Q. 72 Biofilm produced by bacteria is detected by
(A) Saffranin
(B) Malachite green
(C) Basic fuchsin
(D) Congo red
Biofilms are slimy, protective communities formed by bacteria that stick to surfaces, making infections harder to treat. In microbiology labs, detecting these biofilms is crucial for research and medical diagnostics. This article breaks down the standard method to detect bacterial biofilms, including the correct answer to the MCQ question: “Biofilm produced by bacteria is detected by” with options (A) Saffranin, (B) Malachite green, (C) Basic fuchsin, and (D) Congo red.
We’ll explore each option, reveal the right choice, and explain why it’s used in biofilm assays—perfect for students, researchers, and biotech professionals studying microbiology.
Bacterial biofilms represent a major challenge in fields like infection control, medical devices, and industrial microbiology. These structures form when bacteria embed in a self-produced matrix of extracellular polymeric substances (EPS), shielding them from antibiotics and the immune system.
In lab settings, several staining techniques visualize and quantify biofilms. The question—”Biofilm produced by bacteria is detected by”—tests knowledge of these dyes, commonly featured in exams like NEET, CSIR NET, or biotech coursework.
Correct Answer: (D) Congo Red
Congo red is the standard dye for detecting bacterial biofilms. This azo dye binds specifically to the amyloid-like fibrillar structures and polysaccharides in the biofilm matrix, producing a distinctive red or pink color under light microscopy.
The Congo red agar assay is a popular qualitative method: Bacteria are grown on agar containing Congo red (about 40 µg/mL). Biofilm formers absorb the dye, forming red colonies, while non-formers appear white or pale. Quantitative assays involve crystal violet staining followed by Congo red for confirmation, measuring optical density at 490 nm.
This technique was pioneered in studies on Proteus mirabilis and Escherichia coli biofilms, making it a go-to for researchers in plant pathology, medical microbiology, and your genetics or microbiology labs.
Explanation of All Options
Understanding why other dyes don’t fit helps differentiate staining protocols:
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(A) Saffranin: Used as a counterstain in Gram staining to differentiate Gram-negative bacteria (pink/red). It doesn’t specifically bind biofilm EPS and is ineffective for biofilm detection alone. In biofilm assays, it’s sometimes a secondary stain but not primary.
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(B) Malachite green: A counterstain in acid-fast staining (for Mycobacterium) or spore staining, where it colors non-acid-fast cells green. It has antifungal properties but no affinity for biofilm matrices, so it’s irrelevant for bacterial biofilm detection.
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(C) Basic fuchsin: A component of Ziehl-Neelsen stain for acid-fast bacteria, staining them red. It’s also used in endospore staining but lacks specificity for biofilms. No standard biofilm assay relies on it.
| Option | Primary Use | Role in Biofilm Detection? | Why Not Correct? |
|---|---|---|---|
| (A) Saffranin | Gram staining counterstain | Minimal (secondary only) | Non-specific to EPS matrix |
| (B) Malachite green | Acid-fast/spore staining | None | No biofilm binding affinity |
| (C) Basic fuchsin | Acid-fast staining | None | Lacks specificity for biofilms |
| (D) Congo red | Amyloid/EPS staining | Primary method | Correct: Binds biofilm matrix |
Why Congo Red Stands Out in Biofilm Research
Congo red not only detects but also helps study biofilm-related virulence, as seen in Candida albicans or Staphylococcus infections. For advanced work, combine it with crystal violet assay (gold standard for quantification): Fix biofilm, stain with 0.1% crystal violet, wash, and elute for OD reading at 595 nm.
In plant biology (relevant to your interests), Congo red detects biofilms in Xanthomonas on crops, aiding genetic studies on plant immunity.
Pro Tip for Exams/Research: Memorize Congo red for qualitative biofilm detection; use SEM or confocal microscopy for structural details.
This covers the MCQ comprehensively—ideal for revising microbiology concepts in genetics, biochemistry, or plant sciences.
