Science Editor

Salmonella, the bacterium that causes typhoid fever, owes its name in the medical textbooks to U.S. pathologist Daniel E. Salmon (1850-1914).

But in song and story, the eponym really belongs to Irish-born Mary Mallon (1870?-1938), known as "Typhoid Mary." That question mark after her birth date typifies her entire catch-as-catch-can career.

Mallon emigrated to the U.S. in her youth and became a cook. She also became a carrier of Salmonella typhi, and in the early years of the 1900s infected 51 people with typhoid, and caused three deaths. "Like Mary Mallon," observed Swedish research bacteriologist Mikhael Rhen, "there are cases that become infected with S. typhi, and roughly 3 percent of them are at risk of becoming carriers."

Rhen, an assistant professor at the Karolinska Institute in Stockholm, is senior author of a paper in the Proceedings of the National Academy of Sciences (PNAS) dated June 25, 2002, but being released online June 21. It's titled: "Polynucleotide phosphorylase is a global regulator of virulence and persistency in Salmonella enterica," a type species of the genus Salmonella.

"The main finding in this paper," Rhen told BioWorld Today, "is that the single point mutation in S. enterica - a fairly small change in the genome - is located so it can greatly alter the bacterium's pathogenesis. To inactivate its virulence totally wouldn't be that surprising. There seems to be a biological purpose to it: The bacterium doesn't lose anything in the transaction, but gains the ability to cause a persistent rather than a transient infection in the host, which otherwise would clear it."

Rhen defined this critical alteration: "It's a single point mutation that introduces a stop codon in a gene coding for a housekeeping enzyme in Salmonella. That enzyme, polynucleotide phosphorylase [PNP], degrades RNA. Its purpose is not to keep RNA forever, but produce a certain turnover, such that the enzyme is degraded once the gene's not needed any more. What this mutation does is sit in the gene encoding PNPase. We were surprised that the bacterium had actually gained a function. We spontaneously expected that it would be seriously crippled, but it was not."

Bacterial Carriers Don't Know They've Got It

Rhen pointed out, "In Salmonella, the carrier state is usually asymptomatic - as in Typhoid Mary's case. One niche where the bacteria resides," he added, "is the gall bladder. And in consequence the bacteria are re-secreted into the intestine. These carriers, even though they don't experience any symptoms, may secrete these bacteria, or retransmit the typhoid infection. But it's believed that persistence carriers - the ones with bacteria in the gall bladder - are at higher risk of developing hepatobiliary cancer. Generally, within a reasonable time frame, such a person may not experience any symptoms."

Rhen described his in vivo mouse model experiments: "The mouse is called BALBc.' It is not a transgene or knockout that we in the lab would have manipulated. But it's a natural knockout in one of the genes that defends some infectious diseases. These animals developed a disease that resembled typhoid fever," Rhen recounted, "after we infected them by mouth with cultures of the pathogen. We didn't have to manipulate the mice further to provoke the infection. We just returned them to their cages, and followed them up for a month or so.

"At the end of the experiment," Rhen continued, "we sacrificed the mice and determined - by culturing or not being able to culture bacteria from internal organs - how many of them were still infected. In this case, we looked at the gall bladder, liver and spleen, where the bacteria reside if the mice are persistently infected. Persistent carriers could also be recovered from the gall bladder. Roughly 30 [percent] to 40 percent of the mice become carriers. By comparison, around 3 percent of humans become persistently infected with S. typhi.

"The mouse model we are using," Rhen observed, "very much resembles the human infection. Typhoid Mary was presumably infected with another bacterial strain called S. typhi. The fact that we got such a high mouse carrier frequency indicates to me that what we have discovered is significant."

Rhen said, "The pathogenesis of systemic Salmonella is very complicated. The bacterium has managed things in a way that there are different sets of genes, which direct each step of the infections. Many of these genes are organized into large clusters. They are not spread around in the Salmonella genome, but concentrated on certain areas called Salmonella pathogenicity islands [SPI]."

Pathogenicity Islands Have Specialized Missions

"SPI 1, which is affected by this PNPase enzyme," he went on," codes for bacterial invasion. It directs the ability of the bacteria to penetrate the epithelial layers that protect ourselves from incoming bacteria, but then allows Salmonella to enter the host. The other pathogenicity island is called SPI 2, which is at a different location of the chromosome. And its genes are mainly involved in coding proteins that would enable the bacteria to resist macrophage defense functions. When the infection becomes systemic, the bacteria prefer to grow within macrophages.

"That is odd because macrophages also produce a lot of innate defense factors, which would kill the bacteria. And it's believed that the genes from SPI 2 - at least in part - code for functions that would enable the bacteria to prevent these macrophages from actually exerting their killing functions. So the bacteria manipulates the macrophage in order to make it a more friendly environment for the pathogen. And a couple of those functions, from the bacteria's point of view, are encoded by SPI 2."

As for therapeutic or immunological potential, Rhen noted, "It's not only Salmonella typhi in man that causes persistent typhoid fever infection. There's also Mycobacterium tuberculosis, which shares the ability to grow in macrophages. If we could learn how a bacterium avoids immune clearance during a long period of time, that might aid us to clear this infection from persons who are persistently infected. That would be very important in terms of breaking the transmission cycle," he concluded, "because we could clear out the infection from persons who are symptomatic but may present potential risks of spreading the disease."