In this study, we analyzed the gene expression profile of atypical
S. flexneri II:(3)4,7(8) and compared it to that of ATCC 2b as a control strain. We analyzed more than two-fold changed genes. Of the two-fold upregulated genes, the major category of 344 genes was “not in COGs”. The “not in COGs” (including virulence plasmid genes) category contained 41 invasion-related genes of the
ipa-mxi-spatype III secretion system. Among the regulatory genes for the type III secretion system, virF and the MxiE regulon were specifically upregulated in atypical
S. flexneri II:(3)4,7(8). We also confirmed expression of these regulator genes by qRT-PCR in two more atypical
S. flexneri II:(3)4,7(8) and two more 2b serotype strains. Additionally, atypical
S. flexneri II:(3)4,7(8) showed a four-fold increased invasion activity in Caco-2 cells compared to that of typical strains but not in HeLa cells. In the human colonic epithelial cell line Caco-2, which differentiates into a polarized epithelium expressing a well-established brush border, the invasion process occurs through basolateral surfaces and has been used in an invasion study
[30,
31].
The type III secretion system is regulated at several levels. First, transcription of secretion genes is regulated by global regulators, such as H-NS, IspA, Mia, ParC, and ParE, which respond to changes in temperature, pH, or osmolarity
[32]. These global regulators did not show specific changes in the atypical
S. flexneri II:(3)4,7(8) strains. Thus, increased expression of the type III secretion system in atypical strains was low in relation to global regulation. Next, activity of the type III secretion system apparatus is regulated in response to specific stimuli, including contact with host cells, exposure to artificial compounds such as Congo red, and alterations in the growth environment. These stimuli were not altered in atypical
S. flexneri II:(3)4,7(8) strains or in the control strain. Thus, various stimuli did not increase expression of the type III secretion system in the atypical strains. Finally, transcriptional activators such as VirB and MxiE were required for proper expression of the type III secretion apparatus. The atypical
S. flexneri II:(3)4,7(8) strains showed more than 100-fold increases in MxiE and a 96-fold increase in VirB expression compared to those of the control strain; thus, increased type III secretion system expression of the atypical strain originated from increased MxiE and VirB transcription. Most
Shigella spp. show low transcription levels of these regulators and increase only when they contact host epithelial cells or are exposed to artificial compounds such as Congo red. Other reports have shown that increased secretion occurs when some genes such as
ipaB and
ipaD are inactivated, resulting in deregulated secretion
[17,
33]. Inducing or deregulating secretion activity also results in increased transcription of some genes encoding secreted proteins, such as the
virA and
ipaH genes in
Shigella spp.
[13]. In atypical
S. flexneri II:(3)4,7(8), the type III secretion system was induced by an increased expression of the MxiE regulon, but the exact reason for the increased expression of the virulence-related genes is still uncertain in atypical
S. flexneri II:(3)4,7(8). Increased type III secretion system expression and increased invasion ability of human epithelial cells of these atypical
S. flexneri II:(3)4,7(8) strains will be studied specifically in the near future, because this atypical
S. flexneri II:(3)4,7(8) strain is increasing in prevalence in Korea and shows increased antibiotic resistance.
These results of this study will facilitate functional studies of intracellularly regulated genes that may be important for adaptation and growth strategies of this atypical S. flexneri during infection.