Here we describe a protocol that is an adaptable, whole host, high-content screening tool that can be utilized to study host-pathogen interactions and be used for drug discovery.
This method can answer questions in host pathogen interaction and drug discovery, such as the the importance of various virulence factors and the ability of small molecules to ameliorate with the genesis. The main advantage of this technique is that it takes place in a liquid format with small volumes. To begin, while working inside a BSL 2 biosafety cabinet, streak P.aeruginosa from a frozen stock onto an LB Agar plate.
Incubate the plate at 37 degrees C for 16 to 24 hours, then store the plate at four degrees Celsius for up to one week. Two days prior to setting up assay plates, use a single colony from the plate to inoculate three to five milliliters of sterile LB broth. Incubate the culture at 37 degrees Celsius for 12 to 16 hours.
After preparing the slow kill or SK media according to the text protocol, with 350 milliliters of p. Aeruginosa from the fresh overnight LB culture, seed each 10 centimeter SK plate. Use a sterile bacterial spreader to evenly spread the bacteria, and allow the plates to dry in the biosafety cabinet.
Then, incubate the plates at 37 degrees Celsius for 24 hours. To test multiple RNAi bacterial strains in parallel in a single well of a 24 well deep well plate, inoculate a single colony from each clone of previously prepared RNAi containing bacteria into four milliliters of carbenicillin supplemented LB.Place the plates into a shaking incubator optimized for multi-well plates at 37 degrees Celsius and 950 rpm for 16 hours, then collect the bacteria by centrifugation at 2, 000 g for 5 minutes. Decant the supernatant by inverting the plate and vigorously shaking it.
Using 100 microliters of S Basal resuspend the RNAi bacteria. Pipette the resuspended bacteria into an appropriate number of wells of a multi-well NGM plate supplemented with IPTG and carbenicillin and allow the plate to dry. After preparing L1 worms according to the text protocol, prepare plates for basic experiments by pipetting approximately 5, 000 worms per 10 centimeter plate, seeded with RNAi or OP50 superfood.
To set up for an RNAi screen, pipette approximately 300 worms per well in a 24 well plate seeded with RNAi. If the strain being used is temperature-sterile, incubate the worms at 15 degrees Celsius for approximately 16 hours and then transfer them to 25 degrees Celsius for 44 hours to complete development and prevent embryogenesis. To carry out a liquid killing assay use a cell scraper to remove the P.aeruginosa from an SK plateand resuspend the bacteria in approximately 5 milliliters of S.Basal.
Using a spectrophotometer meaure the OD 600 of the bacterial suspension. Prepare 24 milliliters of diluted stock of P.Aeruginosa in S.Basal at an OD 600 equal to 09, then add 21 milliliters of liquid SK media, and, using a multichannel pipette, transfer 45 microliters of bacteria and media to each well of a 384 well plate. Wash the worms from their source into a 50 milliliter conical tube, and allow them to settle under gravitational force.
Aspirate the supernatant to 5 milliliters, then use a total of 50 milliliters of S.Basal to resuspend the worms, and repeat the wash twice more. Using a worm sorter, sort approximately 22 worms into each well of a 384 well plate, according to the text protocol, then use a gas-permeable film to seal the plate and incubate it at 25 degrees Celsius for 24 to 48 hours. At the desired time, use a microplate washer and S.Basal to wash the 384 well plate a total of 5 times.
One of the easiest mistakes to make is to aspirate the 384 well plate before worms have completely settled. It takes practice to accurately see whether the worms have settled completely. After the final wash, aspirate the supernatant down to 20 microliters.
Then, add 50 microliters of 98 micromolar nucleic acid stain per well, for a final concentration of 0.7 micromolar. Incubate the plate at room temperature for 12 to 16 hours. After the desired incubation period, use the microplate washer to wash the plates a minimum of three times.
For a data acquisition use a spectrophotometer or an automated microscope to image both transmitted light and fluorescence. Add one milliliter of S Basal per well of a 24 well plate containing 300 worms per well. Gently agitate the worms by shaking the plate, then transfer worms to an empty, sterile 24 deep well plate.
Allow the worms to settle gravitationally, about five minutes. Aspirate the supernatant, leaving approximately one milliliter per well, Then add 7 milliliters of S Basal to each well. Repeat the wash two more times, and then after the final wash aspirate all but approximately 400 microliters of supernatant.
After pipetting the bacteria into 384 well plates to avoid starvation, using the resampler function of the worm sorter sort 22 worms into each well of a 384 well plate. Finally, following sorting, add small molecules or other experiment specific materials. As illustrated in this graph, when the steps outlined in this video are followed, a time dependent killing of C.elegans will be observed only in the presence of P.aeruginosa.
In contrast, in the absence of key bacterial nutritional supplements, little to no killing will be observed. As shown here, the two step incubation of P.aeruginosa at 37 degrees Celsius and then 25 degrees Celsius, which is critical for conventional slow killing assays, and was originally implemented in liquid killing, is dispensable in this assay. The protocol tolerates a wide range of initial bacterial concentrations, from as low as an OD 600 equal to 0025, and still exhibits time and concentration dependent killing, although the timing does shift.
In addition, as few as four wells is frequently sufficient to obtain statistically significant data. An example of the utility of processing is shown here when the signal-to-noise ratio is high, as in this example, analysis is simple and discriminating between positive and negative conditions is trivial. In these cases, even weak hits can be readily identified.
Once mastered, this technique can be performed in about 60 to 75 minutes of hands on time per 384 well plate, not including sorting. While attempting this procedure, it's critical to remember to add all the components to your media or virulence will be compromised. This assay simplifies the application of whole organism phenotype based screening to drug discovery in host-pathogen research.
After watching this video, you should have a good understanding of how to carry out liquid-based C elegans pathogenesis assays. This procedure can be modified by substituting with other pathogens, such as E.Faecalis for P.Aeruginosa, facilitating the search for treatments for other bacterial infections. Don't forget that working with infectious bacteria like P.Aeruginosa or E.Faecalis can be dangerous.
Appropriate precautions, such as proper training, good personal protective equipment, and proper technique, should always be taken while performing this procedure.
