MTT Cell Viability Assay Protocol: Day 3 Cell Block Analysis

Cell culture is the process of maintaining plant and animal cells alive and dividing in vitro, requiring specific tools including complete cell culture media (basal media plus supplements like serum and glutamine), appropriate culture vessels, and an incubator. Key techniques include subculturing (passaging) cells at optimal confluency (70-80% for adherent cells), practicing strict aseptic technique to prevent contamination, and regularly monitoring cell health through microscopy and media observation. Different cell types (adherent vs. suspension) require different handling protocols, and maintaining genetic stability requires cryopreserving original stocks to prevent drift over multiple passages.

Successful cell culture requires proper laboratory setup with essential equipment including a cell culture hood, incubator, water bath, centrifuge, refrigerator/freezer, hemocytometer, and microscope; maintaining a sterile environment involves opening all materials within the hood, wiping surfaces with 70% ethanol, organizing items systematically (pipettes in front, reagents at back, waste containers in corners), and avoiding breathing or talking into the hood; mammalian cells should be cultured at 37°C with 5-7% CO2 and high humidity, with flasks evenly spaced for proper gas exchange; reagents should be warmed to 37°C for 10-20 minutes but not left excessive time to prevent degradation; daily microscopic examination is essential for monitoring cell health and detecting contamination.

Proper cell culture workstation organization is essential for maintaining sterility and efficiency: work systematically from left to right, keep materials organized within easy reach, never reach across the work area, and keep the hood door closed when not actively working. Cell seeding density is critical for successful culture: fast-growing cells require higher densities (10,000 cells per well) while slow-growing cells need lower densities (1,000 cells per well). Proper seeding density ensures adequate cell attachment and growth without overcrowding.

Cell culture provides a reproducible method for testing drugs, vaccines, and treatments by isolating cells from organisms and creating controlled artificial conditions; successful cell culture requires strict contamination prevention through proper aseptic technique including 70% ethanol sterilization, safety hood usage, and regular UV irradiation, followed by careful cell handling processes such as resuscitating frozen cells by thawing in a 37°C water bath and washing with PBS, then subculturing cells when they reach approximately 80% confluency by detaching them with trypsin-EDTA and transferring them to fresh media at appropriate seeding densities to maintain healthy, proliferating cell populations for experimentation.

Calculate seeding volume using the formula: desired cell count ÷ cell concentration = volume needed. For example, to seed 0.5 million cells from a 1.2 million cells/mL suspension, take 0.42 mL. Add sufficient fresh medium to reach the desired total volume (e.g., 5 mL total = 0.42 mL cells + 4.58 mL medium). Distribute cells evenly using a 'battering' motion: three times down, three times up, three times left-right, three times right-left. Incubate at 37°C with 5% CO2. Do not move cells for several hours post-seeding to allow proper attachment. Work quickly when opening the incubator as temperature and CO2 levels drop rapidly.

Spectrophotometers measure light absorption by passing a beam of incident light through a sample in a cuvette. The machine compares incoming light intensity (I₀) with transmitted light intensity (I). When molecules absorb light, less light reaches the detector. Absorbance (A) is calculated using the logarithmic formula A = log₁₀(I₀/I). For instance, if 100% of light enters and only 1% exits, absorbance equals 2 because 10² = 100. This logarithmic relationship means absorbance increases slowly with increasing light absorption. The machine performs these calculations automatically, displaying the absorbance value directly.

This video explains how microplate readers measure absorbance using the Beer-Lambert Law (A = εbc), where absorbance (A) equals the product of the extinction coefficient (ε), path length (b), and concentration (c); the practical protocol involves preparing 96-well plates with samples and blank controls, using reverse pipetting to avoid air bubbles, configuring Gen 5 software to set wavelengths and plate layouts, and generating blank-subtracted absorbance data for applications like determining critical micelle concentration (CMC) or measuring turbidity without requiring calibration curves.

