Crystal Violet Microtiter Biofilm Quantification of Pseudomonas aeruginosa PAO1 in 96-Well Plates

Each strain/condition is assayed in 6 technical replicate wells per plate across 3 independent biological replicates (separate overnight cultures on separate days). Edge wells are filled with sterile medium and excluded from analysis to avoid evaporation artifacts. A planktonic growth control (OD600 of the supernatant) is read before staining to allow biomass normalization (CV/OD600). Time points at 24 h and 48 h capture maturation. Plate layout is randomized so conditions are not confounded with plate position.

BSL-2 for P. aeruginosa (opportunistic pathogen); handle in a Class II biosafety cabinet and avoid aerosols. PPE: lab coat, nitrile gloves, safety glasses. Crystal violet is a mutagen and skin/eye irritant; glacial/30% acetic acid is corrosive and volatile — handle in a fume hood and avoid inhalation. Collect CV and acetic acid waste in a designated hazardous-waste container; do not pour down the drain. Autoclave or bleach-treat all bacterial waste.

Negative/abiotic control: medium-only wells (no bacteria), processed identically, defining background CV staining to subtract. Biofilm-deficient control: a known low-biofilm mutant (e.g., flgK or pilA) or wild-type treated with 0.5 mM EDTA, expected to stain weakly. Positive control: untreated wild-type PAO1 at peak biofilm. Vehicle control: wells receiving the test-compound solvent (e.g., DMSO <=1%) without compound. Sterility control wells confirm medium is uncontaminated.

Wild-type PAO1 typically yields CV OD595 of 1.0-3.0 (background-subtracted) at 24 h, rising at 48 h. Biofilm-deficient controls <0.3. A clear visible violet ring at the air-liquid interface and well bottom in positive wells. CV/OD600 normalization should distinguish reduced biofilm from simple growth inhibition. Coefficient of variation across technical replicates ideally <20%.

• Pseudomonas aeruginosa PAO1 reference strain (and test mutant/comparator)
• Tryptic Soy Broth (TSB) or M63 minimal medium + 0.2% glucose + 0.5% casamino acids
• Crystal violet solution, 0.1% (w/v) in water
• Glacial acetic acid, 30% (v/v) for dye solubilization (or 95% ethanol)
• Sterile phosphate-buffered saline (PBS), pH 7.4
• Flat-bottom 96-well polystyrene tissue-culture-treated plates with lids
• Sterile multichannel pipette, reservoirs, tips
• LB agar plates for inoculum/CFU verification

To measure adherent biofilm biomass of P. aeruginosa PAO1 grown statically in 96-well polystyrene plates by staining total attached biomass with 0.1% crystal violet, solubilizing the bound dye, and reading absorbance at OD550-595. The assay provides a semi-quantitative, high-throughput readout of biofilm formation suitable for strain comparisons and anti-biofilm compound screening.

1. Grow PAO1 overnight in TSB at 37 C, 200 rpm, ~16 h.
2. Dilute the overnight culture 1:100 in fresh medium to OD600 ~0.05.
3. Add 150 µL of the diluted culture to each test well; add 150 µL sterile medium to abiotic/blank wells.
4. Incubate statically at 37 C for 24 h (and a parallel plate to 48 h with a medium refresh at 24 h if used).
5. Read OD600 of the supernatant (planktonic growth) on a plate reader.
6. Gently aspirate/decant planktonic cells; wash wells 3x by submerging in PBS or pipetting 200 µL, decanting each time.
7. Air-dry plates inverted for 15-30 min, or fix with methanol 15 min.
8. Stain with 175 µL 0.1% crystal violet per well, 15 min at room temperature.
9. Wash 3-4x with deionized water by immersion until runoff is clear; blot and air-dry.
10. Solubilize bound dye with 200 µL 30% acetic acid, 15 min; transfer 125 µL to a clean plate and read OD595.

Independent variable: strain/genotype or test treatment (compound concentration, time point). Dependent variable: CV-stained biofilm biomass (OD595, background-subtracted, optionally normalized to planktonic OD600). Controlled variables: inoculum density (OD600 0.05), medium composition, static incubation at 37 C, wash number/volume, stain concentration and time, solubilizer, and read wavelength.

Wild-type P. aeruginosa PAO1 will form robust biofilm yielding CV OD595 >=4x the abiotic background, while a biofilm-deficient control (e.g., a flagellar/pili mutant or DNase/EDTA treatment) will show significantly reduced staining. Sub-inhibitory test treatments that disperse or inhibit biofilm will reduce normalized CV signal in a dose-dependent manner.

Subtract mean abiotic-control OD595 from each test well. Average technical replicates per biological replicate, then average across biological replicates. Optionally normalize biofilm to planktonic growth (CV OD595 / supernatant OD600) to separate biofilm-specific effects from growth effects. Plot mean +/- SD; for dose-response, fit a four-parameter logistic curve in GraphPad Prism or Python (scipy) to estimate IC50 of anti-biofilm compounds. Classify biofilm formers using the standard ODc cutoff (mean abiotic + 3 SD).

1. High background in abiotic wells -> over-staining or insufficient washing; increase wash cycles and use gentle immersion washing. 2. Variable replicates / detached biofilm -> washing too harsh or aspiration disturbing the biofilm; switch to gentle submersion washes and avoid touching well bottoms. 3. Saturated OD (>3.5) -> dilute solubilized dye 1:2 in 30% acetic acid before reading. 4. Weak signal for known former -> medium or incubation suboptimal; try minimal medium with glucose and confirm static (non-shaking) incubation. 5. Edge-well evaporation -> use only interior wells and fill outer wells with PBS/medium.

Compare two conditions with an unpaired two-tailed t-test on biological-replicate means (n = 3); for multiple conditions use one-way ANOVA with Tukey's HSD (alpha = 0.05). For dose-response, compare fitted IC50 values with an F-test. Report effect sizes and 95% CIs. A priori power analysis (G*Power) targeting 80% power to detect a 30% biofilm change at alpha 0.05 typically requires n>=3 biological replicates given CV<20%. Treat technical replicates as nested, not independent.