Understanding Infectious Diseases: Causes, Transmission & Antibiotic Resistance
Bacterial Diseases
Cholera – Vibrio cholerae
To reach their site of action in the small intestine, Vibrio cholerae bacteria must first pass through the stomach. If the stomach’s contents are sufficiently acidic (pH less than 4.5), the bacteria are unlikely to survive. However, if they reach the small intestine, they multiply and secrete a toxin called choleragen. This toxin disrupts the functions of the epithelium lining the intestine, causing salts and water to leave the blood. This leads to severe diarrhea, and the fluid loss can be fatal if not treated within 24 hours.
Malaria – Plasmodium falciparum
Female Anopheles mosquitoes transmit malaria by feeding on human blood to obtain protein for egg development. If they bite a person infected with Plasmodium, they ingest the parasite’s gametes along with the blood meal. Male and female gametes fuse in the mosquito’s gut, developing into infective stages that move to the salivary glands. When the mosquito feeds again, it injects an anticoagulant to prevent blood clotting, allowing for easier feeding. The infective stages are transmitted to the human bloodstream along with the anticoagulant. The parasites then enter red blood cells and multiply. Plasmodium multiplies in both hosts—humans and mosquitoes—with a significant increase in parasite numbers at each stage, enhancing the chances of infecting new hosts.
Viral Diseases
AIDS – HIV
HIV, a retrovirus, carries its genetic material as RNA instead of DNA. Upon entering a host cell, the viral RNA is reverse transcribed into DNA, which is then integrated into the human chromosomes. HIV specifically targets and destroys helper T cells, crucial components of the body’s immune system. As the number of helper T cells declines, the body’s ability to fight off infections weakens, leading to opportunistic infections. AIDS is not a single disease but a collection of these opportunistic infections resulting from HIV-induced immunodeficiency.
Tuberculosis – Mycobacterium bovis
Tuberculosis (TB) infection can manifest differently in individuals. Some develop active TB quickly, while others harbor inactive bacteria for years. It is estimated that 30% of the global population has latent TB infection without symptoms and cannot transmit the disease. However, the bacteria can become active, particularly in individuals weakened by other diseases, malnutrition, or HIV infection. Active TB often leads to prolonged illness characterized by a persistent cough, fever, appetite suppression, and weight loss.
Measles
Measles, caused by a virus, enters the body and replicates in cells lining the upper respiratory tract. After an 8–14 day incubation period, symptoms such as rash, fever, runny nose, cough, conjunctivitis, and small white spots inside the cheeks appear. Treatment primarily involves bed rest and fever-reducing medications. While measles typically resolves within ten days, complications like pneumonia, ear and sinus infections, brain damage, and convulsions can occur.
Antibiotic Resistance
Antibiotic resistance can develop through spontaneous mutations in bacterial DNA, altering the target protein’s structure and rendering the antibiotic ineffective. When antibiotics are used incorrectly, such as prematurely stopping treatment, susceptible bacteria are killed, but resistant strains can survive and multiply. These resistant bacteria pass on their resistance genes through vertical transmission (from parent to offspring during reproduction) and horizontal transmission (direct transfer of genetic material between bacteria).
Vertical Transmission
Bacteria reproduce asexually through binary fission, where the DNA replicates, and the cell divides into two daughter cells, each receiving a copy of the chromosome. This rapid process can lead to a large population of resistant bacteria from a single resistant ancestor.
Horizontal Transmission
Antibiotic resistance genes are often located on plasmids, small, circular DNA molecules that can be transferred between bacteria, even different species, through conjugation. During conjugation, a tube forms between two bacteria, allowing for the transfer of plasmids and even chromosomal DNA. This process enables the spread of antibiotic resistance genes between bacterial populations.
Mechanisms of Antibiotic Action
- Inhibition of bacterial cell wall synthesis
- Disruption of protein activity in the cell surface membrane
- Interference with enzyme action
- Inhibition of DNA synthesis
- Disruption of protein synthesis