Microbiology Cheat Sheet
Agents, Stains & Transmission
Microbiology agents, stains and transmission for NEET-PG and FMGE: Gram stain, ZN stain, stool microscopy, prions, endotoxin, and routes of disease spread.
MedNext Academy | 3 min read
Agents, Stains & Transmission
Microbiology agents, stains and transmission for NEET-PG and FMGE: Gram stain, ZN stain, stool microscopy, prions, endotoxin, and routes of disease spread.
This topic covers how infectious agents are classified, the core staining and microscopy techniques used to identify them, and the epidemiological principles of disease transmission.
High-yield lines
- Prions are the only infectious agents with no nucleic acid, making them resistant to UV, formalin, and standard autoclaving.
- The Gram stain differentiates bacteria by cell wall structure: Gram-positive cells retain crystal violet and appear purple, Gram-negative cells take up safranin and appear pink.
- Acetone-alcohol decolourisation is the critical and most error-prone step of the Gram stain, and over-decolourisation makes Gram-positive organisms falsely appear Gram-negative.
- Lipid A is the toxic moiety of lipopolysaccharide (endotoxin) in Gram-negative bacteria and triggers TNF-alpha, IL-1, and IL-6 release via TLR4.
- Acid-fastness of mycobacteria on Ziehl-Neelsen stain is due to mycolic acids, which appear as bright red bacilli against a blue background.
- Modified Ziehl-Neelsen stain using 1 percent sulphuric acid detects weakly acid-fast organisms including Nocardia, Cryptosporidium, Isospora, and Cyclospora.
- Staphylococci appear as Gram-positive cocci in clusters, while streptococci appear as Gram-positive cocci in chains.
- Neisseria appear as Gram-negative kidney-bean-shaped diplococci, characteristically intracellular within neutrophils.
- Bacterial endospores of Bacillus and Clostridium contain dipicolinic acid and are the most heat-resistant structures, defining the endpoint for sterilisation.
- Airborne transmission involves droplet nuclei under 5 micrometres, seen in tuberculosis, measles, and varicella, whereas droplet transmission involves larger particles that fall within 1 metre.
- Stool saline wet mount detects motile trophozoites, while iodine mount highlights cyst nuclear detail.
- Mycoplasma lacks a cell wall entirely, making it inherently resistant to beta-lactams and vancomycin.
- Entamoeba histolytica cannot be distinguished from non-pathogenic E. dispar on microscopy alone; antigen detection or PCR is required.
- Faecal-oral transmission underlies cholera, typhoid, hepatitis A and E, polio, and amoebiasis.
Mapped competency codes
- MI1.1
- MI1.2
- MI1.3
Continue into the full chapter
This summary maps to MI1-general-microbiology-and-immunity.
Frequently Asked Questions
Which infectious agent lacks nucleic acid and resists standard sterilisation?
Prions, which are misfolded proteins requiring 1N sodium hydroxide or autoclaving at 134 degrees Celsius for 18 minutes for decontamination.
What is the single most error-prone step of the Gram stain?
The acetone-alcohol decolourisation step, which distinguishes crystal violet retention between Gram-positive and Gram-negative cells.
Why do mycobacteria stain acid-fast?
Their cell wall is rich in mycolic acids, which retain carbol fuchsin despite acid-alcohol decolourisation and appear as red bacilli.
How is Entamoeba histolytica differentiated from E. dispar?
Microscopy cannot distinguish them; antigen detection using the Gal/GalNAc lectin ELISA or PCR is required for definitive identification.
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