1. Dissemination of Antibiotic Resistance
Mechanisms of Bacterial Gene Exchange

2. that requires multiple changes?
How do bacterial cells
acquire resistance
that requires multiple changes?

3. Probability that I will take the red route? decisions independent
1/3
decisions
independent
1/3 X 1/3 X 1/3 = 1/27
1/1,000,000 X 1/1,000,000 X 1/1,000,000 = 1/1,000,000,000,000,000,000

4. Bacterial cells can acquire genes for resistance or for virulence
by obtaining
large pieces of DNA
from other bacteria or bacteriophage
HOW?

5. MODES OF GENETIC EXCHANGE recipient DNA = endogenote
Transformation
Conjugation
Transduction
transfer is one way
donor DNA = exogenote
recipient DNA = endogenote

6. Transformation

7. TRANSFORMATION CLINICAL RELEVANCE
1) naked DNA released by donor cell (lysis)
2) uptake of DNA by recipient cell (competence)
CLINICAL RELEVANCE
Streptococcus pneumoniae, Haemophilus influenzae
& Neisseria gonorrhea
use transformation
Neisseria uses it to alter appearance
of clinically important surface antigens
permits evasion of host immune system*

8. TRANSFORMATION DNA COMPETENT RECIPIENT DONOR DONOR LYSIS NUCLEASES
DETERGENTS
DONOR
naked
DNA
COMPETENT
RECIPIENT
DONOR .
LYSIS
COMPETENCE
FACTOR

9. MECHANISM OF TRANSFORMATION

10. 3 FATES OF THE EXOGENOTE DS EXOGENOTE DEGRADED NUCLEASE
(lost to nucleases)
NUCLEASE
CIRCULARIZED
(lost by dilution)
SS EXOGENOTE
PMF
(uptake
energy)
RECOMBINED
(stably retained)

11. HOMOLOGOUS RECOMBINATIONX
exogenote
endogenote
recombinant

12. Conjugation

13. CONJUGATION RECIPIENT cell-cell contact mating DONOR R plasmid
NUCLEASES
DETERGENTS
RECIPIENT
DONOR
R plasmid
EXCONJUGANT

14. CONJUGATION CLINICAL RELEVANCE
1) rapid dissemination of drug resistance
via conjugal R plasmids or conjugal transposons
2) mediates inter-species transfer
from non-pathogenic Bacteroides
to pathogenic Shigella or E. coli
or Salmonella or worse
normal flora acts as resistance reservoir*

15. R plus R minus Rolling Circle Replication Conjugal Bridge (tube)
Entry &
Replication R
Origin of Transfer
oriT
DNA nicked R
Chromosome

16. Transduction

17. mediated by bacteriophage
TRANSDUCTION
mediated by bacteriophage
composed of:
1) protein capsid
2) DNA or RNA genome

18. BACTERIOPHAGE
Head
nucleases
detergents
DNA
Tail

19. CLINICAL RELEVANCE phage can carry virulence factors
e.g., Beta - toxin produced by
Corynebacteria diptheria Beta - phage or
cholera toxin produced by
Vibrio cholerae CTX phage*

20. Lytic Infection Lysogenic Infection
BACTERIOPHAGE LIFESTYLE CHOICES
Lytic Infection
phage replicates itself
lyses the host cell
releases progeny phage
Lysogenic Infection
phage becomes latent
does not replicate
phage genome called prophage
can circularize or integrate into the host chromosome
cell that carries prophage is called a lysogen

21. TRANSDUCTION or LYSIS REPLICATION INFECTION LYTIC INDUCTION RESPONSE
LYSOGENIC CELL
Prophage
integrated
Prophage
autonomous
i.e., plasmid
ADSORPTION or
LYSOGENIC
RESPONSE

22. Lytic cycle

23. Lysogeny
prophage
lysogen

25. CLINICAL IMPLICATIONS OF GENETIC EXCHANGE
The genetic ecology of antibiotic resistance is complex.
Bacteria co-existing
in the lower GI-tract, soil, or other environments
can exchange DNA encoding diverse antibiotic resistances.
Antibiotic preparations are often contaminated with DNA,
increasing the chance of genetic exchange
via transformation between
antibiotic-producers & pathogens.

26. CLINICAL IMPLICATIONS OF GENETIC EXCHANGE
“Such considerations,
superimposed on our feeble understanding
of the nature, extent, and behavior
of microbial populations in the environment,
make it unlikely that antibiotic resistance and its transfer
can be effectively controlled

27. CLINICAL IMPLICATIONS OF GENETIC EXCHANGE
EXCEPT
by other than good clinical practice.
The answer to maintaining
long-term effective use of therapeutic agents
lies in better, more prudent use of antibiotics
in human and animal health care.”
(Julian Davies, 1994)

28. R plasmid from environment into competent pathogenic recipient
TRANSFORMATION
R plasmid from environment
into competent pathogenic recipient
Contaminating
R plasmid
COMMENSAL
DONOR
PATHOGENIC
RECIPIENT
COMPETENT
PATHOGEN
R plasmid
MULTIPLY RESISTANT
PATHOGEN
MULTIPLY RESISTANT
PATHOGEN
CONJUGATION
R plasmid from commensal donor
into pathogenic recipient