Plasmid Extra chromosomal circular piece of DNA present

Plasmid • Extra chromosomal circular piece of DNA present in many bacteria. • They are autonomously replicating and stably inherited. • Their genetic information is not essential to the host. • Bacterial cell may contains 1 - 40 copy of the same plasmid (copy number). • Curing means elimination of from the host. it can happen naturally or experimentally.

Bacterial plasmid

Classification of Plasmids Ø Transfer properties ØConjugative plasmid ØNonconjugative plasmids v Phenotypic effects Fertility plasmid (F factor) Colicinogenic plasmid (col- factor) Resistance plasmids 7 factors.

Conjugative plasmids The sexual transfer of plasmids to another bacterium through a pilus. Those plasmids, plasmidos F, possess the 25 genes required for transfert.

By function 1. Fertility-(F) plasmids They are capable of conjugation (they contains the genes for the pili). 2. Resistance-(R) plasmids Contain gene (s) that can build resistance against one or several antibiotics or poisons. 3. Col-plasmids, Contain genes coding for colicines, proteins that can kill other bacteria. 4. Degradative plasmids, Able to digest unusual substances, e. g. , salicylic acid. 5. Virulence plasmids, Turn a bacterium into a pathogen. 5. Addiction system. These plasmids produce both a long-lived poison and a short-lived antidote. Daughter cells that retain a copy of the plasmid survive, while a daughter cell that fails to inherit the plasmid dies or suffers a reduced growth-rate because of the lingering poison from the parent cell.

Bacteriophage • Bacteriophages ( phages ) are obligate intracellular parasites that multiply inside bacteria by making use of some or all of the host biosynthetic machinery. They are viruses that infect bacteria.

Composition of Bacteriophage • Nucleic acid: either DNA or RNA but not both – ds DNA, ss RNA, ss DNA – Unusual or modified bases – Encode 3 -5 gene products to over 100 gene products • Protein: Function in infection and protect the nucleic acid.

Structure of Bacteriophage • Structure – Size – Head or capsid – Tail Contractile Sheath Head/ Capsid Tail Fibers Base Plate

• Different sizes and shapes icosahedral filamentous

Phage genome • Encodes proteins needed for making more phages. • Double or single stranded DNA or RNA. • It can be circular or linear. For many phage types, the capsid is attached to a tail structure.

Types of Bacteriophage • Lytic or Virulent Phages. • Lysogenic or Temperate Phage. Lytic or virulent phages are phages which can only multiply in bacteria and kill the cell by lysis at the end of the life cycle. • Life Cycle – Adsorption – Penetration – Biological Synthesis – Maturation and Release

Life cycle of phage May occur as • Lytic cycle which end up with lysis of bacteria • Lysogenic cycle in which phage DNA integrated into genomic DNA of bacteria. Also called temperate phage

1 2 3 5 4

Bacteriophage binding to and injecting their DNA into a bacterial cell

Bacteriophage after contraction of sheath ge a h op r o N m al b ri e t ac

Assay for Lytic Phage Plaque assay

• Lytic phage are enumerated by a plaque assay. • A plaque is a clear area which results from the lysis of bacteria. • Each plaque arises from a single infectious phage. • The infectious particle that gives rise to a plaque is called a pfu (plaque forming unit).

• Lysogenic or temperate phages are those that can • • either multiply via the lytic cycle or enter a quiescent state in the cell. In this quiescent state most of the phage genes are not transcribed; the phage genome exists in a repressed state. The phage DNA in this repressed state is called a prophage because it is not a phage but it has the potential to produce phage. In most cases the phage DNA actually integrates into the host chromosome and is replicated along with the host chromosome and passed on to the daughter cells. The cell harboring a prophage is termed a lysogenic bacterium.

Lysogenic conversion • When a cell becomes lysogenized, occasionally extra genes carried by the phage get expressed in the cell. These genes can change the properties of the bacterial cell. This process is called lysogenic conversion.

Importance of bacteriophage in Bacteriological field: 1 - Phages are used in gene transfer through method called transduction. 2 - Phages are used in typing of many bacteria (phage typing). 3 -Used in treatment of bacterial infections. 4 -Used in molecular biology.