Bacterial genetics and variation

1. Bacterial genetics and
variation

2. Bacterial Genetics
1- chromosome
2- plasmid
3- prophage
4- transposable genetic element

3. Bacterial DNA
Single Haploid Chromosome
Super coiled circular dsDNA=1mm
Exception:
2 chromosomes : Vibro cholerae

4. Extra chromosomal elements
Plasmids
 relaxed: Free Circular dsDNA-In Cytoplasm for several generations 30-50 in
number
Replicate independently in absence of protein
Episome :(stringent )-integrated form depend on chromosomal replication
(1-5number)- large in size –
Not essential for life of bacteria
contain 50-100 genes

5. Classification based on function:
Fertility/F plasmid: contain tra gene or F gene responsible for sex pilli formation:
sex pili expression called F factor ….gene transfer
Resistance/R plasmid (antibiotic resistance) plasmid carry R gene called R factor
…S.aureus(MRSA) ……pencillinase and B.lacatamase……degrading enzymes
Col plasmid (bacteriocin production)
Virulence plasmid (capsule – toxin )
Metabolic plasmid

6. Colicinogenic (col) factor
• Bacteriocins are the antibiotic like substances produced by
one bacterium that inhibit other bacteria
• Bacteriocins produced by coliform bacteria are called as
colicin
• Bacteria other than coliforms also produce similar kind of
substances e.g. pyocin, diphthericin

7. 7
Transposable Elements
• Transposable elements are DNA sequences that can jump from one position to
another or from one DNA molecule to another
• (Non replicate depend on chromosome or plasmid)
• Bacteria contain a wide variety of transposable elements
• Two enzymes
: Restriction enzymes and ligase enzyme (transposase and integrase)
- Four classes
1-The smallest and simplest are insertion sequences, or IS elements, which are 1–3 kb
in length

8. 8
Transposable Elements
• 2- Other transposable elements in bacteria contain one or more genes unrelated to transposition that can be
mobilized along with the transposable element; this type of element is called a transposon
• Transposons can insert into plasmids which can be transferred to recipient cells by conjugation
Transposable elements are flanked by inverted repeats and often contain multiple antibiotic resistance genes

9. 9
3- Integrons
• Integrons are DNA elements that encode a site-
specific recombinase as well as a recognition
region that allows other sequences with similar
recognition regions to be incorporated into the
integron by recombination.
• The elements that integrons acquire are known
as cassettes
• Integron may acquire multiple-antibiotic-
resistance cassettes, which results in the
plasmid resistant to a large number of
completely unrelated antibiotics
• Bacteria with resistance to multiple antibiotics
are an increasing problem in public health

10. 4-Pathogenicity islands
• DNA fragement (10-200 bp)
• Give bacteria more virulent

11. Bacterial Variation
Phenotypic
Genotypic

12. Phenotypic variation:
 Changes of the bacterial character without change in bacterial genes, and usually in response
to environmental factors.
 This variation is reversible when the environmental cause is removed and is not inheritable.
 Examples:
1. Change in culture characteristics (colonial morphology) of bacteria with aging of bacterial cultures
(smooth to rough colony ‘S-R’ variation). Young bacterial culture gives smooth form, while old culture
gives rough form.
2. Bacterial sporulation and vegetation.

13. Genotypic variation:
 Changes of the bacterial character due to change in bacterial genes.
 It is irreversible and heritable.

14. Causes of phenotypic variation:
• Enviromental :
a. Staphylococci produces penicillinase enzyme when they grow in the presence
of sub-therapeutic dose of penicillin.
b. If Staphylococci are cultivated in the media free from penicillinase, they not
give penicillinase.

15. Causes of genotypic variation:
• Mutation:
• Gene transfer

16. Mutation
• Mutation is an inherited change in the base sequence of the nucleic acid
comprising the genome of an organism .
• A strain carrying such changes is called as mutant.
• A mutant may differ from its parent strain in genotype (sequence of nucleotides
in the DNA of the genome) and sometimes in phenotype (observable properties
from its parent) also.
• A nutritional mutant that has a requirement for a growth factor is called an
auxotroph and the wild-type parent from which the auxotroph was derived is
called a prototroph.

17. MUTATION
Random, heritable variation caused by alteration in nucleotide
sequence of DNA
CAUSES
• Spontaneous
• Induced (mutagen) –
• Physical: UV
• Chemical: alkylating agent, 5-FU, acridine dye

18. • Types of mutations:
1.A point mutation, or single base substitution, is a type of mutation that causes the
replacement of a single base nucleotide with another nucleotide of the genetic material,
DNA or RNA.
2.Insertions add one or more extra nucleotides into the DNA.
3.Deletions remove one or more nucleotides from the DNA.

19. Mutation
• Mutation involving change in base pair which codes for a different amino acid is
called missense mutation. Eg. (UAC - Tyrosin; AAC– asparagine).
• Some times a mutation may result in premature termination of translation (as the
base pair alteration contribute to stop codon TAG - UAG (stop codon) resulting in
incomplete protein – such is called non-sense mutation.

