Method Article
A fast impedance-based antimicrobial susceptibility test
https://doi.org/10.21203/rs.3.pex-1088/v1
This work is licensed under a CC BY 4.0 License
published 20 Oct, 2020
posted 01 Oct, 2020
You are reading this latest preprint version
There is an urgent need to develop simple and fast antimicrobial susceptibility tests that allow informed prescribing of antibiotics. In this protocol we describe a method for a label-free AST that can deliver results within an hour, using an actively dividing culture as starting material.
antimicrobial susceptibility test
impedance
cytometry
single cell analysis
In this protocol an actively dividing bacterial isolate is incubated in the presence of the antibiotic for 30 minutes, and then approximately 105 cells are analysed one-by-one with microfluidic impedance cytometry for 2-3 minutes. The measured electrical characteristics reflect the phenotypic response of the bacteria to the mode of action of a particular antibiotic, in a 30-minute incubation window.
Cell media
Tryptic soy broth (TSB) [Sigma: 22092]
Cation Adjusted Mueller Hinton Broth (MHB) [Sigma: 90922]
Hanks media [Sigma: H6648]
Agar plates [Sigma L5542]
Antibiotics
Meropenem [Sigma M2574]
Colistin [Sigma 1148001]
Gentamicin [Sigma G4918]
Ciprofloxacin – [Sigma 17850]
Amoxicillin/Clavulanic acid [Sigma A8523 and Sigma 33454]
Cefoxitin [Sigma 1098107]
Reference Particles
Polybead® Microspheres 1.50μm [Polysciences 17133-15]
Bacterial isolates
Klebsiella pneumoniae strain 43358
Klebsiella pneumoniae strain ATCC 700603
Klebsiella pneumoniae strain 18397
Klebsiella pneumoniae strain 43292
Klebsiella pneumoniae strain 44271
Klebsiella pneumoniae strain KS1
Klebsiella pneumoniae strain KS11
Klebsiella pneumoniae strain K1
Klebsiella pneumoniae strain 1705
Klebsiella pneumoniae strain K14
Acinetobacter baumannii strain NCTC 13424
Acinetobacter baumannii strain ATCC 17978
Klebsiella pneumoniae strain 51851
Klebsiella pneumoniae strain NCTC 13368
Klebsiella pneumoniae strain CFI-014
Pseudomonas aeruginosa strain NCTC 13437
Pseudomonas aeruginosa strain PA01
Escherichia coli strain CFL 161
Escherichia coli strain NCTC 12923
Klebsiella pneumoniae strain51851
Klebsiella pneumoniae strain NCTC 13438
Klebsiella pneumoniae strain 51851
Klebsiella pneumoniae strain TW3
Klebsiella pneumoniae strain NCTC 13368
Klebsiella pneumoniae strain M6
Klebsiella pneumoniae strain NCTC 13368
Klebsiella pneumoniae strain NCTC 5054
Klebsiella pneumoniae strain NCTC 13438
Klebsiella pneumoniae strain M6
Escherichia coli strain LEC001
Escherichia coli strain NCTC 12923
Staphylococcus aureus strain NCTC 13616
Staphylococcus aureus strain NCTC 6571
--Zurich Instruments HF2IS impedance analyser (Zurich Instruments AG, Zurich, Switzerland)
--Shaking incubator set to 36-37oC with an air atmosphere.
--Vortex mixer for falcon and eppendorf tubes
--Single cell impedance cytometer glass chips. Custom - see procedure section for chip fabrication
--Single cell impedance cytometer electronics hardware -see application note here for an example https://www.zhinst.com/sites/default/files/documents/2020-03/zi_hf2is_appnote_microfluidics_2020.pdf
--A syringe pump (set to 30uL/min with a 1mL syringe, e.g. KD Scientific 788101, Harvard Apparatus 70-4500)
--Adjustable pipettes (P10, P20, P300, P1000)
Protocol 1
Step 1. Take a single colony of bacteria from a refrigerated or frozen plate, add to 10mL of Tryptic soy broth (TSB) and place in an incubator overnight at 37oC to revive the cells. This is sample A.
