Typhi colonization factor (Tcf) genetically conserved yet functionally diverse

KEYWORDS:

• Salmonella
• Tcf
• non-typhoidal Salmonella
• fimbria
• host-tropism
• typhoid

This article refers to:

The Typhi colonization factor (Tcf) is encoded by multiple non-typhoidal Salmonella serovars but exhibits a varying expression profile and interchanging contribution to intestinal colonization

Salmonella enterica is a Gram negative species of bacteria containing more than 2600 serovars. Although they are genetically similar, differ in host-specificity and clinical manifestations. Based on the clinical perspectives they are broadly classified into typhoidal and non-typhoidal Salmonella (NTS). Typhoidal Salmonella comprises of serovars Typhi, Sendai and Paratyphi A, B, or C. These serovars are highly adapted to humans and cause enteric fever, commonly known as typhoid fever. The typhoidal strains of bacteria are strictly transmitted among humans and the disease is endemic to developing nations due to lack of clean water and poor sanitation. On the other hand, NTS infections such as S. Typhimurium and S. Enteritidis are pathogens with broad host specificity, cause self-limiting gastroenteritis in humans and are prevalent globally. The major sources of NTS-infection in humans are from animal-derived food products, contaminated-fruits, -vegetables and can be transmitted from animals. However, in 5% of the infected individuals, NTS can breach the intestinal epithelium and cause systemic invasive disease referred to as Invasive NTS (iNTS).¹ iNTS infections are normally associated with immunocompromised conditions such as old age, malnutrition, clinical malaria and HIV infections and elicits an enormous burden especially in sub Saharan Africa.²

The propensity of these pathogens to colonize the intestine primarily depends on the fimbriae (hair-like structures that bind to host-receptors).³ Most of the fimbriae of Gram negative bacteria belong to the chaperone-usher pathway. Chaperone present in the periplasmic space ensures correct folding of the pilin-subunits of fimbriae and an outer-membrane protein called the usher translocates the subunits to the outer membrane and polymerizes into long, linear fimbria.⁴ Typhi colonization factor (Tcf) is a chaperone-usher fimbria expressed by typhoidal strains and has been thought to play a role in host-tropism.⁵ However, more recent studies have shown that it is also expressed by NTS serovars but its regulation and role in NTS-pathogenicity remains elusive.⁶ Azriel et al., have tapped into the available genome resources and identified that NTS serovars contain tcf cluster (tcfA B C D tinR tioA) in a similar genetic orientation as that of S.Typhi and S.Paratyphi; sandwiched between sinR and pagN genes. Although the tcf cluster among the Salmonella strains is highly conserved in genetic orientation and sequence, the expression level of tcfA is highly varied among the NTS strains and dependent on the growth milieu. More specifically, they demonstrate that tcfA expression is highest in S. Infantis among the NTS strains when grown in Luria-Bertani (LB) medium compared to the M93 minimal medium. Furthermore, they provide an interesting correlation that tcfA expression in S. Infantis is elevated when grown in nutrient rich and microaerobic conditions mimicking that of the intestinal milieu of warm blooded animals.

Having found that tcfA expression is dependent on nutrients and oxygen availability, Azriel et al have questioned the regulation of tcf by the well-known bacterial-regulators reported to modulate oxygen homeostasis, Salmonella pathogenicity and leucine-responsive regulatory protein (Lrp), which has been previously demonstrated to regulate the expression of fimbriae in E. coli⁷ and Salmonella.⁸ They convincingly show that Lrp represses tcf in S.Infantis and have identified putative binding sites for Lrp in the promoter region of tcfA. In contrast, Fur was shown to represses tcf in S. Typhi.⁹ Therefore, it remains to be known if the regulation of tcf is specific to serovars of Salmonella and other subunits of the cluster are regulated by a similar mechanism.

Despite the differential regulation of Tcf expression reported, the pathogenic function is conserved between S. Typhi and S. Infantis. Nevertheless, Tcf is dispensable for the adhesion or invasion of S. Infantis and S. Typhi in in vitro grown epithelial cells or macrophages. Tcf is also nonessential for the intestinal colonization of S. Schwarzengrund or S. Heidelberg. However, lack of Tcf results in impaired colonization of S. Infantis in the colon and cecum of mice. The authors attribute the discrepancies to the differential expression of tcfA observed. Moreover, it is important to note that the authors have deciphered significant diversity in the N-terminal region of the Fimbrial adhesion factor TcfD.⁹ Hence, it remains to be investigated if TcfD plays a predominant role in defining host-tropism. On the other hand a recent report from the laboratory of Roy Curtiss 3rd describes functional redundancy in the roles of various fimbriae associated with the virulence of S. Typhimurium,¹⁰ which could possibly explain why Tcf is dispensable for the intestinal colonization of S. Schwarzengrund or S. Heidelberg.

The ability of Salmonella strains to infect various types of hosts and varied clinical outcomes depends on the bacterial factors that enable host specificity. Therefore, unraveling the previously unknown function of Tcf in the pathogenicity of NTS paves way for future studies to understand how fimbriae contributes to disease and vaccine design.

Acknowledgment

Work in the lab of NR is funded by the Deutsche Forschungsgemeinschaft (DFG) Excellence cluster CECAD, University of Cologne (CECAD Research Grant D01), and grants from DFG (SFB 670), Köln Fortune and Maria-Pesch Foundation of the University of Cologne, Germany. Chiara Calabrese from the lab of NR critically read the article. The authors declare no competing financial interests.