## Bacterial Infections
### Cell Membrane Biomarkers  
Cellular membrane macromolecules, especially proteins, have proven to be a major pathogenic biomarker for SERS-based diagnostics [7; 79; 80; 81; 84; 85; 86; 87; 88; 89; 91]. Cell membrane proteins have been used as biomarkers to detect various bacterial infections such as Methicillin-resistant _Staphylococcus aureus_ (MRSA) [73; 86], urinary tract infection (UTI) [85], and step throat [88]. For instance, [PERSON]'s group used silver nanoparticles in solution for the direct, label-free SERS detection of proteins and nucleic acids in MRSA [73]. Cell membrane proteins in _Escherichia coli_ and _Proteus mirabilis_ cells that cause urinary tract infections have also been detected using intrinsic Raman signature (e.g., ring breathing/stretching, carbohydrate, and protein vibrations). In this study, the required sensitivity for intrinsic SERS-based detection was achieved using a SERS platform that was composed of silver nanoparticles decorated on a glass substrate [85], Assay development for cell membrane biomarker detection has been integrated into microfluidic devices [7; 79; 3] and in solution assays [80; 81]. For instance, in 2013, [PERSON] et al. reported the use of intrinsic SERS signature a cell membrane biomarker to detect MRSA with a 10 CFU/mL limit [86]. The SERS spectra produced from the MRSA cell wall consisted of peaks corresponding to ring breathing of tyrosine protein, saturated lipids, and other vibrational modes of cell membrane biomarkers. In order to achieve such high sensitivity in label-free detection, they used graphene oxide modified with an aptamer that selectively binds to Enterotoxin B, which is a prominent MRSA protein, and gold nanopopcorn that provides good resolving power for SERS-based detection. It is reported that a higher analytical sensitivity was able to be achieved because they used the hybrid graphene oxide-based SERS substrates, which exhibited around 2 orders of magnitude higher SERS enhancement than only gold nanoparticle-based SERS substrates. They attributed the higher SERS enhancement effect originated from the hybrid graphene oxide-based SERS substrates to the contribution from both the electromagnetic and chemical enhancement effects simultaneously. This is an example that displays the improvements of the analytical sensitivities achieved by tailored designing of the SERS substrates. In SERS-based diagnostic methods, protein biomarkers offer more advantages; (i) the ability to capture bacterial antigens using antibodies and (ii) the availability and accessibility of a larger quantity of proteins in comparison to nucleic acid biomarkers. [PERSON] et al. used P30 and P65 proteins to detect and differentiate _Mycoplasma pneumoniae_ strains FH, M129, and II-3 from clinical throat swabs and cultured bacteria [80]. They observed a variation in the concentration of proteins of interests between different strains of _M. pneumoniae_. Their results showed \(>\)97% accuracy in identifying the strains from the throat swabs; thus, further displaying the clinical possibilities of SERS-based detection of protein biomarkers. This work shows the high sensitivity of SERS that enables protein biomarker detection that can even discriminate different bacterial pathogenic strains with increased specificity. Antibodies can act as good capture moieties due to specific binding affinity of proteins to antibodies, which ensures high analytical specificity for proteins biomarkers. Peripheral proteins, or surface proteins, are typically targeted as they can be easily accessible by nanoparticles to come in close proximity for SERS signal enhancement [79; 80]. If a specific integral protein or a protein naturally found inside the cell is targeted, then the sample preparation for that method would require protein extraction from the bacterial cells. Added sample preparation can result in POC diagnosis being out of reach, however, protein has still proven to be a good biomarker for possible diagnosis in clinical settings [77; 80; 82; 84; 85; 86; 88; 91]. [PERSON] et al.