Step 2. Add a 1uL aliquot of sample A to an eppendorf containing 1mL Hanks media and vortex for 1 minute to ensure the sample is thoroughly mixed.
Step 3. Dilute sample A 1000-fold (1uL in 1mL Hanks) and measure the concentration of cells using the impedance cytometer (see impedance cytometer measurements section later for further details).
Step 4. Take an aliquot of sample A and dilute such that the final concentration (called sample B) is 5x105 cells/mL; From step 3, if the concentration of sample A is N cells/mL, add a volume of sample A given by V= N / 5x106 (uL) to 10mL MHB, such that the final concentration (called sample B) is 5x105 cells/mL. Typically sample A will be between 108 and 109 cells/mL, and therefore the volume V will be between 10-100uL.
Step 5. Incubate sample B in a shaking incubator for 30 minutes at 37oC.
Step 6. Add aliquots (950uL) to 7 pre-warmed test tubes each containing 50uL MHB and Meropenem at a concentration of 0, 5, 10, 20, 40, 80 and 160 mg/L of meropenem (final antibiotic concentration is 20x lower; i.e. 0, 0.25, 0.5, 1, 2, 4 and 8 mg/L.)
Step 7. Incubate the tubes for 30 minutes at 37oC.
Step 8. Wash the cells and resuspend in Hanks media (spin at 10,000*g for 5 minutes, remove the supernatant and top up to 1mL)
Step 9. Dilute each sample 1:10 (add 100uL from each of the 7 eppendorfs to 7 new eppendorfs containing 900uL Hanks).
Step 10. Add a volume of concentrated bead suspension that contains approximately 104 polystyrene beads (1.5um diameter) to each eppendorf.
Step 11. Load the contents of each tube into a 1mL syringe and measure with the impedance cytometer (see later).
Protocol 2.
Step 1. Streak out a 10uL loop from a frozen glycerol stock onto an agar plate. Place the plate into an incubator overnight at 37oC.
Step 2. Pick three typical colonies from the plate and add to 3mL of MHB - this is sample A.
Step 3. Vortex sample A for 1 minute to ensure that it is well mixed.
Step 4. Dilute sample A 400-fold (2.5uL in 1mL Hanks) and measure using the impedance cytometer to determine the concentration of the bacteria in sample A (see impedance cytometer measurements section later for further details).
Step 5. Assuming the concentration of sample A is N cells/mL, add a volume of sample A given by V=N / 15x105 uL to 3mL MHB, such that the final concentration (called sample B) is 5x105 cells/mL. Typically sample A will be approximately 108 cells/mL, and therefore the volume V will be approximately 15uL.
Step 5. Incubate sample B in a shaking incubator for 30 minutes at 37oC.
Step 6. Add aliquots (500uL) to pre-warmed test tubes each containing 500uL MHB and an antibiotic. Concentrations of antibiotics are 4mg/L and 32mg/L for Meropenem , 2 mg/L for ciprofloxacin, 16 mg/L for gentamicin, 8 mg/L for Colistin, 16 mg/L for ceftazidime, along with a control (no antibiotic). Final concentrations of antibiotics will be half the above concentrations.
Step 7. Incubate the tubes for 30 minutes at 37oC.
Step 9. Dilute each sample 1:10 (add 100uL to an new eppendorfs containing 900uL Hanks media).
Step 10. Add a volume of concentrated bead suspension that contains approximately 104 polystyrene beads (1.5um diameter) to each eppendorf.
Step 11. Load the contents of each tube into a 1mL syringe and measure with the impedance cytometer (see
Single cell impedance cytometry measurements
Step 1. Load the sample into a 1mL syringe.
Step 2. Connect the syringe to the impedance cytometer tubing.
Step 3. Insert the syringe into a syringe pump.
Step 4. Start the syringe running at 30uL/min.
