The most common chemicals used to fix microbial cells are aldehydes (formaldehyde, paraformaldehyde, and glutaraldehyde), alcohols (methanol and ethanol), and osmium tetroxide. Each works differently and is chosen based on what structures need to be preserved and what type of microscopy or staining will follow. These fixatives fall into two broad categories: cross-linking fixatives that lock proteins in place by forming chemical bonds between them, and denaturing fixatives that precipitate proteins by reducing their solubility.
Cross-Linking Fixatives: Aldehydes
Aldehydes are the most widely used chemical fixatives for microbial cells because they preserve the natural shape of proteins, including their three-dimensional folding. Formaldehyde, paraformaldehyde, and glutaraldehyde all work by creating covalent bonds between neighboring protein molecules, essentially stitching cellular structures together so they hold their form.
Formaldehyde reacts with amino groups on proteins in two stages. First, it attaches to reactive sites on amino acids like lysine and cysteine, forming small chemical additions to the protein surface. Then it builds bridges (called methylene bridges) between those attachment points and other nearby molecules, including amino acids like tyrosine and asparagine. The result is a rigid network of cross-linked proteins that keeps the cell’s architecture intact. A saturated solution of formaldehyde gas in water is called formalin, and 10% buffered formalin is one of the most common fixative preparations in any biology lab.
Glutaraldehyde works through a similar cross-linking mechanism but has two reactive ends instead of one, making it a more aggressive fixative. It produces stronger cross-links and is better at preserving fine structural details. Standard concentrations for microbial work are typically around 2.5% glutaraldehyde. Because formaldehyde penetrates cells faster while glutaraldehyde provides stronger fixation, the two are sometimes combined. Karnovsky’s fixative uses both paraformaldehyde and glutaraldehyde together, giving the benefits of rapid penetration and thorough cross-linking in a single step.
The key advantage of aldehyde fixation for bacteria is that it maintains the integrity of membrane lipids and surface structures. Delicate features like flagella and pili remain visible after aldehyde fixation, which matters when you need to study surface ultrastructure.
Denaturing Fixatives: Alcohols and Acetone
Alcohols and acetone fix cells through an entirely different mechanism. Instead of cross-linking proteins, they denature them by disrupting the hydrophobic interactions that hold proteins in their folded shape. This causes proteins to lose solubility and precipitate in place, which anchors the cell to a slide. Methanol, ethanol, and acetone are the most common choices in this category.
Common combinations include methanol and acetone mixed 1:1, and ethanol with acetic acid mixed 3:1 (known as Carnoy’s fluid when combined with chloroform). Typical exposure times are around 10 minutes. These preparations are fast, inexpensive, and widely available.
The tradeoff is significant, though. Alcohol-based fixation strips surface proteins and lipopolysaccharides from bacterial cells. In atomic force microscopy studies, bacteria fixed with alcohols lost their filamentous surface structures entirely, and the remaining cell surfaces appeared as obscure spherical shapes compared to the well-preserved detail seen with aldehyde fixation. For routine procedures like Gram staining, where preserving fine surface detail isn’t critical, alcohol fixation works well. For anything requiring intact surface morphology, aldehydes are the better choice.
Osmium Tetroxide for Lipid Preservation
Osmium tetroxide occupies a unique role in microbial fixation. It reacts primarily with the double bonds in unsaturated lipids found in cell membranes, making it the fixative of choice when membrane structure needs to be preserved for electron microscopy. It also adds electron density to membranes, making them appear dark and clearly defined in electron micrographs.
The initial reaction targets unsaturated lipid chains, converting the osmium from its original state to a reduced form that binds tightly to the membrane. Osmium tetroxide also interacts with proteins through hydrogen bonding, but the predominant reaction is with membrane lipids. Studies on red blood cell membranes confirmed that the fixation signal can be reconstructed from lipid components alone, underscoring how lipid-focused this fixative is. Because it does not penetrate cells quickly and is extremely toxic, osmium tetroxide is almost always used as a secondary fixative after an initial aldehyde step, particularly for transmission electron microscopy of bacteria.
How to Choose the Right Fixative
The downstream application dictates the fixative. For light microscopy and basic staining, methanol or ethanol fixation is fast and sufficient. For scanning or transmission electron microscopy, glutaraldehyde (often followed by osmium tetroxide) provides the structural preservation needed to resolve fine details like pili, flagella, and membrane layers. For fluorescent in situ hybridization (FISH) or immunostaining, fixative choice becomes more nuanced because the chemical must preserve the target molecule (RNA, DNA, or a specific protein) without destroying it. Aldehyde fixatives generally preserve nucleic acids well enough for hybridization, while harsh denaturing fixatives can degrade RNA.
Some specialized protocols use less common chemicals. Potassium permanganate, like osmium tetroxide, is an oxidizing agent that cross-links proteins and is occasionally used for electron microscopy. Picric acid, the active ingredient in Bouin’s fluid, is another cross-linking fixative sometimes paired with formaldehyde. These are niche choices, though. The vast majority of microbial fixation relies on the core group: formaldehyde, glutaraldehyde, methanol, ethanol, acetone, and osmium tetroxide.
Safety Considerations
Several of these chemicals pose serious health risks. Formaldehyde is classified as a carcinogen and causes skin, eye, and respiratory irritation. OSHA limits workplace exposure to 0.75 parts per million averaged over an eight-hour shift, with a short-term ceiling of 2 ppm over any 15-minute period. Labs where airborne formaldehyde exceeds these limits must be designated as regulated areas with restricted access. Glutaraldehyde is also a respiratory and skin sensitizer, though it has lower vapor pressure than formaldehyde, meaning it releases less gas at room temperature. Osmium tetroxide is acutely toxic and can cause severe eye damage even at very low concentrations, requiring work in a fume hood with appropriate protective equipment.
Alcohol-based fixatives are the safest option from a toxicity standpoint, which is one reason they remain popular for routine laboratory work where the highest level of structural preservation isn’t required.