by Jamie Liu
Fun Rating: 3/5
Difficulty Rating: 2/5
What is the general purpose?
This tells the scientist how bacteria grow and respond to different antibiotics. Antibiotics are medicines that doctors prescribe to treat bacterial infections.
Why do we use it?
There are many different species of bacteria, and they all respond differently to the same antibiotic. We use antibiotic kill curves to understand how a certain drug behaves and how effective it may be against bacteria.
How does it work?
A kill curve works by exposing bacteria to a certain antibiotic to observe how well the bacteria grow over time. In order to set up a kill curve, the scientist would need the test antibiotic, bacteria, and growth medium. Growth medium is a broth that provides nutrients for the bacteria to keep them happy and growing. First, an antibiotic solution is made. Antibiotics are usually in powder form, so we need to dissolve it in a solvent like water or alcohol before adding to our growth medium. Kill curves can be done with one or multiple concentrations of antibiotic throughout the entire experiment. Changing the concentration of antibiotics over the course of the experiment can better mimic what antibiotics do in a patient. The antibiotic and bacteria are then added to the growth medium in a flask, and the flask is placed in a warm, shaking incubator. Some bacteria need oxygen to grow, so we shake them because the motion introduces oxygen into the growth medium.
At different time points (1, 2, 4, 6, and 24 hours) after we’ve placed our bacteria into the incubator, we collect samples from each culture. Taking cultures at different time points gives us a snapshot as to how much bacteria have grown over time in the presence or absence of antibiotics. Once we’ve collected our samples, we calculate colony forming units (CFUs). We calculate CFUs to determine the number of bacteria we have in each sample so we know how much of the bacteria are dying in the presence of the antibiotic. To calculate CFUs, we first dilute the sample because there are billions of bacteria in our samples. After, we spread our diluted samples onto a nutrient-rich agar plate. An agar plate is a petri dish with gel that has all the nutrients for our bacteria to grow. The plates go into a warm, static incubator, and the next day, we can count how much bacteria we have at each time point (Figure 1). We can plot the amount of bacteria found at each time point and compare the conditions that have antibiotics and the ones that do not (Figure 2). Antibiotics can have different effects on bacteria – some can be more effective in eliminating the bacteria than others! This information can inform scientists and doctors how antibiotics work and which ones would be better to use to treat patients.
Figure 1. Antibiotic kill curves test how effective antibiotics are against bacteria. First, bacteria and antibiotics are added to nutrient-rich media. The flasks are then placed in a warm, shaking incubator. At designated time points, some of the bacteria is removed from the flask and plated to determine how much bacteria is present. Schematic generated with BioRender.
Figure 2. Antibiotics can affect the growth of bacteria. Bacteria were treated with antibiotic 1, antibiotic 2, or no antibiotic (untreated). Antibiotics 1 and 2 reduced the amount of growth over time, though antibiotic 2 showed a greater reduction. Figure generated with GraphPad Prism.
