The Congo red is an azo dye for proteins, formed by the coupling of a diazonium salt and an active aromatic ring. This substance is capable of absorbing electromagnetic radiation in the visible spectrum, which is why it has an intense color.
It is negatively charged. Therefore, it has an affinity for positively charged cellular components, such as certain protein substances. Its color varies according to the pH. In this sense, if the medium is acidic (< de pH3), el color es azul intenso. Entre pH3 - pH 5,2 es fucsia (zona de viraje), y con >pH 5.2 the color is deep red.
It is a very versatile substance, as it has multiple uses. It has been used as a colorant in the textile industry, and also for cells and fabrics.
Also to make culture media that measure enzymatic action, as a pH indicator, as a control substance in the evaluation of the proper functioning of spectrophotometers, in the study of biofilm formation, or in the diagnosis of amyloid..
Likewise, it has made it possible to distinguish bacterial and fungal serotypes by identifying specific structures in their wall (lipopolysaccharides)..
Article index
This substance was discovered by Böttiger in 1884. It is a diazonium derivative formed by the bis-diazoic acid of benzidine with naphthionic acid. The Congo red molecule measures 21 Armstrong and the molecular weight is approximately 8000 g / mol..
Congo red is characterized by being soluble in water, and even more so in organic solvents such as ethanol, forming a colloidal solution.
Has an affinity for cellulose, amyloid tissue, and positively charged cellular components.
Congo Red is prepared in different concentrations depending on the technique to be used. Most use Congo red at 1%, 2%, 0.1%, among others.
For example, to prepare 2% Congo red, 2 g of dehydrated food coloring should be weighed out and 100 ml of distilled water added. Then it is stored in an amber bottle.
For a time it was widely used in the textile industry due to its fixation on cotton, but currently it is in disuse because it is carcinogenic and also because the color is not stable, becoming discolored by friction..
The biofilm-forming capacity of microorganisms has been shown to be a virulence factor.
In this sense, Congo red dye is used as a method for determining biofilm formation. Congo red binds to the exopolysaccharides present in the biofilm. However, compared to other methods it is the least recommended due to the high false negatives that occur.
The method uses Congo red agar, this is composed of blood agar as a base, glucose (10 g / l) and the Congo red dye (0.4 g / l). The strains to be evaluated are seeded in the medium and incubated for 24 hours at 37 ° C, and subsequently incubated for 48 hours at room temperature..
A positive test is evidenced if crystalline colonies of black color and dry appearance are observed..
To assess whether an absorbance or transaction measuring equipment is complying with the photometric parameters established by international regulations, a simple technique can be used to establish whether the equipment emits results within the ranges of acceptability..
One of the evaluation techniques is using Congo red, based on the isosbestic point.
The isosbestic point is the wavelength at which Congo red emits the same absorbance independently of pH, concentration and temperature. The absorbance value is fixed and can be used as a reference.
The theoretical isosbestic point of Congo red is known to be 541 nm. If the value obtained is different, it is known that the equipment has wavelength drift problems, and it needs to be checked by a specialized technician..
Ortiz et al. Describe a culture medium prepared with Congo red dye and carboxymethylcellulose called CMC agar, in order to detect cellulite microbial strains; that is, cellulase producers (endogluconeses, exoglucanases and ß-glucosidase).
This medium has an intense coloration. The color will be dissipated by the action of the endoglucanase enzyme that breaks the structure of carboxymethylcellulose. This suggests a positive reaction.
The decrease in viscosity and absorbance allows the enzyme activity to be quantified. For example, in strains of Streptomyces sp.
Congo red has an affinity for polysaccharide structures of certain strains, achieving the identification of these microorganisms. Among them are the Escherichia coli, and the Shigella flexneri.
Congo red agar plates are also used to obtain characteristic colonies, as is the case of Azospirillum sp, giving scarlet red colonies, among other.
One of the most common applications of Congo red is its usefulness in the diagnosis of amyloidosis. This strange disease consists of the extracellular accumulation of an abnormal protein in various organs. This abnormal protein is made in the bone marrow and is called amyloid..
Congo red has a high affinity for this substance. This property has been used to show its presence in histological tissue sections. Congo Red is used in conjunction with Hematoxylin / Eosin for this purpose..
The union of the amyloid tissue and Congo red occurs through nonpolar hydrogen bonds, between the carboxyl groups and the amino group. Amyloid protein contributes the carboxyl groups (COOH) and Congo red the amino group.
Amyloid tissue is stained in various hues ranging from pink to deep red seen under the light microscope. In microscopes with doubly polarized light, these preparations are observed with a pathognomonic apple-green birefringence..
That is, they present dichroism, since the amyeloid fibers are anisotropic. This observation confirms the diagnosis.
Tissue staining with Congo red is compatible with other diagnostic methodologies, such as immunocytochemical methods, and it is even possible to recolor.
The property of turning against changes in pH is used by the technique called chromoendoscopy..
This technique uses colorants and pH indicators that allow detecting certain pathologies. Among them is the use of Congo red, which can reveal early cancer foci in the gastric mucosa, being used as a marker of acidity.
The technique is based on the fact that Congo red at acidic pH is black. Therefore, after placing a solution of Congo red on the gastric mucosa, the areas where there is paleness will be chosen to take the sample for the biopsy, that is, where there is no acid production. This suggests the presence of a cancerous focus or loss of parietal cells..
Yet No Comments