In July 1976, the 58th annual convention of the American Legion foregathered at Philadelphia’s Hotel Bellevue. As the event began, the celebrating membership was feeling no pain. Then a sudden, swift, severe pneumonia laid low 182 of the attendees, and brought death to 29 of them.
At first, this unprecedented infection baffled the medical profession, but eventually the etiology of the deadly disease was tracked to the hotel’s rooftop air-conditioning water tanks, and the bacteria that incubated in them invading the convention via the ducts. That pathogen since became famously known as Legionella pneumophila. An estimated 8,000 to 18,000 Americans contract Legionnaires’ disease (LD) each year, and 5 percent to 30 percent of them die.
“Of community-acquired outbreaks of LD,” recalled cell microbiologist Craig Roy, at Yale University in New Haven, Conn., “the most recent that comes to mind is back in April of this year, at a Ford plant in Ohio. A number of auto workers came down with LD, and a couple of them died. The management had to close the plant down and decontaminate all water systems.
“In addition,” Roy continued, “there was a very large LD outbreak a while back at a flower show in Holland. They were using mister sprays, whirlpools and fountains to humidify the environment and create a greenhouse effect in the floral display room. The moisture became contaminated with aerosol-borne Legionella. Hundreds of people came down with symptoms, and there were quite a few deaths.
“When identified very early after infection,” he went on, “Legionnaires’ disease is readily treated with antibiotics. The problem arises because usually LD is not recognized until people develop severe pneumonia, and a number of them come into an area hospital with acute symptoms. At that point, the infection has progressed to a stage where antibiotic therapy may not be sufficient to save their lives.”
Roy, an associate professor of microbial pathogenesis at Yale, is senior author of a paper in today’s Science, dated Jan. 25, 2002. It’s titled: “A bacterial guanine nucleotide exchange factor activates ARF on Legionella phagosomes.”
Co-Conspirator: A Bacterial Protein Called RALF
“The unique thing about this paper,” Roy told BioWorld Today, “is that we’ve determined the existence of a prokaryotic bacterial protein that can be injected into a eukaryotic human host cell by means of a specialized secretion apparatus. This demonstrates that bacteria have the ability to drive the membrane-bound compartment in which they reside to a new location, helped by a protein named RALF, which is directly injected into the host cell. RALF functions by activating the host protein, ARF1, which is a key regulator of membrane transporting in eukaryotic host cells. It’s also involved in the transport of vesicles between two of the cell’s organelles its endoplasmic reticulum [ER] and the Golgi apparatus.
“Interestingly,” Roy explained, “Legionella is a bacterium that can somehow be transported to the ER, rather than being delivered to lysosomes in the cell for destruction digestion by macrophages.” He added: “No one really understood how it can do this, when the ER is an organelle that bacteria shouldn’t end up in under any circumstance. Yet, Legionella figured out how to get into that compartment, which provides us a real clue as to how the bacterium is accomplishing this. It’s not doing it by interfering with normal transport of the pathogen-digesting phagosome. Instead, it stimulates transport by recruiting eukaryotic host proteins, specifically ARF, which are key regulators of driving its compartments to the ER.
“In other words,” Roy went on, “Legionella is really building a Trojan horse here, that the eukaryotic cell thinks is a legitimate type of vesicle, and then brings this vesicle into a location that bacteria shouldn’t ever occupy. In doing so, the cargo of that Trojan horse is Legionella, and it’s delivered to the ER, where it now finds itself in a very rich and non-degradative nutrient environment. This results in bacterial proliferation in the eukaryotic host cell, thus allowing Legionella to cause an infection in humans, which leads to Legionnaire’s disease. But it also gives us clues as to many other bacterial pathogens that can avoid being delivered to lysosomes. So instead of being destroyed by their professional phagocytes, they may be redirecting transport of their membrane-bound compartment after internalization in the cell.
“This isn’t a question only for Legionella,” Roy pointed out. “It’s applicable to a wide variety of intracellular pathogens, including Chlamidia, Mycobacteria, Brucella, Rickettsia. In all of these microbes it’s believed that they’re somehow altering transport of their membrane-bound compartment, but in no case has it been determined that bacterial proteins enable them to do this.
“Another interesting aspect of our study,” Roy noted, “is that Legionella is ubiquitously found in freshwater environments, where they grow in protozoan host cells such as amoeba. When these bacteria gain access to the human lungs, they cannot discriminate between those simple unicellular phagocytes in nature, and the very sophisticated macrophages which are the first line of defense against infection in the human lung. In other words, the genes that encode the specialized transporter are absolutely essential for growth in amoeba as well as in macrophages. This means that Legionella must be targeting host proteins that have remained evolutionarily conserved. That also helps explain why the prokaryotic bacterium can actually parasitize such conserved eukaryotic host cells.”
On Track Of Multi-Pathogen Therapeutic Drugs
“Microbial pathogens usually have conserved mechanisms of virulence,” Roy observed, “meaning that if we can figure out how Legionella transports its phagosomes to a new location in eukaryotic host cells, and determine all the proteins that are essential to this process, we can begin to look at these other pathogens that are not so easy to study, and ask whether similar proteins are found in them. If we can identify a protein that’s shared by a whole array of pathogens, it would become a great target for anti-infective therapy a compound that could inhibit the bacteria’s ability to be transported to a permissive location within the human cells, and thereby block their ability to cause infection and disease.
“Therefore,” Roy concluded, “new compounds might be developed, eventually resulting in a drug that could fight a variety of pathogens, not just Legionella. That’s our ultimate goal.”