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BASIC MEDICAL
MICROBIOLOGY

WHY STUDY MICROBIAL GENETICS

•  provide simple systems for
phenomena

studying genetic

•  understand how microorganisms function
•  genetic traits underlie their harmful activities
•  used for isolation and duplication of genes
from other organisms
•  produce substances of value in industry
•  spread of genes that confer important
properties e
...
antibiotic resistance

DEFINITIONS
•  DNA: genetic material in many organisms
•  Gene: linear sequence of nucleotides that
codes for a protein
•  Genome: the entire genetic material of an
organism/ all the genes present in a cell
•  Genotype: the genetic make-up of an
organism
•  Phenotype: an observable characteristic
of an organism

GENETIC ELEMENTS
PROKARYOTES
–  Chromosome; long, circular, ds DNA
–  Plasmid; short, usually circular, ds DNA
–  Bacteriophages; single or ds DNA or
RNA
–  Transposable element; ds DNA within
another DNA

GENETIC ELEMENTS
EUKARYOTES
–  Chromosome; extremely long, linear,
ds DNA
–  Plasmid; short circular or linear ds DNA
–  Virus; single or ds DNA or RNA
–  Transposable element; ds DNA within
another DNA
–  M i t o c h o n d r i o n o r c h l o r o p l a s t
(organelles); intermediate-length DNA,
usually circular

CENTRAL DOGMA OF MOLECULAR
BIOLOGY
DNA
Transcription

Reverse transcription
RNA

Translation
PROTEIN

The flow of genetic information

CHROMOSOME

CHROMOSOME IN PROKARYOTES
•  No   membrane   separa,ng   chromosome   and  
cytoplasm  

Therefore, messenger RNA produced from the DNA
template can be directly translated by ribosomes
...
Plasmids are usually represented by small, circular DNA
...
(1) Chromosomal DNA
...


PLASMID
•  found in most bacterial species, sometimes
in the nuclei of some eukaryotic cells
eukaryotic organisms (e
...
, the 2micrometre-ring in Saccharomyces
cerevisiae)
 

PLASMID
•  plasmid’s genes
–  control of its own replication
–  provide the cell with one or more useful
functions
•  not essential for cell function
•  a survival advantage in certain
environments (such as in the presence
of antibiotics) over cells lacking the
plasmid
–  unknown function: cryptic

SOME FUNCTIONS ATTRIBUTED TO
BACTERIAL PLASMID GENES
•  promote the transfer of genetic material
from one cell to another
•  provide resistance to antimicrobial agents
(e
...
, penicillin and streptomycin
–  most notorious property
–  responsible to a large extent for the
spread of antibiotic resistance
•  provide resistance to heavy metals (e
...
,
mercury, lead, arsenic)

•  production of certain toxins (e
...
,
coagulase, hemolysin, enterotoxins)
•  nutrient assimilation and metabolism
(e
...
, lactose, nitrogen fixation, urea)
•  induce tumours in plants (e
...
, nodules
and crown galls)
•  degradation of toxic organic materials
(e
...
, camphor, octane, naphtalene)
•  production of pigments
•  production of antimicrobial substances
called colicins (B, I, V, D, K etc
...
Their
only function is to initiate conjugation
•  Resistance-(R-) plasmids: contain genes that can
build a resistance against antibiotics or poisons
•  Col-plasmids: contain genes that code for
(determine the production of) colicines, that can kill
other bacteria
...
g
...


Comparison of nonintegrating plasmids (top)
and episomes (bottom)
...

(2) Plasmids
...

( 4) Chromosomal DNA with
integrated plasmids

BACTERIOPHAGE
•  A virus that infects bacteria (‘phage’)

TRANSPOSABLE ELEMENTS
•  Genes: fixed positions/do not move
sites
•  Mobile DNA sequences=transposable
genetic elements
–  Transposition=process by which
pieces of DNA moves from one place
to another in the genome (for
variation/evolution)

•  Not all genes are capable of transposition
• only those
linked with transposable
element
• genes encoding a transposase flanked
by short inverted terminal repeats (40 to
1000 bp) at both ends
• found in prokaryote and eukaryote
•  3 types of transposable element : insertion
sequences, transposons and some
special viruses

INSERTION SEQUENCE (IS)
•  No genetic information other than that
required to move into new locations
•  Short segments (~1000bp) that can be
integrated at specific sites on genome
•  Found in chromosome, plasmid and
bacteriophage
•  Several distinct IS found, designated with
no
...
g
...
of bp
...
g
...
coli)
–  competence is a brief physiological state
during the exponential growth phase
•  transformation occurs at a low frequency (1
in every 100 cells)
•  C a 2 + i o n s e n h a n c e t h e l e v e l o f
competence
•  In the lab: alternative techniques,
manipulation of the organisms
– Increased/higher frequencies

