"Bring out your dead!"
That gruesome cry of a thousand years ago _ an offer to carry awaythe previous night's victims of the bubonic plague _ remains apersistent echo from those dark ages in our imaginations.
Today, we know what killed the victims piled on the wooden cartshauled away by those early public health officials: Yersinia pestis.But we are only just beginning to understand the fine details of howYersinia and other pathogenic bacteria like Salmonella defendthemselves against the body's immune system. How do they escapethe macrophages that hunt them?
The pathogens bring out their Yops.
Yops are virulence factors, sophisticated bits of molecular weaponrythat make today's weapons designers look like kids playing withdynamite, destructive but crude. Yops protect pathogenic bacteriafrom being eaten by phagocytes.
The code for Yops resides on large hoops of DNA called virulenceplasmids. Some Yops tear down the interior architecture of the whiteblood cells that would engulf and destroy them. Other Yopsdephosphorylate proteins of host cells that need to be phosphorylatedif the hosts are to survive and defend the body. Still other Yopsphosphorylate proteins that need to be dephosphorylated.
Pathogens, of course, tightly regulate the expression and secretion ofthese powerful defensive/offensive molecular weapons. They do notrelease them wastefully into an environment where they would do nogood (or harm, depending on your perspective as host or invader). Itnow appears that Yersinia bacteria, like Salmonella bacteria, turn onthe synthesis and release Yops when and where they will be mosteffective.
The trigger for expression of virulence genes appears to be directcontact with the surface of host cells, according to a report in theAug. 30 issue of Science, by Jonas Pettersson, et al., "Modulation ofvirulence factor expression by pathogen target cell contact."
The authors studied a cousin of the plague bacillus, Yersiniapseudotuberculosis. They equipped the bacteria with a type of signallight which would shine when Yop genes were expressed. Themolecular flashlight is the LuxAB protein which serves as a reporterfor transcription. It emits light upon exposure to n-decanal whengenes are activated. With the right microscope, scientists can use thistechnique to spot a light signal in an individual bacterium. Thetechnique was used to demonstrate that bacteria in contact withmammalian HeLa cells express Yops while those attached to glass donot.
Yop expression was previously shown to be inhibited by the levels ofcalcium typically found in the extracellular environment whereYersinia is known to lurk and replicate. Temperature also was knownto affect Yop gene expression. 37 degrees C is favored by thebacteria. This makes sense if you are a plague bacillus jumping froma cold flea into a warm human being. But body heat itself is notenough to initiate Yop gene expression in a calcium-richenvironment.
The Swedish scientists show that as long as Yersinia is in directcontact with the surface membrane of a host cell, the outside calciumconcentration does not affect the transcription of Yops.
The recent work suggests that after Yersinia docks with a host cell, itsends Yops directly into the mammalian cell by way of a specializedorganelle which serves as a channel.
"This is the first paper actually showing that we have increasedexpression [of Yop genes] upon target cell contact," senior authorHans Wolf-Watz, of the Department of Cell and Molecular Biology,University of Umea, in Umea, Sweden, told BioWorld Today.
Other experiments indicted that the bacterial-to-host contact results inthe export of a negative regulator of Yop expression. This molecularbreak on gene expression is known as LcrQ. With LcrQ out of theway, the bacteria can get on with the job of expressing Yop proteinsand shipping them into host cells to neutralize them. The workindicates that LcrQ is exported by a previously known mechanismcalled the Yop-type III secretion system.
Research May Yield New Antibacterial Agent
"When we talk about Type III secretion, we have shown that is animportant secretion system in Salmonella. And there is in fact a widevariety of different pathogenic organisms of both plant origin andeukaryotic mammalian organisms that have exactly the samesecretion systems. The obvious thing that comes to mind is that thiswould be an interesting goal as a new antibacterial target agent,"Wolf-Watz said.
The group at Umea now is focusing on identifying the ligand on thebacterium and the ligand on the target cell that triggers thisinteraction or up-regulation for opening of the secretion channel. Atthis time they have no hints or clues.
The potential biotechnology applications of this research areintriguing.
"There would be at least two different ways to deal with this. Onewould be to get the bacteria to `puke' at the wrong moment. This isso tightly regulated, it all occurs from the contact with the target cell.So, if you can open the system with a drug at the wrong moment, thatwould kill the bacteria or at least give the host a substantialadvantage during infection. They probably would not be able toattach the cell in fact. The immune system could then take care of thebacterium," Wolf-Watz said.
The other way to deal with the same target would be to block thesecretion, Wolf-Watz suggested. "If you think they have a gatedchannel, you put a plug in the channel." Thus either activating orshutting down the system at the wrong time could help the host andhurt the bacteria.
"The fact that this is so common among these bacteria suggest thatyou should in theory be able to find such an agent or agents. And ofcourse the advantage here is the normal advantage you find in suchstudies: you could foresee it would be difficult for the bacteria todevelop resistance toward this kind of agent since you really areattacking them at the weakest point. It is a specific mechanism theycan not afford to change because they would likely have to changethe mechanism itself," Wolf-Watz said. n
-- Dean A. Haycock Special To BioWorld Today
(c) 1997 American Health Consultants. All rights reserved.