By David N. Leff

Burglars and law enforcers aren¿t the only forms of life that resort to breaking and entering ¿ or exiting.

The smallest life forms of all ¿ bacterial viruses ¿ need to penetrate the stout outer cell walls and fragile inner membranes that envelop and protect those prokaryotic pathogens from the cruel world out there. Once safely inside ¿ by tricks not well known ¿ a bacteriophage spawns its hundreds or thousands of progeny phages.

But what goes in must come out. To escape its bacterial host¿s interior in order to move on and infect the next host organism, newly hatched bacteriophage progeny resort to various strategies for breaking and exiting.

But not all phages are created equal.

These ultra-tiny but genomically sophisticated parasites in bacteria come, roughly, in two persuasions ¿ larger and smaller, double stranded or single stranded, DNA or RNA. Some of the hulking, brute phages escape by smashing right through their host¿s cell wall, whereas wimpier-sized ones take a more passive-aggressive approach that requires only a single gene.

One of the small bacteriophages is well known to virologists as Q-beta. A pioneer in this field is Ryland Young, a biochemist at Texas A&M University in College Station. He is senior author of a paper in the current issue of Science, dated June 22, 2001, titled: ¿A protein antibiotic in the phage Qb virion: Diversity in lysis targets.¿ One of the article¿s two co-lead authors is postdoctoral fellow Ing-Nang Wang.

¿We found,¿ Wang told BioWorld Today, ¿that the way RNA bacteriophage Q-beta lyses its host is similar to the way penicillin-class antibiotics inhibit bacterial cell wall synthesis. After 50 years,¿ he added, ¿we still don¿t know how penicillin lyses its target host, how it penetrates the cell wall and membrane.¿

Most bacteria are wrapped around by a tough wall composed of a peptide-sugar (peptidoglycan) matrix. This integument shields the pathogen¿s inner membrane, and helps maintain its shape. The co-authors discovered a protein within the small Q-beta phage that blocks the ability of the bacterium to synthesize its tough outer wall. So the pathogen blows up, in effect, rather than dividing into more pathogenic cells.

Phages Peek At Antibiotics¿ Hymnal

¿Our finding,¿ Wang continued, ¿provides a new approach for designing drugs to combat many serious bacterial diseases, including E. coli, pneumonia, staph infection, ear infection, Lyme disease and cholera in humans, as well as bacterial diseases in pets, livestock and crops.

¿This protein antibiotic,¿¿ observed Young, ¿is the answer to an old mystery: How Q-beta and other small phages kill bacteria. Basically, they let their host cell commit suicide by dividing without making a new cell wall.¿

He made the point, ¿Ideally, the small bit of protein responsible could be mimicked by a pharmaceutical company, and a drug designed to be general against many bacteria, or specific against a certain pathogen. Even better, it could easily be modified to overcome antibiotic resistance.¿

Wang seconded this prospect by pointing out: ¿As bacteria¿s natural enemies, the potential of bacteriophages as sources for ways to kill those pathogens should have been thoroughly explored long ago. But it¿s only now, with the emerging worldwide crisis in antibiotic resistance, that phages are finally getting attention in their own right. It looks as if small phages are a gold mine for protein antibiotics.¿

Q-beta isn¿t the only nugget in that mine. ¿The important thing,¿ Young observed, ¿is that this is the second small phage we have found to make a protein antibiotic, and other people in our lab are working on a third. Surprisingly,¿ he added, ¿each of these phages makes a different type of cell wall poison, and each is a potential new model for an antibiotic.¿

Wang noted, ¿These are small viruses, and they have a limited amount of genome that can code genes. Q-beta has only four genes, unlike large phages, which run to 160 kilobases. We didn¿t know how Q-beta could get out of the cell, at the end of its replication cycle, because we couldn¿t find any lytic activity in their genes that allows it to punch holes in those cells. So we answered the question by showing that the way they exit is by interfering with cell wall synthesis, which results in cell lysis.¿

This interference with the bacterial enzymes that synthesize precursors of the peptidoglycan sheath, the principal component of bacterial walls, leads to a misconnect between that synthesis and wall construction. The resulting loss of wall weakness causes it to collapse from interior osmotic pressure.

Phage Q-beta, instead of encoding one protein dedicated to cell lysis, produces a multifunctional maturation phage capsid protein called A₂. This 41-kilodalton polypeptide not only lyses bacteria but also binds the phage to the bacterium¿s ¿male¿ sex pilus during infection. As Wang put it, ¿A₂ gives its bacterial host a venereal disease.¿

He concluded: ¿By looking at this bacteriophage, we are looking at potentially new resources, new revenue, for finding new classes of antibiotics, because we know existing antibiotics are usually found in fungi ¿ as are the penicillins ¿ or in bacteria.¿

100,000,000,000,000,000,000,000,000,000,000 Phages?

With three small bacteriophages now on their lab¿s front burners, the Science paper summed up: ¿Small-genome phages thus appear to accomplish host lysis by elaborating polypeptides that inhibit murein [peptidoglycan] synthesis at different steps. This raises the attractive possibility that DNA-encoded oligopeptide antibiotics, subject to facile genetic manipulation, might be designed on the basis of these lysis proteins. Moreover, given that all three prototype small-genome phages attack three different steps of the murein synthesis pathway, it also suggests that a search for new classes of small, lytic bacteriophages, not only of Escherichia coli, but also for other bacteria, is in order.¿

In an accompanying commentary titled ¿The Great Escape,¿ cell biologist Graham Hatfull, at the University of Pittsburgh, made the take-home point: ¿Given the enormous abundance of bacteriophages in the biosphere ¿ estimated to be around 10³¹ and the evident diversity of targets for bacterial lysis, bacteriophage lysins may represent a sizeable, untapped reservoir of new therapeutics.¿