- The resistance to trimethoprim in this scenario is mediated by an alteration in which of the following enzymes?
Answer: Dihydrofolate reductase.
Trimethoprim inhibits bacterial folate synthesis by targeting dihydrofolate reductase (DHFR). A primary mechanism of resistance is the acquisition of a plasmid-borne gene (such as a dfr gene) that encodes a modified DHFR. This altered enzyme is highly resistant to inhibition by trimethoprim but retains its normal enzymatic function, allowing the bacteria to continue synthesizing tetrahydrofolate and survive.
- Which of the following is the most likely mechanism of this organism's resistance to nafcillin?
Answer: Alteration of the drug's target site.
The patient has methicillin-resistant Staphylococcus aureus (MRSA), indicated by nafcillin resistance and the presence of the mecA gene. This gene encodes for a modified penicillin-binding protein (PBP2a), which has a low affinity for beta-lactam antibiotics. This alteration of the target site prevents the antibiotic from effectively inhibiting cell wall synthesis.
- The development of ciprofloxacin resistance in this organism is best explained by an alteration in which of the following?
Answer: DNA gyrase.
Fluoroquinolones, such as ciprofloxacin, exert their effect by inhibiting bacterial DNA gyrase (topoisomerase II) and topoisomerase IV. Resistance commonly arises from point mutations in the genes encoding these enzymes, such as gyrA for DNA gyrase. These mutations alter the drug's binding site, reducing its inhibitory effect.
- The mechanism of carbapenem resistance related to OprD is best described as which of the following?
Answer: Reduced drug influx.
Pseudomonas aeruginosa is an opportunistic pathogen known for its intrinsic and acquired resistance. One important mechanism is its relatively impermeable outer membrane. OprD is a specific outer membrane porin channel through which carbapenems preferentially enter the cell. Downregulation or mutational inactivation of the oprD gene leads to decreased permeability and reduced influx of the drug, resulting in resistance.
- The resistance to sulfamethoxazole in this isolate is most likely due to which mechanism?
Answer: Production of an alternative dihydropteroate synthase.
Sulfonamides are structural analogs of PABA and competitively inhibit dihydropteroate synthase, a key enzyme in the bacterial folate synthesis pathway. The most common mechanism of resistance is the acquisition of a plasmid-encoded gene (e.g., sul1, sul2) that produces an altered dihydropteroate synthase. This modified enzyme has a much lower affinity for sulfonamides but can still efficiently use PABA, thus bypassing the drug's inhibitory effect.
- Plasmid-mediated penicillin resistance in Neisseria gonorrhoeae is most commonly due to which of the following?
Answer: Production of beta-lactamase.
Neisseria gonorrhoeae has developed resistance to penicillins through multiple mechanisms. A historically important and common plasmid-mediated mechanism is the production of a beta-lactamase enzyme (penicillinase) that cleaves the beta-lactam ring of penicillin, inactivating the drug. Chromosomal mutations leading to altered PBPs or porin channels also contribute to resistance but beta-lactamase production is the classic plasmid-mediated mechanism.
- The mobility of this resistance gene between a plasmid and the chromosome is characteristic of which of the following genetic elements?
Answer: Transposon.
The vanA gene cluster, which confers high-level vancomycin resistance, is often located on a transposon (e.g., Tn1546). Transposons are mobile genetic elements ('jumping genes') that can move from one location to another within the genome, such as from a plasmid to a chromosome or between plasmids. This mobility greatly facilitates the dissemination of resistance genes.
- This mutation confers resistance to oseltamivir by altering which viral protein?
Answer: Neuraminidase.
Oseltamivir is a neuraminidase inhibitor. Neuraminidase is a viral enzyme that cleaves sialic acid residues, allowing newly formed virions to be released from the infected host cell. The H275Y mutation occurs in the neuraminidase enzyme, altering its active site. This change reduces the binding affinity of oseltamivir, allowing the enzyme to function despite the presence of the drug, leading to resistance.
- Which of the following best describes the mechanism of this intermediate-level vancomycin resistance?
Answer: Trapping of vancomycin molecules in the outer layers of the cell wall.
This patient has vancomycin-intermediate S. aureus (VISA). The mechanism of resistance is distinct from the high-level resistance seen in VRE. In VISA, there are mutations that lead to a disordered, thickened cell wall with an increased number of D-Ala-D-Ala residues. Vancomycin molecules get trapped in this thick outer layer, binding to these false targets and are thus prevented from reaching their site of action at the cell membrane.
- What is the most likely mechanism for chloramphenicol resistance in these isolates?
Answer: Enzymatic inactivation by acetylation.
The most common mechanism of plasmid-mediated resistance to chloramphenicol is enzymatic inactivation. The plasmid carries the gene for chloramphenicol acetyltransferase (CAT). This enzyme transfers an acetyl group from acetyl-CoA to the chloramphenicol molecule, rendering it unable to bind to the 50S ribosomal subunit and inhibit protein synthesis.
