Question

1. A woman has recently been dealing with an ear infection and has been prescribed the antibiotic amoxicillin. However, after 3 days into a 10-day regimen, she feels better and opts to stop taking her meds. A few days later, she experiences a Clostridium difficile (C. diff) infection. Laboratory testing of a sample of C. difficile from the woman to determine which antibiotic to use shows that the bacteria are mostly resistant to amoxicillin. A sample of the amoxicillin-resistant C. difficile reveals the presence of a unique plasmid in the cells. Using your knowledge of plasmids, discuss what genes and regulatory sites you expect to find on this unique plasmid. For each type of gene or site, describe why you would expect to find it on the plasmid. (2 PTS)

Answer 1

Plasmids are extracheomosomal circular DNA molecules naturally found in bacteria. They exist independently and hence in order to survive in a bacterial cell they must have region where it could recruit the protein machinery for its replication. This stretch of sequence is called an origin of replication (Ori) and is specific for a given strain or species of bacteria. Most often plasmids are seen to carry genes which render resistance to the host bacterium against certain antibiotics. These genes either encode an ABC transporter which act by pumping out the antibiotic from the bacterial cell or a beta-lactamase enzyme which cleaves beta-lactams (bactericidal drugs).

Hence, in order to render resistance to C. difficile against amoxicillin (a beta-lactam drug), the new plasmid must, in the first place, possess a Ori specific for C. difficile in order to propagate itself in these bacteria. Secondly, it should contain a gene encoding beta-lactamase enzyme to target and hydrolyse the beta-lactams. Thirdly, it requires a promoter upstream of the beta-lactamase gene which can recruit the RNA polymerase to its site for transcription.