By David N. Leff
A virus that kills the bacterium it lives on is on the way to joining up with antibiotics in a one-two punch against infection in humans. What¿s more, this secret bacteriophage weapon seems proof against succumbing to the drug resistance that threatens antibiotics today.
The secrecy lies in the hiding place where Streptococcus pneumoniae lurks, waiting to pounce. The notorious pathogen holes up by the millions in half of everybody¿s nasopharynx a moist warm area of mucous membrane between the back of the nose and the throat. From that colonizing sanctuary, the bacteria strike out and cause ear infections, pneumonia and meningitis.
The Centers for Disease Control and Prevention (CDC) indict S. pneumoniae as a leading worldwide cause of disease and death in young children, the elderly and immunocompromised people. It¿s a special scourge in nursing homes, hospitals and day-care centers ¿ environments that tend to foster bacterial infections. Because they use large quantities of antibiotics, the number of people infected with drug-resistant strains of S. pneumoniae continues to rise.
Until now, there has been no strategy to expunge S. pneumoniae from its safe, snug, smug nasopharyngeal reservoir, observed bacteriologist Vincent Fischetti, at Rockefeller University in New York. All the while, the bacterium¿s constant, strain-specific expunger, its bacteriophage, has been right at hand ¿ Fischetti¿s hand.
He is senior author of a paper in the current issue of Science, dated Dec. 7, 2001, titled: ¿Rapid killing of Streptococcus pneumoniae with a bacteriophage cell wall hydrolase.¿
¿For the first time ever,¿ Fischetti told BioWorld Today, ¿we have a reagent that can control colonizing pathogenic bacteria. We¿ve never had anything like this before. We¿re taking advantage of a substance ¿ an enzyme ¿ that bacteriophages naturally produce. These viruses have been associated with bacteria for millions of years, and we are just now utilizing them for our own purposes. We¿ve never used such a compound before, taking advantage of a phage enzyme to kill bacteria this way.¿
The killer enzyme in question goes by the acronym ¿Pal.¿ ¿It stands for Pneumococcal lytic enzyme,¿¿ Fischetti explained. He described how that viral protein, of 30,000 molecular weight, can wipe out its thousandfold bulkier symbiotic pathogen.
Unique Attack From Within Prey
¿When the S. pneumoniae-dedicated bacteriophage infects its bacterium,¿ Fischetti recounted, ¿it replicates inside that microorganism. It takes over the machinery of the bacterium to produce more virus particles. And at the end of that takeover process, which can last anywhere from 30 minutes to an hour, the phage has a problem: how to get out of its host bacterium. It solves the problem by using its Pal enzyme to kill the bacterium by cleaving the peptidoglycan of its cell wall. That releases the phage progeny to start the cycle again.
¿Inside the cell wall is the membrane, and since that wall is actually protecting the membrane, when Pal punctures a hole in the wall, the membrane comes out and by osmotic lysis, explodes. That releases its cytoplasmic contents, thus killing the bacterium.¿
Armed with this novel weapon, Fischetti and his co-authors ran in vitro and in vivo experiments to test Pal¿s capabilities. ¿The in vitro experiment,¿ he related, ¿was to take a culture of organisms in a test tube and add varying concentrations of Pal enzyme. Then we measured the viability of the bacteria in the tube. Basically, we could take 107 organisms ¿ 10,000,000 bacteria ¿ down to zero in a few seconds. Pal is probably the most active killing agent for a bacterium other than chemical agents.
¿In vivo, in mice, we predosed the animals with pneumococci, and five hours later treated them, via nose drops, with Pal. Then we examined their nasal mucous membrane to see if they were infected. After treatment with Pal, we were able to eliminate the infectious organisms speedily.
¿So far,¿ Fischetti went on, ¿we have not seen drug resistance. That¿s the other interesting feature of these enzymes. We¿ve looked very hard to try to identify resistant organisms, but we cannot find them. We believe it¿s the case because these viral enzymes are designed with a binding domain and a catalytic domain. The catalytic domain is specific for cleaving the peptidoglycan of the bacterial cell walls. The binding domain is specific for binding choline, a cell-wall receptor critical for the viability of pneumococci.
¿We believe the phage, once it penetrates into the bacterium, does not want to get trapped inside. So it has devised ¿ over the evolutionary eons of association with the bacterium ¿ the binding domain, which is a critical component of the bacterial cell wall,¿ he said. ¿Thus it becomes difficult for the bacterium to mutate away from that lipotrophic choline. So we believe that if this mutation to resistance is going to occur, it will be a rare event.¿
Clinicals In 6 Months; Start-Up In 3
Fischetti anticipates human trials of the antibacterial phage enzymes ¿probably in six months. We will likely have to do more preclinicals, showing them to be completely safe. Also, we would have to devise a more efficient way of producing large quantities of the enzyme. At present, we get it from recombinant clones in E. coli.¿
He noted that while current pneumococcal vaccine may confer lifetime protection against pneumonia on an individual, ¿it doesn¿t cover all the bacterial serotypes that are out there in the environment. And it doesn¿t eliminate your ability to carry pneumococci in your nasal cavity. So you could be a carrier for certain pneumococci, and spread that organism to other people. Using the nasal spray with the enzyme,¿ he suggested, ¿you¿d simply eliminate the carriage of this organism in the population, particularly the pediatric population.
¿We have 12 issued patents on the whole technology,¿ Fischetti pointed out. ¿And we¿re starting a start-up company in the next three or four months, called Enzybiotics.
¿Ours is a platform technology that can be used in a wide range of fields,¿ Fischetti summed up, ¿not only in the medical field but also in the food industry for decontamination of food, and the dairy industry for controlling organisms in milk and cheese production. Wherever there are bacteria that need to be killed, this technology can be applied, without the use of antibiotics.¿