Bacteriophage Biology and Infection Mechanisms

How Bacteriophage Infect Bacteria

Introduction to Bacteriophage

Bacteriophages, or phages, are viruses that specifically infect bacteria. They play a crucial role in microbial ecology, genetics, and biotechnology. The study of bacteriophage infection mechanisms is fundamental in microbiology.

• Bacteriophage: A virus that infects bacteria.

• Obligate intracellular parasite: Requires a host cell for replication; cannot reproduce independently.

• Lacks independent metabolism: Phages do not carry out metabolic processes on their own.

Discovery and Classification of Viruses

Historical Perspective

The first virus discovered was the plant virus Tobacco Mosaic Virus (TMV) in 1900. This was followed by the discovery of animal viruses and then bacterial viruses (bacteriophages).

• TMV: First identified virus, infects plants.

• Bacteriophage: Viruses that infect bacteria, discovered after plant and animal viruses.

Overview of Typical Virus Structure

Virus Components

Viruses are non-cellular particles composed of genetic material and a protein coat. Some viruses possess an additional lipid envelope derived from the host cell membrane.

• Nucleic acid: Can be DNA or RNA, single-stranded (ss) or double-stranded (ds).

• Capsid: Protein shell composed of capsomers; protects the viral genome.

• Nucleocapsid: Complex of nucleic acid and capsid.

• Envelope: Lipid bilayer membrane acquired from the host cell during viral release; contains glycoproteins and spike proteins for host recognition.

Unenveloped viruses consist only of nucleocapsid, while enveloped viruses have an additional membrane.

Viral Genomes

Genome Diversity

Viral genomes vary widely in size and structure, influencing their replication strategies and host range.

• Size: Range from 2 to 1,000 genes.

• Types:

  • Single-stranded DNA (ssDNA)

  • Double-stranded DNA (dsDNA)

  • Single-stranded RNA (ssRNA)

  • Double-stranded RNA (dsRNA)

• Most DNA viruses: dsDNA

• Most RNA viruses: ssRNA

Generalities of Viral Genomes

Distribution Among Hosts

• dsDNA: Most common among bacterial viruses (bacteriophages).

• ssRNA: Most common among plant viruses.

• Animal viruses: All genome types are represented.

Genome structure: DNA may be linear or circular; RNA is always linear and may be segmented.

Bacteriophage Infection Cycle

General Viral Lifecycle

Bacteriophage infection follows a series of steps, leading to the production of new virions and often the destruction of the host cell.

1. Attachment (Adsorption): Phage recognizes and binds to specific bacterial cell surface receptors (e.g., porins, transporters, LPS).

2. Penetration: Phage injects its genome into the host cell, often using a syringe-like mechanism. Enzymes degrade peptidoglycan to facilitate entry.

3. Synthesis: Host cell machinery is redirected to synthesize viral components. Early genes promote genome replication; middle and late genes encode structural proteins.

4. Assembly: Capsids and other viral structures are assembled, and the genome is packaged into new virions.

5. Release: Host cell is lysed by viral enzymes, releasing mature virions to infect new cells.

Types of Bacteriophage Life Cycles

Lytic Cycle (Virulent Phage)

In the lytic cycle, phages rapidly replicate and lyse the host cell, releasing progeny virions.

• Example: T4 phage infects Escherichia coli.

• Outcome: Host cell death and release of new phages.

Lysogenic Cycle (Temperate Phage)

Temperate phages can integrate their genome into the host chromosome, becoming a prophage and replicating with the host cell. Under certain conditions, they can switch to the lytic cycle.

• Example: Lambda phage infects E. coli.

• Prophage: Integrated phage genome within the host DNA.

• Lysogeny: State where the phage genome is maintained in the host without causing lysis.

• Induction: Process by which the prophage is activated to enter the lytic cycle, often triggered by host stress (e.g., DNA damage, nutrient deprivation).

One-Step Growth Curve of Viral Replication

Phage Replication Dynamics

The one-step growth curve is used to measure the replication time and burst size (number of virions released per cell) for a given phage.

• Eclipse period: No infectious virions are detected; viral components are being synthesized.

• Maturation phase: Assembly and release of virions occur.

• Latent period: Time from infection to release of new virions.

• Burst size: Number of virions released per infected cell.

Example: T4 phage latent period is ~25 minutes; most bacteriophages require 20-60 minutes to replicate, while animal viruses may require 8-40 hours.

Bacterial Defense Mechanisms Against Phage Infection

Physical and Biochemical Defenses

• Receptor modification: Bacteria may lose or alter the receptor required for phage adsorption.

• Restriction-modification systems: Bacteria produce restriction enzymes that degrade foreign DNA. Methylation protects host DNA from these enzymes.

• CRISPR-Cas system: Adaptive immune system in bacteria; incorporates short sequences from invading phages (spacers) into CRISPR loci, allowing recognition and destruction of future invaders.

CRISPR-Cas System

CRISPRs are DNA regions with repeated sequences interspersed with spacers acquired from phages. Upon reinfection, the system uses these spacers to recognize and degrade matching viral DNA.

• Immunization: Acquisition of new spacers from invading phages.

• Interference: Recognition and destruction of phage DNA using Cas proteins.

• Application: CRISPR-Cas is now widely used as a gene editing tool in biotechnology.

Table: Comparison of Lytic and Lysogenic Cycles

Feature	Lytic Cycle	Lysogenic Cycle
Phage Type	Virulent	Temperate
Host Cell Fate	Lysis and death	Survival, phage genome integrated
Genome Integration	No	Yes (prophage)
Induction	Not applicable	Triggered by host stress
Examples	T4 phage	Lambda phage

Key Terms and Definitions

• Bacteriophage: Virus that infects bacteria.

• Virion: Complete, infectious virus particle.

• Capsid: Protein shell of a virus.

• Prophage: Phage genome integrated into bacterial DNA.

• Lysogeny: State of phage genome maintenance in host without lysis.

• Induction: Activation of prophage to enter lytic cycle.

• CRISPR: Clustered Regularly Interspaced Short Palindromic Repeats; bacterial adaptive immune system.

Formulas and Equations

• Burst size: $\text{Burst size} = \frac{\text{Total number of released virions}}{\text{Number of infected cells}}$

• Latent period: $\text{Latent period} = \text{Time from infection to first release of virions}$

Summary

Bacteriophages are essential agents in microbiology, with diverse infection strategies and significant roles in bacterial evolution and biotechnology. Understanding their structure, lifecycle, and bacterial defense mechanisms provides foundational knowledge for advanced studies in microbiology.