The Microbial World

The Microbial World
Introduction to Viruses and Bacteriophage
Definition of a Virus
Viruses are segments of nucleic acid enclosed in a protein coat.
Poliovirus
Properties of Viruses
Small size: range ~0.02 - 0.3 micrometers
Picornavirus (“little RNA virus”) is one of the smallest viruses, about
20 nanometers in diameter.
Smallpox virus, one of the largest
viruses, about 300 nanometers, near
the resolution of the light microscope.
Properties of Viruses
Various morphologies
polyhedral
helical
spherical
filamentous
complex
Ebola virus
Rabies virus
Poliovirus
Herpes virus
Coronavirus
Lassa virus
Properties of Viruses
Obligate intracellular parasites

Human adenoviruses
growing within the nucleus
of an infected cell
Properties of Viruses
Lack membranes and a means to generate energy
Schematic Illustration of a of Hepatitis virus
Properties of Viruses
Lack metabolic and biosynthetic enzymes
Schematic illustration of
Herpes simplex virus Type 1 (HSV1)
Properties of Viruses
Lack ribosomes
and a mechanism for their own protein synthesis
Arenaviruses are an exception;
they contain all classes of ribosomal RNA derived from the host,
Properties of Viruses
Do not grow in size

Viruses grow by independent synthesis and assembly of their components inside of a host cell.
Human adenoviruses growing in the nucleus of their host cell
Structure of Viruses
The viral genome is DNA or RNA.
Most bacterial viruses contain double-stranded DNA.
Many animal viruses contain ds DNA or ssRNA.
Structure of Viruses
Most common morphologies are polyhedral (icosahedral) and helical
Structure of Viruses
Some viruses have additional structures: animal viruses may have envelopes and “spikes”.
Structure of Viruses
bacterial viruses may have tails and related structures.
Classification of Viruses
Criteria:
Type of nucleic acid
Manner of nucleic acid replication
Size and morphology
Additional structures such as envelopes and tails
Host range > refers to the range of cells that can be infected by the virus, most often expressed as bacteria, insect, plant and animal hosts > not the most reliable criterion for classification
Classification of Viruses
Comparative size and shape of various groups of viruses representing
diversity of form and host range.
Some Families of Bacteriophage
Some Families of Animal Viruses
Some Families of Animal Viruses (continued)
Bacteriophages
Viruses that infect bacterial cells

Two types of infections:

1. Lytic infection: phage
replicates its DNA and
lyses the host cell

2. Lysogenic infection:
phage DNA is maintained by
the host cell, which is
only rarely lysed
Bacteriophage T4 infects cells of E. coli
Bacteriophages
Lytic infections

The bacteriophage always lyses its host cell.


Bacteriophage T4
Electron micrograph and schematic drawing of bacteriophage T4, a lytic phage of E. coli
Other Lytic phages
Clockwise: Pseudomonas aeruginosa phage; Aeromonas
phage; Salmonella phage p2; Listeria phage
Life Cycle of a Lytic Phage
Life Cycle of a Lytic Phage
Step 1 Adsorption: virus attaches to the cell wall surface


Step 2 Penetration: entry of the viral DNA
Life Cycle of a Lytic Phage
Step 1 Adsorption: virus attaches to the cell wall surface

Step 2 Penetration: entry of the viral DNA
Phage T4 adsorption to the cell wall of E. coli
Life Cycle of a Lytic Phage
Step 3 Synthesis of early viral proteins and

Step 4 Replication of viral DNA
Life Cycle of a Lytic Phage
Step 3 Synthesis of early viral proteins

Step 4 Replication of viral DNA
Phage T2 attacks E. coli
Life Cycle of a Lytic Phage
Step 5 Synthesis of late viral proteins
(structural proteins)
Life Cycle of a Lytic Phage
Step 6 Assembly
The viral nucleic acid becomes enclosed in the virus coat (capsid)

Life Cycle of a Lytic Phage
Step 7 Lysis and release of mature viruses
Life Cycle of a Lytic Phage
Step 5 Synthesis of late viral proteins

Step 6 Assembly

Step 7 Lysis and release of mature viruses
Lysis of E. coli cell by Phage T4
Life Cycle of a Lytic Phage
Plaques bacteriophage T4, a lytic phage, growing on a
“lawn” of E. coli cells
Bacteriophages
Lysogenic infections

Lysogenic phages rarely lyse their host cell

Lysogenic phages are also called temperate phages


Phage P2, a lysogenic phage of Salmonella
Life Cycle of a Lysogenic Phage
Lysogeny
Lysogenic infection begins like a lytic infection with adsorption of the virus and penetration of the viral DNA



Lambda phage, adsorbed to the surface of E. coli, injecting Lambda DNA
Life Cycle of a Lysogenic Phage
Phage DNA is injected into the host bacterial cell
Life Cycle of a Lysogenic Virus
The phage chromosome integrates into the bacterial DNA (chromosome), where it is referred to as a prophage.

The bacterium is said to be lysogenized and this state is called lysogeny.
Lysogeny
After penetration, phage DNA interates into the bacterial chromosomal DNA

Integrated phage DNA is called prophage

Prophage genes for DNA replication and coat proteins are repressed



Phage lambda, a lysogenic phage of E. coli
Lysogeny
Bacterial cell containing prophage DNA is lysogenized

Lysogenized bacteria replicate the prophage DNA

Lysogenized bacteria divide normally and appear normal
Phage mu,another lysogenic phage of E. coli
Life Cycle of a Lysogenic Virus
Each time the bacterium divides, it reproduces the prophage chromosome
Life Cycle of a Lysogenic Virus
Occasionally (1 in 10,000 cell divisions) the prophage detaches from the host bacterial chromosome and enters into a lytic cycle which produces mature infectious bacteriophage

Life Cycle of a Lysogenic Virus
and enters into a lytic cycle which produces mature infectious bacteriophage

Lysogeny
The lambda prophage
deintegrates (excises) from the bacterial chromosome

The process is called derepression and it leads to a lytic cycle that reproduces more phage particles
A lambda particle reeling in a headfull of DNA during an occasional lytic cycle in E. coli
Life Cycle of a Lysogenic Virus
Lysogenic Conversion
A lysogenized bacterium exhibits a change in phenotype

Important in medical microbiology because prophage may contain DNA for toxin production or other virulence determinants
Corynebacterium diphtheriae, the agent of diphtheria,
only produces the diphtheria toxin when lysogenized
by Beta phage.
Bacteriophage as agents of transduction
Generalized transduction: usually mediated by lytic phages. Phage transducing particles can transfer any gene between a donor and recipient
Bacteriophage as agents of transduction
Specialized transduction: mediated by lysogenic (temperate) phages. The phage transfers only genes physically located adjacent to the point of prophage integration into the host bacterial chromosome.
Uses for Bacteriophages
Phages as vectors in genetic engineering and biotechnology designs
Phage lytic enzymes to control infections
Phage therapy in animals and other uses of phage in agriculture
Bacteriophage therapy
Phages for detection of pathogenic bacteria