Broth Microdilution MIC Determination for Ciprofloxacin Against Escherichia coli per CLSI M07

Each isolate is tested across an 11-point two-fold dilution series of ciprofloxacin from 0.008 to 8 µg/mL, plus a growth (no-drug) and sterility (no-inoculum) control well, in a single 96-well row. Each isolate is run in technical duplicate within a plate and biological triplicate across three independent days using freshly prepared inoculum. E. coli ATCC 25922 is included as a QC strain on every plate. Final inoculum is standardized to ~5 x 10^5 CFU/mL per well. Reading is performed by a single trained reader; a second blinded reader independently scores 20% of plates to assess concordance.

BSL-2 for clinical E. coli isolates; work in a Class II biosafety cabinet for inoculum prep when handling unknown clinical strains. Wear lab coat, nitrile gloves, and eye protection. Ciprofloxacin is harmful if ingested and is a reproductive-hazard concern; weigh powder in a fume hood with a dust mask and avoid skin contact. Decontaminate all plates, tips, and reservoirs by autoclaving or 10% bleach (>=20 min contact) before disposal as biohazard waste. NaOH stock is corrosive; handle with care.

Positive growth control: drug-free well that must show visible turbidity for the test to be valid. Sterility control: uninoculated CAMHB that must remain clear (detects medium contamination). QC reference strain E. coli ATCC 25922 on every plate, whose MIC must fall within the CLSI published range (0.004-0.015 µg/mL) or the entire plate is discarded. A known resistant isolate (e.g., gyrA mutant) serves as a high-MIC assay positive. Inoculum colony count (CFU plate) confirms 2-8 x 10^5 CFU/mL final density.

E. coli ATCC 25922 MIC of 0.004-0.015 µg/mL (mode ~0.008). Susceptible clinical isolates typically <=0.25 µg/mL; resistant isolates >=1 µg/mL, often 4 to >8 µg/mL. A clean two-fold endpoint with a sharp turbid-to-clear transition; no skipped wells (a clear well below a turbid one indicates an error). Growth control strongly turbid (OD600 >0.4 if measured); sterility control clear. Inoculum count 2-8 x 10^5 CFU/mL.

• Cation-adjusted Mueller-Hinton Broth (CAMHB), prepared per manufacturer, Ca2+ adjusted to 20-25 mg/L and Mg2+ to 10-12.5 mg/L
• Ciprofloxacin reference standard powder (USP grade), stock 1280 µg/mL in 0.1 N NaOH then diluted in sterile water
• E. coli ATCC 25922 (QC strain) and test isolates
• Mueller-Hinton agar (MHA) plates for purity/colony counts
• Sterile 0.85% NaCl for inoculum suspension
• 0.5 McFarland turbidity standard (BaSO4 or latex)
• Sterile round-bottom 96-well polystyrene microtiter plates with lids
• Sterile reagent reservoirs, multichannel pipette (10-300 µL), sterile tips
• Adhesive plate seals (gas-permeable)

To quantify the lowest concentration of ciprofloxacin (in µg/mL) that visibly inhibits growth of Escherichia coli after 16-20 h incubation, using a standardized two-fold dilution series in cation-adjusted Mueller-Hinton broth (CAMHB) in a 96-well plate, and to interpret results against current CLSI M100 breakpoints. This provides a reproducible, reference-standard antimicrobial susceptibility measurement suitable for surveillance and comparison across laboratories.

1. Streak each isolate onto MHA and incubate 18-24 h at 35 +/- 2 C to obtain isolated colonies; confirm purity.
2. Prepare ciprofloxacin working stock at 16 µg/mL (2x final top concentration) in CAMHB.
3. Dispense 50 µL CAMHB into wells 2-11; add 100 µL of 16 µg/mL drug to well 1 and 50 µL to well 2.
4. Serially transfer 50 µL across wells 2-11, discarding 50 µL from well 11, to create a two-fold series (8 to 0.008 µg/mL after inoculation).
5. Suspend 3-5 colonies in saline and adjust to 0.5 McFarland (~1-2 x 10^8 CFU/mL).
6. Dilute 1:100 in CAMHB to ~1 x 10^6 CFU/mL; add 50 µL to each test well for a final ~5 x 10^5 CFU/mL.
7. Include a growth control (50 µL CAMHB + 50 µL inoculum, no drug) and sterility control (100 µL CAMHB, no inoculum).
8. Plate 10 µL of the diluted inoculum on MHA for a colony count to verify inoculum density.
9. Seal, incubate at 35 +/- 2 C in ambient air for 16-20 h.
10. Read the MIC as the lowest concentration with no visible turbidity against a dark background.

Independent variable: ciprofloxacin concentration (0.008-8 µg/mL, two-fold steps). Dependent variable: presence/absence of visible growth per well, yielding the MIC endpoint. Controlled variables: medium (CAMHB with defined cation content), inoculum density (5 x 10^5 CFU/mL), incubation temperature (35 +/- 2 C), atmosphere (ambient air), incubation time (16-20 h), plate geometry (round-bottom), and reader.

Susceptible E. coli isolates will exhibit MIC values at or below the CLSI susceptible breakpoint for ciprofloxacin (<=0.25 µg/mL), while quinolone-resistant isolates carrying gyrA/parC mutations or qnr genes will display MICs at or above the resistant breakpoint (>=1 µg/mL). The reference strain E. coli ATCC 25922 will reproducibly fall within its published QC range (0.004-0.015 µg/mL), confirming assay validity.

Record the MIC per replicate as the lowest clear concentration. The modal MIC across replicates is reported; if results span more than one dilution, report the mode and note the range. Categorize each isolate as susceptible, intermediate (susceptible-dose-dependent), or resistant using the current CLSI M100 ciprofloxacin/E. coli breakpoints. Tabulate MIC50 and MIC90 for isolate collections. Inoculum CFU is back-calculated from colony counts x dilution factor. Data captured in a spreadsheet or LIMS with plate maps; optional OD600 reads (BioTek/Tecan) support a turbidity audit trail.

1. QC strain MIC out of range -> verify cation adjustment of CAMHB and check drug stock potency/expiry; remake drug dilutions. 2. Skipped wells (clear well below turbid) -> usually a pipetting/contamination error; repeat the row. 3. Growth control negative -> inoculum too dilute or non-viable; re-streak and re-standardize to 0.5 McFarland. 4. Trailing/hazy endpoints -> read as the first dramatic reduction (~80% inhibition) for bacteriostatic-appearing growth; ensure round-bottom plates and dark-field reading. 5. Sterility control turbid -> medium or plate contaminated; discard lot and remake.

MIC distributions are summarized as MIC50/MIC90 and essential agreement (within one two-fold dilution) versus a comparator method, with target essential agreement >=90% per ISO 20776-2. Inter-reader concordance is assessed by weighted Cohen's kappa (target >=0.8). For comparing MIC distributions between isolate groups, a Mann-Whitney U test on log2-transformed MICs is used with alpha = 0.05; for more than two groups, Kruskal-Wallis with Dunn's post hoc and Benjamini-Hochberg correction. n = 3 biological replicates per isolate minimum.