Dissemination of Antibiotic Resistance Mechanisms of Bacterial Gene Exchange

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

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

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

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

Transformation

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*

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

MECHANISM OF TRANSFORMATION

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)

HOMOLOGOUS RECOMBINATION X exogenote endogenote recombinant

Conjugation

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

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*

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

Transduction

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

BACTERIOPHAGE Head nucleases detergents DNA Tail

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*

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

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

Lytic cycle

Lysogeny prophage lysogen

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.

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

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)

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