment of clinical pathogens against common first line antibiotics treatment [17], to complement previous efforts [18; 123]. AMR is considered as one of the biggest public health threats where microbes elude antibiotics designed to kill them. AMR makes it difficult or impossible to treat some invasive microbial infections. To improve the management of endemic AMR, [PERSON] et al. developed a sensitive SERS biosensor to detect biochemical signatures and status of bacterial infections and AMR rapidly [17]. This SERS biosensor not only enumerated cells to assess infection severity but also distinguished live (resistant) from dead (sensitive) _E. coli_ cells challenged with antibiotics based on differential spectral signatures as illustrated in Figure 3; a result which is practically impossible to achieve by DNA tools like PCR [108]. Whilst lengthy conventional culturing methods can also differentiate live and dead cells for cultivable organisms, it gives false negative results for viable but non-culturable pathogens. The practical advantage with SERS here is that unlike in dead cells, metabolically active microbes can incorporate NPs to their biomolecules in the cell envelope or cytoplasm more effectively that provide larger and distinctive EF shown in Figure 3. This provides access to vital information about microbial viability state which is invaluable for tracking the response of causative microbes to prescribed antimicrobial therapy.