SopA is expressed mainly at the early stages of infection. These results are consistent with data reported earlier (Drecktrah et al., 2005; Giacomodonato
et al., 2007; Patel et al., 2009) and indicate that the expression of SopB can be induced and maintained in vivo under environmental conditions different to those found in the intestinal milieu. In agreement, our in vitro experiments Thiazovivin showed that SopB can be expressed and secreted in growth conditions that resemble early and late intracellular niches (Fig. 1). Concurrently, we investigated the in vivo translocation of SopB in the cytosol of infected cells isolated from MLN during murine Salmonella infection. Gentamicin experiments revealed that 80% of bacteria recovered from MLN were intracellular. This result was confirmed by electron microscopy (data not shown). As shown in Fig. 3b (lane 2), Trichostatin A supplier translocation of SopB in infected cells recovered from MLN was evident for at least 24 h after animal infection coincident with the peak of expression (Fig. 3a). At later time points we were not able to detect SopB in the cytosol of infected cells. On the other hand, although SopA is expressed at day 1 (Fig. 3a, lane 1), it could not be detected in the eukaryotic cytosol of infected cells (Fig. 3b,
lane 6). Again, we observed that the dual effector SopD is translocated during the first 24 h after inoculation (Fig. 3b, lane 4). To the best of our knowledge, this is the first time that the translocation of Salmonella SPI-1 effector proteins has been assessed in vivo. Altogether, our results are consistent with those reported earlier showing that sopB continues to be transcribed and translated in vitro for many hours after bacterial internalization (Knodler et al., 2009). Our work acknowledges the significance of analyzing protein expression
and O-methylated flavonoid translocation, in vivo, in the context of bacteria–host interactions. For instance, attenuated Salmonella carrier vaccines have the potential to be used as delivery systems for foreign antigens from pathogens of viral, bacterial and parasitic origin (Everest et al., 1995). In this regard, Panthel et al. (2005) proposed SPI-1 and SPI-2 type III effector proteins as carrier molecules for heterologous antigens. Taking into account our results, SopB appears as an attractive carrier, potentially able to translocate heterologous antigens at different time points of the Salmonella infection cycle. Moreover, Nagarajan et al. (2009) have recently highlighted the importance of understanding the time and the compartment in which expression of SPI-1 and SPI-2 proteins occurs in selecting vaccine candidates; the authors proposed Salmonella Typhimurium sopB as a potential DNA vaccine.