Microbiology and Immunology

Phage therapy WORLD OF MICROBIOLOGY AND IMMUNOLOGY

434

•

pathway is followed, transcriptionof the remainder of the

viral genes occurs, and assembly of the virus particles will

occur. The N protein functions in this process, ensuring that

transcription does not terminate.

The path to lysogenyoccurs differently, involving a

protein called cI. The protein is a repressor and its function is

to bind to operator sequences and prevent transcription.

Expression of cI will induce the phage genome to integrate

into the host genome. When integrated, only the cI will be pro-

duced, so as to maintain the lysogenic state.

The virus adopts the lytic or lysogenic path early fol-

lowing infection of the host bacterium. The fate of the viral

genetic material is governed by a competition between the cro

and cI proteins. Both can bind to the same operator region.

The region has three binding zones—cro and cI occupy these

zones in reverse order. The protein, which is able to occupy

the preferred regions of the operator first, stimulates its further

synthesis and blocks synthesis of the other protein.

Analysis of the genetics of phage activity is routinely

accomplished using a plaqueassay. When a phage infects a

lawn or layer of bacterial cells growing on a flat surface, a

clear zone of lysis can occur. The clear area is called a plaque.

Aside from their utility in the study of gene expression,

phage genetics has been put to practical use as well. Cloning

of the human insulin gene in bacteria was accomplished using

a bacteriophage as a vector. The phage delivered to the bac-

terium a recombinant plasmid containing the insulin gene.

M13, a single-stranded filamentous DNA bacteriophage, has

long been used as a cloning vehicle for molecular biology. It is

also valuable for use in DNA sequencing, because the viral

particle contains single-stranded DNA, which is an ideal tem-

plate for sequencing. T7 phage, which infects Escherichia

coli,and some strains of Shigellaand Pasteurella,is a popu-

lar vehicle for cloning of complimentary DNA. Also, the T7

promoter and RNA polymerase are in widespread use as a sys-

tem for regulatable or high-level gene expression.

See alsoBacteriophage and bacteriophage typing; Microbial

genetics; Viral genetics

PPhage therapyHAGE THERAPY

Bacteriophageare well suited to deliver therapeutic payloads

(i.e., deliver specific genes into a host organism).

Characteristic of viruses, they require a host in which to make

copies of their genetic material, and to assemble progeny virus

particles. Bacteriophage are more specific in that they infect

solely bacteria.

The use of phage to treat bacterial infections was popu-

lar early in the twentieth century, prior to the mainstream use

of antibiotics. Doctors used phages as treatment for illnesses

ranging from cholera to typhoid fevers. Sometimes, phage-

containing liquid was poured into the wound. Oral, aerosol,

and injection administrations were also used. With the advent

of antibiotic therapy, the use of phage was abandoned. But

now, the increasing resistance of bacteria to antibiotics has

sparked a reassessment of phage therapy.

Lytic bacteriophage, which destroy the bacterial cell as

part of their infectious process, are used in therapy. Much of

the focus in the past 15 years has been on nosocomial, or hos-

pital-acquired infections, where multi-drug-resistant organ-

isms have become a particularly lethal problem.

Bacteriophage offer several advantages as therapeutic

agents. Their target specificity causes less disruption to the

normal host bacterial flora, some species of which are vital in

maintaining the ecological balance in various areas of the

body, than does the administration of a relatively less specific

antibiotic. Few side effects are evident with phage therapy,

particularly allergic reactions, such as can occur to some

antibiotics. Large numbers of phage can be prepared easily

and inexpensively. Finally, for localized uses, phage have the

special advantage that they can continue multiplying and pen-

etrating deeper as long as the infection is present, rather than

decreasing rapidly in concentration below the surface like

antibiotics.

In addition to their specific lethal activity against target

bacteria, the relatively new field of genetherapy has also uti-

lized phage. Recombinant phage, in which carry a bit of non-

viral genetic material has been incorporated into their genome,

can deliver the recombinant DNAor RNAto the recipient

genome. The prime use of this strategy to date has been the

replacement of a defective or deleterious host gene with the

copy carried by the phage. Presently, however, technical safety

issues and ethical considerations have limited the potential of

phage genetic therapy.

See alsoBacteriophage and bacteriophage typing; Microbial

genetics; Viral genetics; Viral vectors in gene therapy

PPhagocyte and phagocytosisHAGOCYTE AND PHAGOCYTOSIS

In the late 1800s and early 1900s, scientific researchers

worked to uncover the mysteries of the body’s immune sys-

tem—the ways in which the body protects itself against harm-

ful invading substances. One line of investigation showed that

immunityis due to protective substances in the blood—anti-

bodies—that act on disease organisms or toxins.

An additional discovery was made by the Russian-

French microbiologist Élie Metchnikoff(1845–1916) in the

1880s. While studying transparent starfish larvae, Metchnikoff

observed certain cells move to, surround, and engulf foreign

particles introduced into the larvae. Metchnikoff then

observed the same phenomenon in water fleas. Studying more

complicated animals, Metchnikoff found similar cells moving

freely in the blood and tissues. He was able to show that these

mobile cells—the white blood corpuscles—in higher animals

as well as humans also ingested bacteria.

The white blood cells responded to the site of an infec-

tion and engulfed and destroyed the invading bacteria.

Metchnikoff called these bacteria-ingesting cells phagocytes,

Greek for “eating cells,” and published his findings in 1883.

The process of digestion by phagocytes is termed

phagocytosis.

womi_P 5/7/03 11:09 AM Page 434