0.4% Trypan Blue Solution

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• Cell Culture ContaminationCell culture contamination falls into six categories: bacteria, yeast, filamentous fungi and moulds, mycoplasma, viruses, and cross-contamination by another cell line. Bacteria, yeast and fungi announce themselves within one to five days through turbidity, a pH shift or visible particles under the microscope, and are handled by discarding the culture and finding the technique failure that let them in. Mycoplasma, viral contamination and cell line misidentification produce no visible change at all and are found only by testing — PCR or DNA stain for mycoplasma, STR profiling for identity. The correct response to any confirmed biological contamination is to discard the affected culture, decontaminate the incubator, and restart from a clean frozen stock.
• Counting Cells with a HemocytometerA hemocytometer is a thick glass slide bearing a precisely etched grid over which a coverslip creates a chamber exactly 0.1 mm deep, so that a known volume of cell suspension can be counted under a microscope and converted to a concentration. For mammalian cells you count the four 1 mm2 corner squares, take the mean, multiply by the dilution factor, and multiply by 10^4 to obtain cells per mL, because each corner square encloses exactly 10^-4 mL. Mixing the sample 1:1 with 0.4% trypan blue before loading gives a viability percentage from the same count, since only dead cells with compromised membranes take up the dye.
• Trypan Blue and the Dye Exclusion Viability AssayTrypan blue is a diazo dye used at 0.4% (w/v) to separate live from dead cells by dye exclusion: an intact plasma membrane keeps the charged dye out, so viable cells stay clear and refractile, while cells with a damaged membrane take the dye up and stain blue. In practice the cell suspension is mixed 1:1 with 0.4% trypan blue, loaded into a hemocytometer or an automated cell counter, and counted within 3-5 minutes to give both a cell concentration and a percent viability. Because it reports membrane integrity only, trypan blue counts cells that have already died -- it does not detect apoptotic or metabolically failing cells that still have an intact membrane, so it reads high compared with metabolic or flow-cytometry viability assays.
• ACK lysis bufferACK (Ammonium-Chloride-Potassium) lysis buffer is an isotonic ammonium chloride solution used to remove red blood cells from leukocyte preparations such as mouse spleen, bone marrow and buffy coat. The standard composition is 150 mM ammonium chloride (8.02 g/L NH4Cl), 10 mM potassium bicarbonate (1.0 g/L KHCO3) and 0.1 mM disodium EDTA, adjusted to pH 7.2-7.4. Red cells lyse osmotically within one to five minutes at room temperature because their band 3 anion exchanger and high carbonic anhydrase activity drive ammonium chloride and water into the cell, while leukocytes, which lack that transport capacity, survive.
• Puromycin for Mammalian Cell SelectionPuromycin is an aminonucleoside antibiotic from Streptomyces alboniger that mimics the aminoacyl end of tRNA, enters the ribosomal A site and causes premature release of a truncated peptide chain, killing cells that cannot inactivate it. In mammalian cell culture it is used as a selection agent at 0.5-10 ug/mL, most commonly 1-2 ug/mL, and it kills non-resistant cells faster than any other common selection antibiotic -- typically within 2-5 days. Resistance is conferred by the pac gene encoding puromycin N-acetyltransferase, which acetylates the drug and renders it inactive. Because the effective concentration depends strongly on cell line, cell density and serum content, a kill curve must be run for each new cell line and each new lot of puromycin.
• Sf9 Cells and the Baculovirus Expression SystemSf9 is a clonal insect cell line isolated from Sf21, which was established from pupal ovarian tissue of the fall armyworm *Spodoptera frugiperda*. It is the standard host for the baculovirus expression vector system, used to produce recombinant proteins, virus-like particles and AAV. Sf9 cells grow at 27 C without CO2 in phosphate-buffered insect media at pH 6.2-6.4, adapt readily to serum-free suspension culture at 100-130 rpm, and double every 18-24 hours. Cultures are passaged when they reach roughly 2.5-3.5 x 10^6 cells/mL and diluted back to about 1 x 10^6 cells/mL, and infections are performed on healthy log-phase cells above 95% viability.
• DF-1 Chicken Embryo Fibroblast CellsDF-1 (UMNSAH/DF-1, ATCC CRL-12203) is a spontaneously immortalised chicken embryo fibroblast line derived from East Lansing Line 0 embryos, a chicken line free of endogenous avian leukosis virus loci. Because it carries no endogenous retrovirus and was immortalised without viral or chemical transformation, it is the standard continuous substitute for primary chicken embryo fibroblasts in avian virology, vaccine development and recombinant virus production. DF-1 cells are cultured in high-glucose DMEM with 10% fetal bovine serum at 37 C in 5% CO2, grow faster than primary CEF, transfect efficiently, and support replication of avian influenza, infectious bursal disease virus, avian leukosis and sarcoma viruses and many other avian pathogens at titres equal to or higher than CEF.
• Subculture of Cells (Passaging)Subculture of cells, also called passaging, is the transfer of cells from a culture that is approaching confluence into fresh vessels with fresh medium so that growth can continue. Adherent cells are detached first, usually with 0.25% or 0.05% trypsin-EDTA or a non-enzymatic dissociation reagent, then reseeded at a lower density set by a split ratio such as 1:4; suspension cells are simply diluted into fresh medium without any dissociation step. Most continuous adherent cell lines are subcultured at 70-80% confluence, which normally means two or three passages per week.
• Freezing Medium for Cells and Cryopreservation ProtocolsCell freezing medium is a cryoprotectant-containing solution that allows cells to survive freezing and long-term storage, most commonly built from a base medium or serum plus 5-10% dimethyl sulfoxide (DMSO), with 10% the standard starting point. DMSO permeates the cell and modifies how water freezes, preventing the intracellular ice crystals that otherwise rupture membranes during cooling. Cells are frozen at a controlled rate of approximately 1 C per minute to -80 C and then transferred to the vapour phase of liquid nitrogen, since long-term storage requires temperatures below about -135 C to prevent ice recrystallisation. Thawing is the opposite -- as rapid as possible in a 37 C water bath, followed by prompt dilution to remove the DMSO, which is toxic to cells above freezing temperatures.