1  Operational factors

1.1  The smear is too thick. During smearing, if too many bacteria are picked up, they cannot be separated; consequently, during decolorization, the crystal violet stain from the first step cannot be removed, which may cause Gram-negative bacteria to appear purple, leading to a misidentification as Gram-positive.

1.2  Bleaching time Gram-positive bacteria have a relatively impermeable cell wall and are resistant to decolorization, whereas Gram-negative bacteria possess a more permeable cell wall and are readily decolorized. However, these observations hold only under fixed experimental conditions; if the decolorization time is extended, the decolorizing agent can penetrate the cell wall of Gram-positive bacteria, causing the retained crystal violet to lose its color and resulting in a pink or red counterstain, thus falsely classifying them as Gram-negative. Conversely, if the decolorization period is too short, the crystal violet stain on Gram-negative bacteria may not be fully removed, leaving a purple hue that could be misinterpreted as indicating Gram positivity.

1.3  Fixing method Fixation not only kills bacteria and alters their permeability to dyes, but also causes the bacteria to adhere to the slide, preserving their original morphology. The standard fixation procedure involves passing a dried bacterial smear over a flame at moderate speed three times, until the slide feels warm—not hot—against the back of the hand. However, in practice, some students hold the slide in the flame for too long, which can disrupt the bacteria’s natural permeability and alter their staining characteristics, thereby compromising the results of the stain.

2  Bacterial factors

2.1  Bacterial culture time The typical morphology and staining characteristics of bacteria are observed during the logarithmic growth phase; bacterial cells harvested at this stage yield an accurate reflection of their true staining properties. If the culture is allowed to grow for too long and the cells enter the stationary or senescent phase, their staining behavior may change, potentially causing Gram-positive bacteria to appear Gram-negative.

2.2  Components of the culture medium It is generally believed that Gram-positive bacteria contain a unique complex of nucleoprotein–magnesium salts and polysaccharides, which binds very tightly to iodine–crystal violet complexes and resists decolorization. However, if the culture medium lacks nucleoprotein–magnesium salts, the bacterium cannot synthesize these essential components; consequently, the bacterial–iodine–crystal violet complex becomes unstable and readily loses its color, potentially causing Gram-positive bacteria to appear as Gram-negative.

3  Dye solution factors

3.1  Iodine solution Iodine solution, acting as a mordant, enhances the affinity between the dye and the substrate. If the iodine solution is left standing for too long or exposed to sunlight, it may lose its effectiveness as a mordant, resulting in a weaker bond between crystal violet and the bacteria. This can lead to easy decolorization, causing Gram-positive bacteria to appear Gram-negative.

3.2  Crystal violet If the concentration of crystal violet is too high, the stain may not be easily decolorized by alcohol, potentially causing Gram-negative bacteria to appear Gram-positive.

3.3  Alcohol Alcohol serves as a decolorizing agent; the concentration of alcohol used for decolorization in Gram staining is 95% At this concentration, the crystal violet stain on Gram-positive bacteria is not easily decolorized and remains purple, whereas the crystal violet stain on Gram-negative bacteria is readily decolorized, leaving a red color. However, when the alcohol concentration is… 70% At this point, its decolorizing power is at its strongest, potentially stripping even the crystal violet stain from Gram-positive bacteria and causing them to appear as Gram-negative.

In summary, other factors influencing staining include the structure of bacterial cells and the permeability of their outer membranes—such as membrane permeability, the size of membrane pores, and the integrity of cellular architecture—all of which play a role in the staining process. Furthermore, the composition of the culture medium, the age of the culture, and the concentration of electrolytes in the staining solution also have an impact. pH Temperature, the effects of reagents, and other factors can also influence bacterial staining; therefore, when performing staining, it is important to consider these variables if the observed results differ from the expected outcome.

Deer DL-DYE fully automatic dyeing machine, The staining process is fully automated—simply load the slides, and no further manual intervention is required. Upon completion, the slides are centrifuged and dried, ready for direct microscopic examination following a standard staining protocol: fixation, primary staining with crystal violet, rinsing, mordanting with iodine solution, rinsing, decolorization followed by counterstaining, and drying. The micro‑immersion staining technique ensures thorough contact between the stain and the smear, minimizing reagent consumption while preventing cross‑contamination between slides. This approach effectively reduces factors that can compromise staining outcomes, thereby ensuring the accuracy of the results.