Introduction to microbiology

Differences between bacterial endotoxins and exotoxins

Microbial Pathogenicity
Endotoxins
Exotoxins
Lipopolysaccharides in nature
Protein in nature
Part of Gram-negative bacterial cell wall
Secreted both by Gram-positive and negative bacteria; diffuse into surrounding medium
Released by cell lysis, Not by secretion
Actively secreted by the bacteria
Highly stable
Heat labile destroyed at 60 oC
Mode of action – Induces ↑IL-1 and TNF
Mostly enzyme like action
Nonspecific (fever, shock, etc.)
Specific action on particular tissues
No specific affinity for tissues
Specific affinity for tissues
Poorly antigenic
Highly antigenic
Neutralization by antibodies is ineffective
Neutralized by specific antibodies
No effective vaccine is available using endotoxin
Toxoid forms are used as vaccine, e.g. tetanus toxoid


Structure of Prokaryotic genome
1- Plasmids:

2- Transposons:

3- Integrons: mobile genetic elements,
• Consist of an integrase gene
• Series of antibiotic resistance gene
• Plus insertion sequences they are not capable of independent replication.

• Episome: Plasmid may integrate with chromosomal DNA

of Bacteria.

Bacterial Genetics

Gene Transfer in Bacteria:

• Transposons:
• Structurally / Genetically – Discrete sequence of DNA – Move around in a cut and paste manner between Chromosomal and Extra chromosomal DNA molecules (plasmid) within cells.
• “Jumping Genes”
• First discovered in corn
• Genetic transfer due to Transposition
• Not self replicating and depend on Plasmid or Chromosome for replication.
• Moves other genes from place to place on the chromosome or plasmid.
• Moved genes may be regulated differently


Bacterial Genetics

• Genotypic Variation

• Genotypic by transfer of genes
• Mutations
• Gene Transfer in Bacteria:
• Transformation
• Transduction
• Conjugation

Bacterial Genetics

Bacterial Genetics

Bacterial Genetics

Gene Transfer in Bacteria:

1- Transformation:
• Naked DNA is taken up from the environment by bacterial cells.
• It has been studied so far only in certain bacteria: Streptococcus, Bacillus, Haemophilus, Neisseria, Acinetobacter and Pseudomonas.


Bacterial Genetics

Gene Transfer in Bacteria:

Bacterial Genetics
Transformation

Gene Transfer in Bacteria:

2- Transduction:
• is defined as transmission of a portion of DNA from one bacterium to another by a bacteriophage.
• Types of transduction
• Generalized transduction: It involves transfer of any part of the donor bacterial genome into the recipient bacteria.
• Restricted or specialized transduction: Here, only a particular genetic segment of the bacterial chromosome that is present adjacent to the phage DNA is transduced.
• Role of transduction
• In addition to chromosomal DNA, transduction is also a method of transfer of episomes and plasmids.
• Drug resistance, e.g. plasmid coded penicillin resistance in Staphylococci.
Bacterial Genetics

• Lysogenic Conversion:

• During the temperate or lysogenic life cycle, the phage DNA remains integrated with the bacterial chromosome as prophage.
• Prophages: Are bacteriophage genomes integrated into the host genome
• The prophage acts as an additional chromosomal element which encodes for new characters to the daughter cells.
• Imparts toxigenicity to the bacteria: Phage DNA may code for various toxins abbreviated as ABCDE:
• A and C of Streptococcus pyrogenic exotoxin (SPE), Botulinum toxin C and D, Cholera toxin, Diphtheria toxin and E. coli.


Bacterial Genetics

Gene Transfer in Bacteria:

3- Conjugation
• Transfer of genetic material from one bacterium (donor or male) to another bacterium (recipient or female) by mating with each other and forming the conjugation tube.

Bacterial Genetics

Gene Transfer in Bacteria:
3- Conjugation

• Transfer of genetic material from one bacterium (donor or male) to another bacterium (recipient or female) by mating with each other and forming the conjugation tube.
• DNA is directly transferred
• Plasmids carry genetic information necessary for conjugation to occur.
• Only cells that contain such plasmids can act as donor. Cells lacking the corresponding plasmid act as recipient.
• Requires direct contact between donor and recipient

Bacterial Genetics

Gene Transfer in Bacteria:
3- Conjugation
High frequency recombination (Hfr) cell conjugation and F-prime (F`) cell
• F factor being a plasmid, it may integrate with bacterial chromosome and behave as episome.

Bacterial Genetics

Gene Transfer in Bacteria:
3- Conjugation
• Such donor cells are able to transfer chromosomal DNA to recipient cells with high frequency in comparison to F+ cells, therefore, named as Hfr cells (high frequency of recombination).
• During conjugation of Hfr cell with an F- cell, only few chromosomal genes along with only a part of the F factor get transferred. Hence, F- recipient cells do not become F+ cells.
• Conjugation plays an important role in the transfer of plasmids coding for antibacterial drug resistance [resistance transfer factor (RTF)].

Bacterial Genetics

Mutations:
• Is a random, undirected, heritable variation,
• Caused by alteration in the nucleotide sequence at some point of DNA which can occur due to
• Addition
• Deletion
• Base Substitution
• Insertion
• Rearrangement
• Mutagens:
• UV. Light is a physical mutagen
• Nitrous acid (HNO2)
• Hydroxyle groups for amino groups on DNA basis
• Acridine Dyes
Bacterial Genetics


Bacteriophages
Bacteriophages resemble most viruses in having a protein coat called a capsid that surrounds a molecule of DNA or RNA. They look almost like spiders with long skinny necks.
Bacterial Genetics

Bacteriophages

• The phage will bind by its tail fibers to specific receptors on the
bacterial cell surface. This is called adsorption.

• The phage then undergoes penetration.

• The injected DNA takes over the host bacteria's RNA polymerase for the transcription of phage DNA to messenger RNA (mRNA).
• There are two types of phages, virulent phages and temperate phages.
Bacterial Genetics