# Proteomics Analysis in Dairy Products: Cheese, a Review
## 7 Starters and Proteomics
### Lactobacillus  
_Lactobacillus_ is the largest genus within lactic acid bacteria with over 170 species and subspecies and the most common starter used for fermented food products such as fermented dairy, meat and other foods, such as sourdough and yogurt enriched with probiotics. Aside from fermentation they are used as probiotics and in compounding bioactive and antimicrobial molecules [57; 58; 59]. The strains of _Lactobacillus_ are recognized as safe for human consumption and they are able to metabolize various carbon sources into lactic acid, a pathway with great significance in food technology [58; 60; 61]. The ability of _Lactobacillus_ to rapidly lower pH during lactic fermentation contributes to spoilage prevention and food shelf-life extension, while other metabolic activities improve organoleptic characteristics of food [62]. Since _Lactobacillus_ are widely used in food industry and are linked to human health, many experiments have been designed using comparative proteomics, in order to reveal those adaptive and metabolic mechanisms under optimal laboratory and food-like conditions. The latter aims to recognize all those surface-exposed proteins, also known as secretome, that differentiate each strain and make it suitable for starter or probiotic [57]. One of these studies had been conducted by [PERSON] et al. [53] and the authors tried to elucidate the metabolic and proteomic adaptive pathways of _L. rhamn_ strains isolated at different ripening stages of Parmigiano Reggiano (PR). In order to mimic the ripening environment in PR the strains' cultivation was conducted under optimal conditions (de Man, Rogosa, and Sharpe (MRS) agar) or under environmental conditions (cheese broth, CB). The protein identification was performed with MALDI-TOF-MS and multidimensional liquid chromatography (MDLC) coupled to nano-ESI-MS/MS to obtain comprehensive understanding of the mechanism of environmental adaptation in _L. rhamn_ strains [53]. A variety of compounds originating from ripening cheese, such as N-acetyl-D-galactosamine, D,L-lactic acid, thymidine, uridine, L-alanyl-L-histidine, glycolyl-L-methionine, and FAA, are catabolized by many strains of _L. rhamn_[63; 64]. The consumption of free amino acid was increased during the late stages of ripening by the predominant bacteria. The amount of proteins responsible for protein biosynthesis, such as Protein translation elongation factor (EF-G), Transcription elongation factor NusA (NusA), 30S ribosomal protein S3 (RpsC) and 30S ribosomal protein S6 (RpsF), was elevated in strains cultivated in laboratory conditions and isolated shortly after the beginning of ripening. CB grown strains (e.g., PR1473) showed increased levels of proteins (RPL1, EF-P, RP 50 SL5, Rps10, and Adt) with similar function to the above-mentioned ones. During cheese ripening, the proteolytic system of lactic acid bacteria (LAB) degrades caseins into small peptides and FAA, which fulfill the nutritional and organoleptic requirements [65; 66]. The results obtained from [PERSON] et al. (2012) [53] indicated that the amount of protease (PepS16), aminopeptides(PepA and MAP), endopeptidases (PepS, PepS24, and PepM16) and proline-specific peptides (PIP and PepQ) was increased during cultivation on CB. The _L. rhamn_ strains also demonstrated an increase in the amount of proteins responsible for amino acids catabolism (e.g., MetC, SDH), favoring the synthesis of lactic acid, acetic acid, and ATP via the pyruvate pathway. Cultivation on cheese broth resulted in augmentation of the enzymes responsible for the catabolism of aspartic acid (AspAT and OadA), glutamic acid (GDH), arginine (ADI), aromatic amino acids (ArAT), methionine and cystathionine (MetC), branched-chain amino acids (BcaT) and serine (SDH), as well as proteins responsible for citrate catabolism, acetate production, proteolytic activity, and amino acid catabolism.