Colistin Case Study

Q1A: What is the mechanism of action of colistin? Colistin is an antibiotic that works best against Gram-negative bacteria. It works by binding to LPSs (lipopolysaccrides) and phospholipids in the outer cell membrane of the bacteria. This, in turn, disrupts the outer cell membrane by displacing cations and leaking the intracellular contents, combining it with outer cellular contents, causing the bacteria to be unable to differentiate the bacteria’s intra and outer cellular contents from one another. This ultimately leads to the bacteria’s death.
Q1B: What infections are treated with colistin? Colistin is used to treat infections caused by gram-negative or MDRO (Multidrug-Resistant Organisms). Occasionally, professionals will choose to avoid

Q2F: Minimum inhibitory concentration (MIC)
- Minimum inhibitory concentration is the minimum concentration of the substance necessary to prevent microbial growth.
Q2G: Lipopolysaccharide
- Lipopolysaccharide is a complex lipid structure containing unusual sugars and fatty acids found in most gram-negative Bacteria and constituting the chemical structure of the outer membrane.
Q2H: Enterobacteriaceae
- Enterobacteriacaea is a family of gram-negative, anaerobic, rod-shaped bacteria that are usually motile and consist of saprophytes and parasites of worldwide distribution. They can be found in soil, water, plants and animals.
Q2I: Mutation
- Mutation is an inheritable change in the base sequence of the genome of an organism. Question Set 3:
Q3A: The authors hypothesized that colistin resistance was spreading by horizontal gene transfer as opposed to mutation. What specific observation led to this

Afterwards, with the use of Glimmer 3.02 and BLAST, gene prediction and annotation were completed with another plasmid, pHN122-1, as a reference. To confirm the role of the gene that caused polymyxin resistance and contained mcr-1, the gene and its sequence were place into a cloning vector pUC18 that yielded pUC18-mcr-1. With this yield and electroporation, it was used to transform an E.coli strain, revealing its ability to confer colistin resistance.
Q3D: By what mechanism do the authors propose that the mcr-1 gene confers colistin resistance, and what evidence do they use to support this assertion?
The protein sequence of mcr-1 showed its similarity to the polymyxin-producing bacterium, Paenibacillus spp., which showed the possibility of gene transfer occurring. The mcr-1 gene enables protection from polymyxin. The mechanism that the authors proposed on how the mcr-1 gene confers colistin resistance is that mcr-1 causes a modification in lipid A, present in the lipopolysaccharides of most bacteria, which leads to lessened polymyxin affinity. The lipid A has phosphoethanolamine added to it, which in turn, inhibits the bacteria from any attachment.
Q3E: What is the origin of the mcr-1 gene, and what evidence do the authors use to support this