Rats infested the little German town of Hamelin some 700 years ago.The impudent, swarming rodents "fought the dogs, killed the cats andbit the babies in their cradles."
So the town fathers hired an itinerant piper, who contracted to charmall the rats out of Hamelin by playing on his magic flute.
But when the musical rat-catcher claimed his fee, the mayor andcouncil welshed on the deal. So the swindled pied piper charmedaway all of Hamelin's children, who were never heard of again.
If those legendary rats had been infected with the bubonic plaguebacillus, Yersinia pestis, their mass departure would have left thefleas that transmit plague from rat to rat without a meal ticket. Crazedby hunger, the infected insects would have jumped on and bitten thenearest warm-blooded alternative, namely humans.
In historical fact, six hundred years ago, between 1346 and 1350A.D., plague blew away one-quarter of Europe's entire population,25 million victims _ presumably because a sudden die-off in the ratpopulation left the Yersinia-infected fleas no other dietary option.
In 1665, plague killed an estimated 100,000 people in London alone;the pandemic devastated other cities on the European continent aswell.
Today, improved sanitation and housing abate the danger of plaguepandemics. Y. pestis still simmers in parts of India, Peru and Africa,and 2,000 cases are reported annually worldwide. In the U.S. itinfects a dozen victims a year on average, virtually all in theAmerican southwest _ mainly Colorado, New Mexico and Arizona.Instead of rats, the pathogen's rodent vector consists of groundsquirrels and prairie dogs.
Each of these animals nourishes its own species-specific flea,observed medical microbiologist B. Joseph Hinnebusch, a stafffellow at the Rocky Mountain Laboratories (RML) of the NationalInstitute of Allergy and Infectious Diseases (NIAID) in Hamilton,Mont.
No Y. pestis In U.S. Inner-City Rats
"In this country," Hinnebusch told BioWorld Today, "urban rats arenot infected with plague. Wild rodents are much more important."
While modern medical science keeps plague infection under controlwith antibiotics and vaccines, it's only beginning to seek answers asto just how the rodent-flea-rodent transmission cycle works _ withthat occasional side-track when fleas in a feeding frenzy go afterhuman blood.
At RML, Hinnebusch said, "Our laboratory of microbial structureand function is looking at the basic biology, particularly themolecular genetics, and also the mechanism in the bacillus itself.We're asking what specific bacterial genes are required in the flea toset up colonization and transmission."
He and his colleagues are focusing on the hemin storage of Y. Pestis.Hemin is the iron-containing moiety of hemoglobin molecules thatbinds oxygen.
A trio of genes in Yersinia's genome, Hinnebusch explained, "encodethe bacterium's surface protein, which actively takes up hemin andstores it in its outer wall. That much is known.
"We had a likely candidate in those three hms genes," he added,"which together comprise less than one percent of the Yersiniagenome. Their 102-kilobase locus is a well-known phenotype [anorganism's specific characteristics] of the plague pathogen that waslooking for a function. We had some indication," he pointed out,"that hms might be important, simply because that phenotype isexpressed only at the lower temperature one would encounter in aflea, not in a mammalian host."
Hinnebusch is the primary author of a paper in today's Science titled:"Role of the Yersinia pestis hemin storage (hms) locus in thetransmission of plague by fleas."
He and his co-authors tested the effect on Oriental rat fleas(Xenopsylla cheopis) of mutant Y. pestis organisms lacking the hmslocus. Co-author Robert Perry, a microbiologist at the University ofKentucky, engineered these hms-minus variants by either outrightgene deletion or transposon insertion, which obliterated theirexpression.
Flea's Standing-Start Long Jump: 1.5 Feet
X. cheopis is the classical plague-spreading flea of past pandemics.Its overall length is about 3/32 of an inch, and it can jump 200 timesthat distance _ a good 1.5 feet. Once landed on a mammal's skin, beit rat or human, the blood-thirsty insect pierces and bites withfishhook-like mouth parts containing backward-pointing barbs that letit hang in there as it sucks away. The blood it swallows goes down itsesophagus.
After the flea ingests a blood meal from an infected rat, the bacteriamultiply in the insect's proventriculus, a spined foregut between theesophagus and midgut. When the growing mass reaches a volumethat blocks the proventriculus, the constipated flea can no longerpump blood down its esophagus and on to the midgut.
Starving to death, the desperate insect bites a new host. In its futileefforts to feed, it up-chucks clotted Y. pestis organisms from theblocked proventriculus into the bite site. This efficiently transmits theplague infection. The flea eventually dies of starvation.
"Those normal hms genes," Hinnebusch said, "were specificallyrequired to form this block, which is a well-known requirement fortransmission by fleas. The hms-minus mutants," he continued, "wereable to colonize the fleas' midguts, but were completely unable toform that blockage, so the bacilli would never be transmitted."
His Science paper calls these findings "the first genetic locus of anarthropod-borne bacterium that has been shown to be required fortransmission-competent infection in the vector."
Now Hinnebusch and his collaborators are going after the nextunknown: "We haven't completely established," he said, "why, orexactly how, the expression of those protein genes enables theblockage to occur in the fleas." It's the team's impression that "whenthose hms genes are expressed, the cells don't merely bind hemin butalso become very hydrophobic and auto-aggregative. The bacteriaexpressing those genes tend to clump, which physically block the fleagut.
"Acquiring those genes early in their evolution," Hinnebuschsurmises, "would have been a critical step in being able to utilize thefleas in their transmission."
He concluded: "Not many pathogens are transmitted by fleas,because very few bacteria have been able to pull that off." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.