High Throughput, Real-time, Dual-readout Testing of Intracellular Antimicrobial Activity and Eukaryotic Cell Cytotoxicity

The overall goal of this protocol is to use a single combination real-time assay to identify specific small molecule inhibitors of intracellular bacterial growth that are non-toxic to host cells. This method can help answer key questions in the antimicrobial discovery field. Specifically, what types of compounds can penetrate into intracellular compartments and kill pathogens without harming the host cell?

The main advantages of this technique are the bacterial growth mammalian cell toxicity are detected simultaneously using non-destructive, real-time assays. Generally, individuals new to this method will struggle with the scalability that it affords. Assaying greater than ten thousand compounds in a session requires a whole new set of techniques and equipment.

In addition to Lucius, demonstrating a communal commitment to academic antimicrobial discovery and translational diagnostic microbiology are post-doctoral fellows, KP Smith, Yoon-Suk Kang, Jennifer Tsang, Thea Brennan-Krohn, and undergraduate Kat Truelson. To prepare host cells, culture J774A. 1 macrophage is in suspension in RPMI 1640, with nine percent iron supplemented calf serum.

Initially passage cells in tissue culture flasks. After cells have become confluent in a 75 square centimeter tissue culture flask in 15 milliliters of medium, split the cells by scraping and diluting them to 65 milliliters with the same type of medium. Return 15 milliliters to the tissue culture flask, and transfer 50 milliliters to a 250 milliliter bacterial shaker flask.

Aerate by setting rotation speed at approximately 120 revolutions per minute. For consistent growth, incubate at exactly five percent carbon dioxide and 37 degrees Celsius. Harvest the cells when they reach a density in the range of two point five million cells per milliliter, to five million cells per milliliter.

Ensure dead cell percentage does not exceed 25 percent, as dead cells will increase background noise in cytotoxicity assays. It's critical to prevent cell settling in reservoirs when you're dispensing large volumes of macrophages and bacteria with liquid handlers. Swirl reservoirs every few minutes while dispensing to avoid non-uniformity and assay plate wells.

Plate the cells in white tissue culture treated, 384 well microplates at 50, 000 cells in 30 microliters of tissue culture medium per well. Incubate microplates overnight to achieve ninety percent confluence on the day of the experiment. Prepare patches of bacteria for experiments, by spreading organisms thickly on a new BCYE plate and incubating one day to obtain confluent growth.

Re-suspend the organisms in the same tissue culture medium used for the J774A. 1 cells. To perform the macrophage infection, add test compounds of interest, including screening compounds, and positive and negative controls for bacterial growth inhibition in eukaryotic cell lysis.

Stock solutions should be dissolved at greater than or equal to 500x in DMSO or an aqueous solution, to allow sufficient dilution of the vehicle. Dilute luminescent bacteria to a target of two point five million CFU per millileter in tissue culture medium. Then, add appropriate non-toxic membrane impermeant nucleic acid finding dye at two point five x final assay concentration.

Add 20 microliters of this mixture to each culture well. The final assay volume is 50 microliters and the final bacterial concentration should be one million CFU per milliliter for the lux epron reporter, or four million CFU per milliliter for the fluorescent protein reporter. Incubate at 37 degrees Celsius, for one to three days in five percent carbon dioxide at 100 percent relative humidity to prevent evaporative edge effects.

To avoid temperature associated edge effects when reading assay results, thermally equilibrilate microplates prior to luminescence reading by placement in a single layer on a lab bench with lids ajar for approximately 20 minutes. Read bacterial growth and eukaryotic cell toxicity on a microplate luminometer and flourometer as appropriate for the reporters being used. Return microplates to the incubator if real-time readout at later time points is desired.

Analyze the data as described in the text protocol. For dose response and synergy testing experiments, prepare macrophages in bacteria as before. Just prior to macrophage infection, or axenic incubation, use an automated liquid handling system to facilitate single dose response in combinatorial synergy testing set up through direct automated addition of antimicrobial dilutions according to manufacturers protocol.

Add antimicrobials of experimental interest in a serial doubling dilution series. The goal is to span, on the high end, a concentration that completely eliminates growth, and on the low end, a concentration that shows no obvious activity. Shown here, are representative results over time for bacterial luminescence and cytotoxicity associated fluorescence.

Note the contrasting effects of antimicrobial treatment and eukaryotic cell cytotoxic detergent. Dual IC50 and CC50 dose response determination in the same screening wells was used to determine selectivity for the antibiotic Doxycycline. Bacterial replication was inhibited by intermediate concentrations of antibiotic.

However, cytotoxicity was observed at high concentrations consistent with the selectivity of approximately 100. The same assay format was used to test two dimensional dose response for combinations of Minocycline and Azithromycin. Isocontours connect points of equal intracellular growth inhibition.

Here, concave isocontours suggested strong synergistic effects for the two drugs. Digital dispensing robotics for assay setup real-time readout, and high-throughput format enabled combinatorial triple synergy testing. Here, observation of a distinctly concave surface suggests a high degree of synergistic effect for combinations of Minocycline, Azithromycin, and Rifampin against intracellular growth of legionella pheumophila.

After watching this video, you should have a good understanding of how to combine either fluorescent or luminescent assay read-out with real-time, cytotoxicity testing in high-throughput assay format. Following this procedure, other methods like confocal microscope can be applied answer questions about biological event in the whole cell, which are also sited with their antimicrobe activity or whole cell cytotoxicity. Though we studied the effects on legionella replication, the same techniques can also be applied to other intracellular pathogens such as Brucella, and microbacteria.

We first conceived to this protocol when trying to figure out an assay simple and reliable enough to use in a very high-throughput screening format. Don't forget that working with bacterial pathogens, cell lines, and robotic equipment has some inherent risks and appropriate precautions should be taken, such as use of appropriate personal protective equipment while performing this procedure.