Microbiology

Gram-negative bacteria are characterized by having a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides. This structure gives them a pink color when subjected to the Gram staining technique. Among the options provided:
1. Lactobacillus: A Gram-positive bacterium, commonly found in probiotics and involved in lactic acid fermentation.
2. Escherichia coli (E. coli): A Gram-negative bacterium found in the gut of warm-blooded organisms. It is distinguished by its thin peptidoglycan wall and outer membrane.
3. Bacillus subtilis: A Gram-positive bacterium, widely studied and used in industrial processes.
4. Staphylococcus aureus: Another Gram-positive bacterium, known for causing infections like MRSA. Thus, the correct answer is Escherichia coli, which is a well-known Gram-negative bacterium.

All the diseases listed are caused by viruses:
- Yellow fever: Caused by the Yellow fever virus, transmitted by Aedes mosquitoes. It primarily affects the liver and can lead to jaundice.
- Hepatitis B: Caused by the Hepatitis B virus, which affects the liver and can lead to chronic liver disease or cancer.
- Chickenpox: Caused by the Varicella-zoster virus, it leads to itchy blisters and can reactivate later in life as shingles.
- Rabies: Caused by the Rabies virus, it is transmitted through animal bites and affects the nervous system, leading to fatal encephalitis if untreated.

Lederberg and Tatum discovered conjugation in bacteria, a process where genetic material is transferred between bacterial cells through direct physical contact using a pilus. This discovery demonstrated that bacteria are capable of sexual reproduction and horizontal gene transfer, influencing bacterial evolution and genetic diversity.

- D’Herelle and Twort: Independently discovered bacteriophages, viruses that infect bacteria, which are now used in phage therapy.
- Beadle and Tatum: Introduced the one-gene-one-enzyme hypothesis, demonstrating that genes dictate enzyme production, foundational to molecular biology.
- Fleming: Discovered penicillin, the first antibiotic, revolutionizing the treatment of bacterial infections.
- Griffith: Conducted experiments showing bacterial transformation, where genetic material from one bacterium can alter another.

AIDS, caused by HIV, remains without a vaccine due to the virus's high mutation rate and ability to evade the immune system. In contrast, effective vaccines exist for Hepatitis B, Influenza, and Mumps, significantly reducing their global burden.

IgM is the first antibody produced in response to an infection, playing a vital role in the primary immune response. It is large in size, allowing it to form pentamers that enhance pathogen neutralization. IgG, the most abundant antibody, follows IgM and provides long-term immunity.

The Gram-stain procedure involves:
1. Staining with Crystal Violet: This is the primary stain that binds to the peptidoglycan in bacterial cell walls.
2. Staining with Gram’s Iodine: The iodine acts as a mordant, forming a complex with Crystal Violet, increasing the dye’s binding strength.
3. Decolorization: Alcohol or acetone is used to remove the stain from Gram-negative bacteria due to their thinner peptidoglycan layer.
4. Staining with Safranine: The counterstain provides color to Gram-negative bacteria, making them appear pink under a microscope, while Gram-positive bacteria remain purple.

Yellow fever, a viral hemorrhagic disease, is primarily transmitted by the Aedes aegypti mosquito. This mosquito acts as a vector, spreading the virus between humans or from infected monkeys to humans. The disease affects the liver and kidneys, causing jaundice and sometimes fatal complications. Other organisms listed, such as Culex, are vectors for other diseases, and Mycobacterium tuberculosis is a bacterium unrelated to yellow fever.