Step 5. Start the ziControl software which controls the Zurich Instruments impedance analyser, and apply a measurement voltage (see measurement settings below).
Step 6. Record data for 2-3 minutes (depending on the required number of bacteria to measure).
Step 7. Turn off the measurement voltage.
Step 8. Flush through with a suitable cleaning reagent (e.g. buffer, NaOH, NaClO).
Repeat steps 1-8 for each remaining sample.
Measurement settings (ziControl)
Sample rate: 230k samples/second
Applied frequencies: 5MHz and 40MHz
Applied voltage: 2v pk-pk
Data analysis
Step 1. Determine the impedance of each particle by extracting the peak signal amplitude using convolution for each applied frequency.
Step 2. Determine the mean signal of the 1.5um beads
Step 3. Normalise the opacity-cell size scatterplot by a single linear multiplier for each axis to ensure the mean of the beads is at Opacity =1 and diameter = 1.5um.
Step 4. Define a contour is defined around the population of cells not exposed to antibiotic. This is termed the unexposed contour should include 50% of the cell population.
Step 5. For each antibiotic exposed sample, normalise the data (as per step 3) and determine the number of cells that fall within the unexposed contour.
Single cell impedance cytometer chip fabrication
Step 1. Deposit 10nm titanium and the 200nm platinum onto two 6 inch glass substrates
Step 2. Spin-coat a suitable photoresist (e.g. S1813) and pattern by exposure to UV light through a 7 inch photomask
Step 3. Develop the resist by washing with a suitable resist developer.
Step 4. Pattern the meta layers using Ion beam milling.
Step 5. Strip the photoresist.
Step 6. Spin-coat SU-8 to the desired channel thickness (e.g. 30um) on to one of the two patterned wafers
Step 7. Expose to UV light through a 7 inch photomask and develop.
Step 8. Align and bond the second wafer to the first wafer by vacuum bonding at 180°C, 10kN for 6 hours.
Step 9. Carefully scribe and dice the wafer to release the individual cytometer chips.
If the microcytometer chip gets blocked, try flushing with buffer from outlet back to the inlet. If this is unsuccessful, replace with a new chip. If the cytometer is frequently clogging, filter all reagents with a 0.22um (sterile) filter.
A populations of beta-lactam exposed cells from an isolate that is sensitive to the antibiotic are expected to increase in low-frequency impedance and decrease in electrical opacity relative to an unexposed sample of the same isolate. A populations of beta-lactam exposed cells from an isolate that is resistant to the antibiotic are expected to not change significantly in low-frequency impedance or electrical opacity relative to an unexposed sample of the same isolate.
A populations of aminoglycoside exposed cells from an isolate that is sensitive to the antibiotic are expected to decrease in low-frequency impedance and increase in electrical opacity relative to an unexposed sample of the same isolate. The bacterial concentration is expected to decrease relative to the unexposed aliquot. Populations of aminoglycoside exposed isolates that are resistant to the antibiotic are expected to not change or decrease slightly in low-frequency impedance and not change significantly in electrical opacity relative to an unexposed sample.
A populations of Colisitin exposed cells from an isolate that is sensitive to the antibiotic are expected to decrease in low-frequency impedance and increase in electrical opacity relative to an unexposed sample of the same isolate. The bacterial concentration is expected to decrease relative to the unexposed aliquot. Populations of Colistin exposed isolates that are resistant to the antibiotic are expected to not change or decrease slightly in low-frequency impedance and not change significantly in electrical opacity relative to an unexposed sample.
A populations of fluoroquinolone exposed cells from an isolate that is sensitive to the antibiotic are expected to increase in low-frequency impedance and not change significantly in electrical opacity relative to an unexposed sample of the same isolate. Populations of fluoroquinolone exposed isolates that are resistant to the antibiotic are expected to not change significantly in low-frequency impedance or in electrical opacity relative to an unexposed sample.
We thank Katie Chamberlain for fabricating the cytometer chips.
published 20 Oct, 2020
posted 01 Oct, 2020
You are reading this latest preprint version
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