Absorbance microplate readers measure optical density by passing light through samples and detecting absorbed wavelengths at specific positions. Key components include lamps, filters, and detectors enabling flexible kinetic assays. Standard wavelengths cover ELISA applications (405-630nm), protein assays (562-595nm), MTT/XTT cell viability (490-570nm), and nucleic acid quantification (260nm). Advanced features include dual-mode reading with reference subtraction, built-in shakers, single/dual-channel detection, and eight-position filter wheels. Data analysis encompasses pre-processing operations and kinetic parameters: average rate, maximum rate, time to peak, and absorbance kinetics. Practical applications demonstrate the technology's versatility: histamine detection in food samples uses competitive ELISA where HRP-labeled conjugates compete with sample histamine for antibody binding sites, measured at 450nm reference 630nm. In antibiotic resistance research, microdilution methods determine minimum inhibitory concentrations using MTT-based viability assays measured at 570nm. These examples illustrate how standardized ELISA protocols enable quantitative analysis across food safety, clinical microbiology, and environmental health research domains.

Spectrophotometry is a technique that measures optical density (OD), defined as the ratio of incident light intensity to transmitted light intensity (OD = I_incident/I_transmitted), which indicates a substance's ability to absorb or resist light; according to Beer-Lambert's law, cell mass and solute concentration are directly proportional to optical density, meaning higher concentrations absorb more light and produce higher OD values, and the spectrophotometer instrument uses a light source, collimator, monochromator with prism, wavelength selector, sample compartment, detector, and display to measure these properties.

Laboratory rules include mandatory obligations (enter 5 minutes early, wear laboratory coat, follow instructions, use equipment properly, borrow tools from staff, report damage immediately, clean after use, dispose trash properly, check laboratory condition before leaving, notify in writing if unable to attend) and prohibited actions (no smoking/eating/drinking, no open shoes, no joking around, no jewelry, no touching equipment without permission). Safety protocols include studying hazard labels, washing hands before/after handling chemicals, wearing masks and gloves, tying back long hair, handling glass equipment carefully, using appropriate tools, rinsing with water if chemicals contact body, contacting staff for injuries. Chemical handling procedures include working in fume hoods for hazardous chemicals, not mixing strong acids/bases without knowing reactions, pouring only needed amounts, not returning chemicals to original bottles, diluting concentrated solutions before discarding, wafting chemicals instead of smelling directly, directing tube mouths away from face when heating, and adding chemicals drop by drop through tube sides.

Chemical solvents like DMSO can facilitate the absorption of other substances through the skin, creating serious safety hazards. The hosts explain that DMSO is a polar solvent that can carry other chemicals through the skin into the bloodstream, potentially causing serious health problems. They share personal experiences of working with solvents and emphasizing the importance of proper protective equipment when handling chemical solvents. The hosts recommend that anyone working with chemical solvents should always wear appropriate protective equipment and understand the specific hazards associated with each solvent.

This section covers essential laboratory safety protocols and solution preparation procedures. Three critical safety measures include wearing lab coats to protect clothing and skin from chemical spills and hot liquids, wearing gloves to protect hands from harmful chemicals and hot surfaces, and wearing safety goggles to protect eyes from chemical splashes and flying debris. For preparing solutions from solids: weigh the required mass using an electronic balance, transfer to a volumetric flask using a funnel, add distilled water to dissolve, fill to the calibration mark, stopper and invert to mix, then label with chemical formula and molar concentration. For dilution: use a volumetric pipette to measure the concentrated solution volume, transfer to a new volumetric flask, add distilled water to mix, fill to the calibration mark, stopper and invert to mix, then label with the diluted solution's formula and concentration.

Air-sensitive chemicals require immediate covering after use; leaving them exposed causes degradation (e.g., iron(II) chloride oxidizes to brown iron(III) chloride). Always use TA-prepared stock solutions in labeled beakers rather than direct pipetting from stock containers to prevent contamination. When adding chemicals, always add concentrated solutions to less concentrated ones—never add water to acid, which causes dangerous splashing.