•  certain bacteria
–  c o m p e t e n c e i s a s s o c i a t e d w i t h t h e
production of double stranded DNA-binding
proteins on the plasma membrane of
competent cells and single-stranded DNAbinding proteins in the cytoplasm of the cell
•  present only at specific stages of the life
cycle (e
...
, E
...
g
...


reversible binding of double-stranded DNA
molecules to cell surface receptor sites
(membrane-bound dsDNA-binding protein)

2
...


conversion of donor DNA into single-stranded
molecules via degradation of one strand by a
nuclease

MECHANISM OF DNA TRANSFER IN
BACTERIA BY TRANSFORMATION
4
...

The integrated
ssDNA is converted into dsDNA by DNA
polymerase I

5
...


CONJUGATION
•  o n e w a y t r a n s f e r , n o r e c i p r o c a l
exchange of information
–  v a l u a b l e m e a n s o f g e n e t i c
manipulations in vivo
•  transfer of genes of importance from
one host to another
•  common in E
...
aeroginosa and
Vibriocomma

CONJUGATIVE AND NON-CONJUGATIVE
PLASMIDS
•  Conjugative plasmids
–  contain a tra-gene, which can initiate
conjugation, the sexual exchange of
plasmids, with another bacterium
•  Non-conjugative plasmids
–  incapable of initiating conjugation,
hence they can only be transferred
together with conjugative plasmids,
during conjugation

CONJUGATIVE (TRANSMISSIBLE)
PLASMIDS
•  provide conjugation facilities for other
non-conjugative plasmids (cannot
mediate DNA transfer on their own)
•  allow movement of non-conjugative
plasmid from one host to another: drivers
or mobilizing plasmids

The F+/F- conjugation process (E
...
  formation of donor/recipient pairs following
effective cell/cell contact
–  by specialized sex pili (specialized cell
surface hairlike tubes: F or R pili (the synthesis
is controlled by an F or R plasmid)
•  initial contact and into close contact
c
...
  prepare their own DNA for transfer
•  a plasmid encoded protein makes a
single-stranded nick in the transfer origin
(oriT) of the plasmid
2
...
  DNA transfer and start of replication
•  transfer of the 5’ end of the nicked strand
through the pilus into the recipient
•  form a ‘rolling circle replication’: the
displaced strand is replicated in the
recipient and the circular strand is
replicated in the donor
b
...
  the F factor (blue circle, with four reference points a, b, c
and d) integrates into the bacterial chromosome (the white
circle with the numbers 1-6 representing different parts of
the chromosome) to form an Hfr
(Note: the arrow head represents the first part of the DNA to
be transferred during conjugation

TRANSDUCTION
n 

n 

involves the use of viruses that infect
bacteria: bacteriophage (phage)
n  bacterial DNA is transferred by phage
particles from a donor bacterium to a
recipient cell
two types of transducing phage
n  generalized transducing phage:
(generalized transduction)
n  s p e c i a l i z e d t r a n s d u c i n g p h a g e :
(specialized transduction)

GENERALIZED TRANSDUCING
PHAGE
•  produce particles that contain only DNA
obtained from the host bacterium rather
than phage DNA
•  the DNA may derive from any part of
the bacterial chromosome

SPECIALIZED TRANSDUCING
PHAGE
•  produce particles that contain both
phage and bacterial DNA linked together
to form a single molecule
–  the bacterial genes are obtained from
a particular region of the bacterial
chromosome

MECHANISM OF TRANSDUCTION
•  depends on how phage grows in bacteria
–  the lysogeny cycle
–  the lytic cycle

GENERALIZED TRANSDUCTION
•  Mediated by virulent/lytic bacteriophage
•  Some of host’s DNA is packaged into the
virus
•  Formation of a generalized transducing
phage
–  Initiate infection but not its replication
and lysis of the host cell
–  Serves as vehicle for the transfer of the
host DNA

GENERALIZED TRANSDUCTION
§  i f d o n o r g e n e s d o n o t u n d e r g o
homologous recombination with the
recipient’s, they will be lost as they
cannot replicate independently and are
not part of a viral genome

GENERALIZED TRANSDUCTION

SPECIALIZED/RESTRICTED
TRANSDUCTION
•  Lysogenic bacteriophage transfers a
specific gene at high frequency
•  Upon infection, the viral DNA is incorporated
into the host’s genome
•  Through the process of induction (for e
...
,
exposure to UV light), the prophage genome
is excised from that of the host and undergo
a lytic life cycle

SPECIALIZED
TRANSDUCTION



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