13.3 Bacteriophages and Their Applications

Bacteriophage Structure and Life Cycle

Bacteriophages (phages) are viruses that specifically infect bacteria. They've been central to some of the most important discoveries in genetics, and they remain powerful tools in genetic engineering and medicine. Understanding how phages work gives you a strong foundation in bacterial genetics, gene transfer, and modern biotechnology.

Structure and Lifecycle of Bacteriophages

A typical bacteriophage (like T4) has three main structural components:

• Head (capsid): A protein shell that encloses the phage's genetic material, which can be either DNA or RNA depending on the phage type.
• Tail: A hollow tube through which the genetic material passes into the host cell. Some phages (like T4) have a contractile sheath around the tail that acts like a syringe to inject DNA.
• Tail fibers: Protein structures at the base that recognize and bind to specific receptors on the bacterial surface, such as lipopolysaccharides (in Gram-negative bacteria) or teichoic acids (in Gram-positive bacteria). This receptor specificity determines which bacteria a given phage can infect, known as its host range.

Phages reproduce through one of two life cycles:

Lytic Cycle

This cycle destroys the host cell. It proceeds in five steps:

1. Attachment (adsorption): Tail fibers bind to specific receptors on the bacterial surface.
2. Penetration: The phage injects its genetic material through the tail into the host cell. The protein coat stays outside.
3. Replication and synthesis: The phage hijacks the host's machinery to replicate phage DNA and produce new phage proteins.
4. Assembly: New phage particles are assembled inside the host cell.
5. Lysis: The host cell bursts open, releasing the new phages. The number of phages released per cell is called the burst size (often 100–200 for T4). The timing of these events can be tracked using a one-step growth curve, which plots phage numbers over time and reveals the latent period (before lysis) and the burst.

Lysogenic Cycle

Instead of immediately destroying the host, some phages (called temperate phages, like lambda $\lambda$) can integrate into the host genome:

1. Attachment and penetration occur just as in the lytic cycle.
2. Integration: The phage DNA recombines into the host chromosome, becoming a prophage.
3. Latency: The prophage is passively replicated along with the host DNA every time the bacterium divides. The bacterial cell carrying a prophage is called a lysogen.
4. Induction: Environmental stressors like UV light or certain chemicals can trigger the prophage to excise from the chromosome and enter the lytic cycle.

The lysogenic cycle matters because it can change the properties of the host bacterium. This phenomenon, called lysogenic conversion, can give bacteria new traits, such as toxin production (the diphtheria toxin gene, for example, is carried by a prophage).

Bacteriophages in Genetic Discoveries

Transduction

Transduction is the transfer of bacterial DNA from one cell to another via a phage. It was discovered by Norton Zinder and Joshua Lederberg in 1952 using Salmonella. There are two types:

• Generalized transduction happens when a phage accidentally packages a random fragment of the host's bacterial DNA instead of phage DNA during lytic replication. When this defective phage particle infects a new bacterium, it injects that bacterial DNA, which can then recombine into the new host's chromosome. Any gene can potentially be transferred this way.
• Specialized transduction occurs only with temperate phages. When a prophage excises imprecisely from the host chromosome, it takes a small piece of adjacent bacterial DNA with it. As a result, only genes near the prophage integration site get transferred. For example, phage $\lambda$ integrates between the gal and bio genes in E. coli, so it can transduce those specific genes.

Transduction is an important mechanism of horizontal gene transfer in bacteria and can spread traits like antibiotic resistance between cells.

The Hershey-Chase Experiment (1952)

This experiment provided definitive evidence that DNA, not protein, is the genetic material. Alfred Hershey and Martha Chase used T2 phage and two radioactive labels:

1. They labeled phage DNA with $^{32}P$ (phosphorus is abundant in DNA but not in protein).
2. They labeled phage protein with $^{35}S$ (sulfur is found in protein but not in DNA).
3. They allowed the labeled phages to infect E. coli, then used a blender to separate phage coats from the bacteria, followed by centrifugation.
4. Result: $^{32}P$ (DNA) was found inside the bacterial cells in the pellet, while $^{35}S$ (protein) remained in the supernatant with the empty phage coats.

The conclusion: DNA enters the host and directs phage reproduction, confirming it as the genetic material. This built on the earlier work of Avery, MacLeod, and McCarty (1944).

Bacteriophages in Genetic Engineering

Phage Display

Phage display is a technique for studying protein interactions and screening large protein libraries. Here's how it works:

1. A gene encoding a protein of interest (or a library of random gene variants) is fused to a gene encoding a phage coat protein (typically gene III or gene VIII of filamentous phage M13).
2. When the phage assembles, the foreign protein is displayed on the phage surface.
3. These phages are then screened against a target molecule (an antigen, receptor, etc.) through rounds of binding, washing, and amplification, a process called biopanning.
4. Phages that bind the target are selected and enriched over multiple rounds.

This technique is widely used to discover new antibodies, identify enzyme variants with desired properties, and map protein-protein interactions. George Smith and Gregory Winter shared the 2018 Nobel Prize in Chemistry for developing and applying phage display.

Phage Vectors

Phages can serve as cloning vectors to carry foreign DNA into bacteria. Lambda ($\lambda$) phage and cosmid vectors are common examples. Phage vectors have some advantages over plasmid vectors:

• They can accommodate larger DNA inserts (up to ~23 kb for lambda replacement vectors, compared to ~10 kb for most plasmids).
• Phage infection is highly efficient, which improves the success rate of getting DNA into cells.

Recombineering

Recombineering (recombination-mediated genetic engineering) uses phage-derived recombination proteins to make precise modifications to bacterial genomes. The most commonly used system is the Red recombination system from phage $\lambda$, which includes three proteins (Exo, Beta, and Gam). This approach allows you to insert, delete, or modify specific sequences without needing restriction enzymes or ligases, making genome editing faster and more flexible.

Medical Applications of Bacteriophages

Phage Therapy

Phage therapy uses phages to kill pathogenic bacteria. It was first explored in the early 1900s, fell out of favor with the rise of antibiotics, and is now experiencing renewed interest due to antibiotic resistance. Key advantages include:

• High specificity: Phages typically infect only one species or even one strain of bacteria, so they don't disrupt the beneficial microbiota the way broad-spectrum antibiotics do.
• Self-amplifying: Phages replicate at the site of infection, so their numbers increase exactly where they're needed.
• Co-evolution potential: Phages can evolve alongside resistant bacteria, unlike static antibiotic molecules.

Challenges remain significant, though:

• Regulatory frameworks for phage therapy are still developing in most countries.
• Treatment often requires personalized phage cocktails matched to the patient's specific bacterial strain.
• There's a risk that phages could transfer virulence or antibiotic resistance genes between bacteria through transduction.

Targeted Drug Delivery

Phage particles can be engineered as drug delivery vehicles:

• Phage capsids can be loaded with therapeutic molecules and protect them from degradation in the body.
• Surface proteins on the phage can be modified (using phage display principles) to bind specific receptors on target cells, such as tumor-specific markers on cancer cells.
• Potential applications include targeted cancer therapy and delivery of gene-editing tools like CRISPR-Cas components or RNA interference (RNAi) molecules.

This area is still largely in the research stage, but it represents a promising intersection of phage biology and nanomedicine.