Characterisation of proteins extracted from the surface of Salmonella Typhimurium grown under SPI-2-inducing conditions by LC-ESI/MS/MS sequencing

Salmonella enterica has two pathogenicity islands encoding separate type three secretion systems (T3SS). Proteins secreted through these systems facilitate invasion and survival. After entry, Salmonella reside within a membrane bound vacuole, the Salmonella containing vacuole (SCV), where translocation of a second set of effectors by the Salmonella pathogenicity island 2 (SPI-2) T3SS is initiated. SPI-2 secretion in vitro can be induced by conditions that mimic the Salmonella containing vacuole. Utilising high-throughput mass spectrometry, we mapped the surface-attached proteome of S. Typhimurium SL1344 grown in vitro under SPI-2-inducing conditions and identified 108 proteins; using secretion signal prediction software, 43% of proteins identified contained a signal sequence. Of these proteins, 13 were known secreted effector proteins including SPI-2 effector proteins SseB, SseC, SseD, SseL, PipB2 and SteC, although surprisingly five were SPI-1 proteins, SipA, SipB, SipC, SipD and SopD, while 2 proteins SteA and SlrP are secreted by both T3SSs. This is the first in vitro study to demonstrate dual secretion of SPI-1 and SPI-2 proteins by S. Typhimurium and demonstrates the potential of high-throughput LC-ESI/MS/MS sequencing for the identification of novel proteins, providing a platform for subsequent comparative proteomic analysis, which should greatly assist understanding of the pathogenesis and inherent variation between serovars of Salmonella and ultimately help towards development of novel control strategies.