Antibiotic Resistance and Susceptibility Testing

This flashcard set covers the mechanisms of antibiotic resistance, historical evidence of AMR, and various laboratory methods for determining antimicrobial susceptibility.

Inherent resistance

A form of resistance where bacteria are naturally resistant to certain antibiotics regardless of prior exposure, such as Pseudomonas aeruginosa's resistance to penicillins.

Mutation

Spontaneous and random genetic changes in DNA that allow a bacterium to survive and thrive in the presence of drugs, following Darwin's principle of 'Survival of the Fittest'.

Bacterial transformation

A method of horizontal gene transfer involving the uptake of released DNA from a donor cell by a recipient cell.

Bacterial transduction

A method of horizontal gene transfer where genetic material is transferred from a phage-infected donor cell to a recipient cell via a phage.

Bacterial conjugation

A method of horizontal gene transfer involving the direct transfer of genetic elements like plasmids or transposons from a donor cell to a recipient cell.

Plasmid

An extra-chromosomal, self-replicating genetic element made up of double-stranded DNA ($ds-DNA$) that is extremely common in bacteria.

$\beta$-lactamase

An enzyme secreted into the bacterial periplasmic space that inactivates antibiotics by breaking down the antibiotic molecules.

Efflux pumping

An active transport mechanism requiring ATP that pumps antibiotics out of the bacterial cell to keep the concentration below lethal levels.

mecA

A gene acquired by MRSA that codes for a modified Penicillin-Binding Protein ($PBP$), allowing the bacterium to escape the activity of penicillin.

Minimally Inhibitory Concentration (MIC)

The lowest concentration of an antibiotic that produces no visible growth after overnight incubation of a standard inoculum.

Breakpoint

A chosen concentration ($\text{mg/L}$) of an antibiotic used in laboratory tests to define whether a species of bacteria is susceptible or resistant.

Etest (epsilometer test)

A phenotypic susceptibility test using a plastic reagent strip with a predefined gradient of antibiotic to determine the MIC.

Disc diffusion test (Kirby Bauer test)

A standardized susceptibility test that uses filter paper discs containing antibiotics to measure the zone of inhibition produced on an agar plate.

0.5 McFarland turbidity standard

A standard used to ensure a pure bacterial culture suspension is roughly equivalent to $150 \times 10^6$ cells per mL before susceptibility testing.

Zone of inhibition

The area around an antibiotic disc where bacteria have not grown, the diameter of which is measured and compared to standards to determine susceptibility.

Target overproduction

A resistance mechanism where bacteria produce additional amounts of a target molecule, such as chromosomal DHFR, to overcome competitive inhibition by drugs.

Genotypic susceptibility methods

Molecular methods such as PCR or whole-genome sequencing used to detect specific resistance genes like mecA, VanA, or rpoB.