In earlier studies, phosphoglycerate kinase was reported on the surface of S. pneumoniae, was antigenic in humans, and elicited protective immune responses in mouse model  [see Additional file 6]. Also in Schistosoma mansoni, phosphoglycerate kinase has been identified as a protective antigen . Another surface protein, EF-G, identified in this study was found to be immuno-reactive against sera from broiler
chicken immune to necrotic entritis . The protein was secreted into the culture supernatant and unique to virulent C. perfringens strain CP4 causing necrotic entritis. Notably, EF-G is regulated Selleck PF-6463922 by the VirR-VirS virulence regulon of C. perfringens . Moreover, EF-G has been demonstrated as an immunogenic protein and was identified in both cell surface and extracellular fraction
of B. anthracis [9, 29]. Further, choloylglycine hydrolase family protein, cell wall-associated serine proteinase, and rhomboid family protein can be excellent surface protein markers for specific BAY 11-7082 in vitro detection of C. perfringens from environment and food as they share very low percent amino acid sequence identity with there nearest homologs (<50%) and are conserved among the C. perfringens strains [see Additional file 6]. Some of the surface proteins from C. perfringens ATCC13124 showed metabolic functions that would typically place them in the cytoplasm. Moreover, except for N-acetylmuramoyl-L-alanine amidase and cell wall-associated serine proteinase, these proteins have no N-terminal signal peptide and do not possess the canonical gram-positive anchor motif LPXTG [see Additional file 7]. Several surface-associated cytoplasmic proteins reported in this study were also detected on the bacterial surface in previous proteomic analysis [see Additional file 6]. For example, phosphoglycerate kinase was reported on the surface of S. pneumoniae , S. agalactiae , S. pyogenes , and S. oralis [see Additional file 6] and also as secreted protein in B. anthracis . Increasing number of reports have shown presence of proteins on the surface of Gram positive bacteria or secreted into the medium that one would otherwise
expect to be cytoplasmic [25, 29, 36, 37]. In a previous study, the culture supernatant of C. perfringens at the late selleck compound exponential Cepharanthine growth phase was shown to contain intracellular proteins that had no putative signal sequences, such as ribokinase, β-hydroxybutyryl-coenzyme A dehydrogenase, fructosebisphosphate aldolase, and elongation factor G . In other studies also, a significant number of cytoplasmic proteins have been identified as cell-wall associated proteins/immunogens [25, 37]. In spite of a growing list of cytoplasmic proteins identified on the bacterial surface, the mechanism of their surface localization and attachment to the bacterial envelope remain unclear. Internal signal sequences, posttranslational acylation, or an association with a secreted protein are hypothesized as possible means .