Bacteriophages are viruses that specifically infect bacteria and have no direct effect on humans. In fact, with an estimated 1031
phage on earth it would be impossible to avoid ingesting phage regularly. After replicating inside its bacterial host the phage is faced with a problem, it needs to efficiently exit the bacterium to disseminate its progeny phage to begin a new cycle. To solve this, double-stranded DNA phages have evolved a lytic system to weaken the bacterial cell wall resulting in bacterial lysis. Phage lytic enzymes or lysins are highly efficient molecules that have been refined over millions of years of evolution for this very purpose. These enzymes target the integrity of the cell wall, and are designed to attack one of the five major bonds in the peptidoglycan. With few exceptions,1
lysins do not have signal sequences, so they are not translocated through the cytoplasmic membrane to attack their substrate in the peptidoglycan, this movement is tightly controlled by a second phage gene product in the lytic system, the holin.2
During phage development in the infected bacterium, lysin accumulates in the cytoplasm in anticipation of phage maturation. At a genetically specified time, holin molecules are inserted in the cytoplasmic membrane forming patches, ultimately resulting in generalized membrane disruption,3
allowing the cytoplasmic lysin to access the peptidoglycan, thereby causing cell lysis and the release of progeny phage.2
In contrast to large DNA phage, small RNA and DNA phages use a different release strategy. They call upon a phage-encoded protein to interfere with bacterial host enzymes responsible for peptidoglycan biosynthesis4,5
resulting in misassembled cell walls and ultimate lysis.
The scientific community has been aware of the lytic activity of phage for nearly a century, and while whole phage have been used to control infection,6
not until recently have lytic enzymes been exploited for bacterial control in vivo.7-9
One of the main reasons that such an approach is now even being considered is the sharp increase in antibiotic resistance among pathogenic bacteria. Current data indicate that lysins work only with gram-positive bacteria, since they are able to make direct contact with the cell wall carbohydrates and peptidoglycan when added externally, whereas the outer membrane of gram-negative bacteria, with rare exception,10
prevents this interaction. This review will outline the remarkable potency these enzymes have in killing bacteria both in vitro and in vivo.
Most human infections (viral or bacterial) begin at a mucous membrane site (upper and lower respiratory, intestinal, urogenital and ocular). In addition, the human mucous membranes are the reservoir (and sometimes the only reservoir) for many pathogenic bacteria found in the environment (i.e., pneumococci, staphylococci, streptococci, hemophilus) some of which are resistant to antibiotics. In most instances, it is this mucosal reservoir that is the focus of infection in the population.11-13
To date, except for polysporin and mupirocin ointments, which are the most widely used topically primarily to remove methicillin resistant Staphylococcus aureus
(MRSA) and other colonizing staphylococci from the anterior nares, there are no anti-infectives that are designed to control colonizing pathogenic bacteria on mucous membranes14
; we usually first wait for infection to occur before treating. Because of the fear of increasing the resistance problem, antibiotics are not indicated to control the carrier state of disease bacteria. It is acknowledged however, that by reducing or eliminating this human reservoir of pathogens in the community and controlled environments (i.e., hospitals and nursing homes), the incidence of disease will be markedly reduced.11,14
Toward this goal, lysins have been developed to prevent infection by safely and specifically destroying disease bacteria on mucous membranes. For example, based on extensive animal results, enzymes specific for S. pneumoniae
and S. aureus
may be used nasally and orally to control these organisms in the community as well as in nursing homes and hospitals to prevent or markedly reduce serious infections caused by these bacteria. This has been accomplished by capitalizing on the efficiency by which phage lysins kill bacteria.15
Like antibiotics, which are used by bacteria to control the organisms around them in the environment, phage lysins are the culmination of millions of years of development by the bacteriophage in their association with bacteria. Specific lysins have now been identified and purified that are able to kill specific gram-positive bacteria seconds after contact.7,16
For example, nanogram quantities of lysin could reduce 107
by > 6 logs seconds to minutes after enzyme addition. No known biological compounds, except chemical agents, kill bacteria this quickly. Because of their highly effective activity against bacteria for the control of disease, the term “enzybiotics” was coined7
to describe these novel anti-infectives.