30 March 2015
By Fernanda Schreiber
Flagella are noodle-like surface structures that bacteria use to move and to attach to cells in their hosts. Salmonella typhi, the bacterium that causes typhoid fever, has three different types of flagella. However, while one can be found all over the world, the other two variants are confined to Indonesia.
Why? What kind of advantage is obtained by having a particular type of flagellin? That was the question that kicked off my PhD.
Typhoid fever, which is caused by the ingestion of food or water contaminated with Salmonella enterica serovar Typhi (S. Typhi), remains an important health threat in lower-income countries with poor sanitation. Many travellers know about it, as they need to get vaccinated if they are going to exotic places.
The principal component of the flagellar filament is a protein called flagellin. This protein is used for Salmonella classification, with most S. Typhi strains producing Hd. However, isolates from Indonesia, a country with high incidence of typhoid, show alternative flagellins: Hj and Hz66. Hj arises from a naturally occurring deletion in the Hd fliC gene. Hz66, on the other hand, is encoded by a different gene, fljB, located on a plasmid.
Using strains of S. Typhi that had been modified to express the different flagellin variants, we looked at the structure of the flagellum and the ability of the bacteria to invade host cells.
Flagella from Hj bacteria were shorter and thinner than the other two variants, but that did not impair their ability to move around. In fact, Hj bacteria were more motile and they were more effective at invading epithelial cells. We thought this might be because the filaments of Hj were better at attaching themselves to cells. However, bacteria we had genetically modified without flagella could stick to cells just as well as their hairy counterparts.
We noticed that Hj bacteria induced massive ruffles on the membrane of cells, which could explain their increased ability to invade. When we infected cells lacking key genes responsible for ruffle formation with Hj, ruffles still formed. This leads us to suspect that Hj bacteria could be exploiting alternative signals to produce ruffles that other Salmonella do not use.
Hd expressing S. Typhi were better at invading macrophages, a type of white blood cell that engulfs and digests foreign substances, which could partly explain their worldwide success. However, additional factors such as host genetics and environmental factors may play an even greater role.
Looking at samples from typhoid patients infected with strains expressing Hj or Hz66 flagellins, we found that they were older than those infected with the more common Hd strains and they had anti-Hd antibodies. This suggests that those individuals had a prior encounter with an Hd strain, symptomatic or not, and that Hj and Hz66 strains are behaving opportunistically.
Could this explain why these two strains appear only in Indonesia? If people are infected with the common strain and develop antibodies to fight it, Hd expressing S. Typhi can no longer infect them. As the number of people infected in Indonesia is so high, the bacteria in this area may have been forced to mutate to ensure they could still get round their hosts’ defences. The Hj and Hz66 strains may be taking the opportunity to re-infect those who would previously have been immune.
This is an important consideration for those designing vaccines in this area, as protection from Hd strains may just make life easier for the Hj and Hz66 strains to thrive.
Fernanda Schreiber is a Posdoctoral Fellow working as part of the CTTV team on the development of intestinal organoids as models for IBD research. She completed her PhD in Gordon Dougan’s lab on host-pathogen interactions, in particular the role of flagella during Salmonella Typhi infection. After her PhD, Fernanda worked in the Cardiovascular Division in Addenbrooks looking at the effect of cholesterol loading in macrophages.
- Schreiber F (2015). The Hd, Hj, and Hz66 flagella variants of Salmonella enterica serovar Typhi modify host responses and cellular interactions. Scientific Reports. DOI:10.1038/srep07947