A simple metachromatic and fluorescent staining method for microorganisms using carbocyanine dye

The cationic carbocyanine dye, 1-ethyl-2-[3-(1-ethylnaphtho[1, 2d]-thiazolin-2-ylidene)-2-methylpropenyl]-naphtho[1, 2d]thiazolium bromide, interacts with several classes of anionic polymers, exhibiting metachromasia. We were able to stain various kinds of microorganisms with this dye. Gram-negative bacteria were stained reddish purple, while Gram-positive bacteria were stained violet or bluish purple. Stains of molds were of various colors. Yeast vegetative cells were stained reddish purple, but zygotic asci were bluish. Chlamydia trachomatis inclusions, which are surrounded by cytoplasmic membranes, were also stained red. Microorganism and cell stains have different features and can be identified also by use of fluorescent microscopy. The new staining method we report here is rapid and simple enough for routine microscopical examinations of smears of clinical specimens including microorganisms.