20. Mutations
• Agents that induce mutations are called mutagens which
may be chemical or physical agents. Eg.
• Chemical mutagens – Nitrous acid (HNO3),
Hydroxylamine (NH2OH), alkylating agents.
• Physical mutagens – UV and ionizing radiation (x-rays)

21. Genetic recombination(Integration of Donor DNA to recipient
chromosome)
or Gene Transfer in Bacteria
Horizontal gene transfer (HGT)
• In prokaryotes, genetic recombination occurs because fragments of homologous
DNA from a donor chromosome are transferred to a recipient cell by any of the
three following processes.
• Transformation – Transfer of bacterial genes involving free DNA.
• Transduction – Transfer of host genes from one cell to another medicated by a virus.
• Conjugation – Transfer or genes from one cell to another involving cell to cell contact and a
plasmid.

22. Transformation
• A cell that is able to take up a molecule of DNA and be transferred is called
competent cell.
• Bacteria differ in the form in which DNA is taken up. In Gram negative bacteria
(eg. Haemophilus) only DS DNA is taken up into the cell, however only SS – DNA
segment is incorporated into the genome.
• In Gram positive bacteria (Streptococcus sp. and Bacillus) only SS – DNA is taken
up.
• 1928: Frederick Griffith (London): First demonstrated bacterial transformation

23. Transformation
- random uptake of free / naked DNA incorporation into chromosome
•

24. An "S" or SMOOTH coat strain, which is lethal to mice.

25. An “R" or rough coat strain, which is NOT lethal to
mice.

26. Griffith found that he could heat inactivate the smooth strain.

27. heat-inactivated S strain, mixed with the R strain, the mouse would die.
Thus there was some Material in the heat-killed S strain that was responsible for
"transforming" the R strain into a lethal form.

28. 2- Transduction- Transfer of genetic material through
bacteriophage (bacterial virus)

29. • Transduction: using phage to map bacterial genes
• General transduction: any genes can be transferred
• Specialized transduction: only a few genes can be transferred
Techniques for the study of bacteriophages

33. Bacterial Transduction

34. Lysogenic Conversion
• In Lysogenic bact prophage acts as additional segment of bact
chromosome-new characters-lysogenic conversion eg. C.diphtheriae and
its bacteriophage
• Phage coded Toxins:
• Diphtheria toxin
• cholera toxin
• Verocytotoxin of E. coli
• Streptococcus pyrogenic exotoxin A & C
• Botulism toxin C & D

35. • Lysogenic conversion: Phage DNA itself behave as new genetic element
• Transduction: Phage act as vehicle carrying bacterial gene

36. 3- Conjugation
• Conjugation or mating – involves the transfer of DNA from a donor to a recipient
by cell to cell contact through the F (Fertility) pilus, followed by recombination
within the recipient bacterial cell.
• Pili are involved in attachment processes.
• F pili specifically join mating bacteria. When an F pilus joins with the mate, there
is a change in plasma membrane permeability so that DNA can move from one
cell to another.
• Bacteria that produce F pili are donors and are designated F+ strains.

37. Conjugation
• During mating, a single strand of donor DNA is replicated, and this copy is
transferred to the recipient where the complimentary strand is synthesized.
• Bacteria are designated Hfr (high frequency recombinant) if the F plasmid DNA is
incorporated into the bacterial chromosome.
• Bacteria lacking F pili are recipient strains and are designated F – strains. When
F+ cell mates with F– cell, the F plasmid DNA is copied and transferred from donor
to the recipient. This results in F+ strains.
• The F plasmid confers the genetic information for acting as a donor strain.

38. Bacterial conjugation

42. FATE OF DONOR DNA:

43. • Bacterial variations include:
1. Variation in bacterial morphology.
2. Variation in culture characters.
3. Variation in biochemical activities.
4. Variation in Antigenicity.

44. 1- Variation in bacterial morphology.
1-Shape:-
• Typical morphology means shape of bacteria under suitable conditions.
 Cocci- Bacilli- Coccobacilli- Medium sized bacilli- Large sized bacilli.
• But under unsuitable conditions, the bacterial shapes becomes pleomorphic (Bacteria from the
same colony has different morphology).
2-Capsule:-
 Pasteurella (capsulated) by culturing in ordinary media Results in loss of capsule.
 But when it cultured in enriched media (contain nutrients needed for formation of the capsule)
it becomes capsulated.
3- Fimbriae
4- Flagella

45. 2-Variation in culture characters (Smooth-Rough variations; (S-R
variations)).
Items Smooth form Rough form
1. colony surface and edge Smooth and entired Rough and serrated
1. Suspension of colonies in Saline Give homogenous turbidity No homogenous turbidity
1. Growth in liquid media Homogenous or uniform turbidity Precipitation
1. Bacterial antigenicity - All antigens are present
a. K antigen in capsulated bacteria
b. H antigen in flagellated bacteria
- Loss of antigens
a. Absence of K antigen.
b. Absence of H antigen.
c. Absence of spore antigen.
1. Pathogenicity It is highly pathogenic if its of pathogenic type Loss of pathogenicity, even if it is pathogenic.
1. Biochemical activities Highly Active Less active
1. Morphology - Capsule is present if it is capsulated.
- Flagella is present if it is motile
- Capsule is lost, even if it is capsulated.
- Flagella are lost, even if it is motile.
1. –Autoagglutination - Not present - Present

47. MECHANISMS OF RESISTANCE
 Enzymatic inhibition
 Alteration of bacterial membranes
 Outer membrane permeability
 Inner membrane permeability
 Rapid ejection of the drug [efflux] or reduced drug influx.
 By pass of antibiotic inhibition.
 Alteration of target sites
 Altered ribosomal target sites
 Altered cell wall precursor targets
 Altered target enzymes