- The mutation in the rpoB gene confers resistance to rifampin by altering which of the following cellular components?
Answer: DNA-dependent RNA polymerase.
Rifampin functions by inhibiting bacterial DNA-dependent RNA polymerase, thereby blocking transcription. The rpoB gene encodes the beta subunit of this enzyme. Mutations in this gene are the primary mechanism of rifampin resistance in M. tuberculosis, as they alter the drug-binding site on the RNA polymerase, preventing the drug's inhibitory action.
- The overexpression of these genes most likely contributes to fluconazole resistance through which mechanism?
Answer: Increased production of drug efflux pumps.
A major mechanism of azole resistance in Candida albicans is the upregulation of genes encoding ATP-binding cassette (ABC) transporters (e.g., CDR1, CDR2) and major facilitator superfamily (MFS) transporters (e.g., MDR1). These transporters function as multidrug efflux pumps, actively removing azole antifungals from the cell, which prevents them from reaching their target, lanosterol 14-alpha-demethylase.
- What is the biochemical basis for this organism's resistance to vancomycin?
Answer: Modification of the peptidoglycan precursor target.
Vancomycin-resistant enterococci (VRE) with the vanA gene cluster exhibit resistance by altering the drug's target. The terminal D-alanyl-D-alanine (D-Ala-D-Ala) of the peptidoglycan precursor is modified to D-alanyl-D-lactate (D-Ala-D-Lac). This change significantly reduces the binding affinity of vancomycin, rendering it ineffective.
- Which of the following is the mechanism of resistance conferred by the mef gene in this Streptococcus pneumoniae isolate?
Answer: Active drug efflux.
The mef (macrolide efflux) gene encodes a drug efflux pump that actively transports macrolides out of the bacterial cell, keeping intracellular concentrations below effective levels. This results in the M phenotype of resistance (resistant to macrolides but susceptible to lincosamides like clindamycin), as opposed to the erm gene which causes methylation of 23S rRNA and results in the MLS-B phenotype (resistance to macrolides, lincosamides, and streptogramin B).
- The spread of this resistance gene between different bacterial species is most likely facilitated by which of the following processes?
Answer: Conjugation.
The scenario describes the transfer of a resistance gene on a large, self-transmissible extrachromosomal DNA element (a plasmid) between different bacterial species (Acinetobacter and Klebsiella). This process is conjugation, which involves the transfer of genetic material, typically a plasmid, from a donor to a recipient cell through direct cell-to-cell contact via a sex pilus. It is a major mechanism for the spread of antibiotic resistance among gram-negative bacteria.
- Which of the following mechanisms, when encoded on a plasmid, is most likely responsible for the high-level resistance to gentamicin in this organism?
Answer: Formation of a drug-inactivating enzyme.
A common and highly effective mechanism of resistance to aminoglycosides like gentamicin is enzymatic modification of the drug. Bacteria can acquire plasmids encoding enzymes that inactivate aminoglycosides through acetylation, phosphorylation, or adenylation. This modification prevents the drug from binding to its target, the 30S ribosomal subunit.
- The resistance of this E. coli isolate to ceftriaxone is most likely due to which of the following mechanisms?
Answer: Production of an extended-spectrum beta-lactamase.
The susceptibility pattern (resistance to third-generation cephalosporins like ceftriaxone but susceptibility to carbapenems) and a positive double-disk synergy test are characteristic of an extended-spectrum beta-lactamase (ESBL)-producing organism. ESBLs are enzymes that hydrolyze and inactivate penicillins and extended-spectrum cephalosporins.
- Which of the following is the most likely mechanism of resistance in this isolate?
Answer: Production of a carbapenemase.
Resistance to carbapenems (e.g., imipenem, meropenem) in Klebsiella pneumoniae is most commonly due to the production of carbapenemases, such as Klebsiella pneumoniae carbapenemase (KPC). These enzymes are beta-lactamases that can hydrolyze a broad spectrum of beta-lactam antibiotics, including the carbapenems, rendering them ineffective.
- How does a mutation in the katG gene lead to isoniazid resistance?
Answer: It impairs the enzymatic activation of the prodrug.
Isoniazid is a prodrug that must be activated within the mycobacterium to become effective. The bacterial enzyme catalase-peroxidase, encoded by the katG gene, is responsible for this activation. Mutations that inactivate or reduce the function of KatG prevent the conversion of isoniazid to its active form. As a result, the drug cannot exert its therapeutic effect, which is the inhibition of mycolic acid synthesis.
- The presence of the tet(A) gene confers resistance to doxycycline through which of the following mechanisms?
Answer: Creation of an energy-dependent efflux pump.
The most common mechanism of tetracycline resistance is the acquisition of genes, such as tet(A), that encode for membrane-associated efflux pumps. These pumps use energy (e.g., proton motive force) to actively transport tetracycline molecules out of the bacterial cell, preventing the drug from reaching the high intracellular concentration needed to inhibit protein synthesis at the 30S ribosome.
