Biology XII: Microbiology and Infectious Disease
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Bacteria
- Structure: cell wall (peptidoglycan), plasma membrane, nucleoid, plasmids, ribosomes (70S), capsule, flagella, pili. Shapes: cocci, bacilli, spirilla.
- Gram staining: Gram-positive bacteria (thick peptidoglycan) retain crystal violet and appear purple; Gram-negative bacteria (thin wall and outer membrane) appear pink.
- Growth: binary fission; the growth curve has lag, exponential (log), stationary, and death phases. Number after n divisions = N₀ × 2ⁿ.
- Culturing: nutrient media (agar), aseptic technique, streak plates, pure cultures. Sterilization: autoclave (about 121 °C for 15 to 20 minutes), dry heat, filtration, radiation; disinfectants and antiseptics (Madigan et al., 2021).
- Genetic exchange: transformation, transduction, conjugation; the spread of antibiotic resistance.
- Antibiotics: mechanisms (cell wall synthesis inhibitors such as penicillin, protein synthesis inhibitors, and others); resistance arises by selection, so use antibiotics only when prescribed and complete the course. Antimicrobial resistance is a serious global health threat.
Viruses
Viruses are acellular, made of nucleic acid (DNA or RNA) in a protein capsid, sometimes with an envelope. They replicate only in host cells. Bacteriophages show the lytic and lysogenic cycles. Retroviruses such as HIV use reverse transcriptase. Viruses cause influenza, dengue, hepatitis, rabies, and COVID-19 (SARS-CoV-2). Prevention: vaccines, hygiene, and vector control; treatment: antivirals for some.
Fungi and Protozoa
- Fungi: yeast and moulds; uses in baking, brewing, and antibiotic production (penicillin, discovered by Fleming in 1928); diseases such as ringworm and candidiasis; plant diseases.
- Protozoa: Plasmodium causes malaria, transmitted by Anopheles mosquitoes. The World Health Organization certified Sri Lanka as malaria-free in 2016, an achievement that requires continuing vigilance against imported cases (WHO, 2016). Entamoeba (amoebic dysentery) and Giardia are intestinal parasites.
Infectious Diseases of Public Health Importance
| Disease | Agent | Transmission and control |
|---|---|---|
| Dengue | Dengue virus | Aedes mosquitoes; remove breeding sites; recurrent outbreaks occur in Sri Lanka |
| Leptospirosis | Leptospira bacteria | Contact with water contaminated by animal urine; protective measures for farmers |
| Tuberculosis | Mycobacterium tuberculosis | Airborne; early detection, treatment, and BCG vaccination |
| Cholera and typhoid | Vibrio cholerae; Salmonella Typhi | Contaminated water and food; sanitation |
| HIV/AIDS | HIV | Sexual contact, blood, mother to child; prevention and treatment |
| COVID-19 | SARS-CoV-2 | Respiratory; vaccines, masks, hygiene |
| Rabies | Rabies virus | Animal bites; post-exposure prophylaxis; vaccinate pets |
Learn the terms epidemic, pandemic, endemic, reservoir, vector, incubation period, and herd immunity. Public health measures include surveillance, vaccination, sanitation, vector control, and health education.
Microbes in Food, Industry, and the Environment
- Food: fermented foods (yoghurt, cheese, bread, curd), spoilage and preservation (refrigeration, drying, salting, pasteurization, canning), food poisoning, and food safety.
- Industry: antibiotics, enzymes, organic acids, vaccines, biofuels.
- Environment: decomposition and nutrient cycling, nitrogen fixation, biofertilizers, sewage treatment, bioremediation, and biogas production.
- Normal flora and the microbiome contribute to health.
Common Mistakes
- Confusing Gram-positive and Gram-negative colours.
- Saying that antibiotics work against viruses (they do not).
- Mixing up the vectors of different diseases.
CHAPTER 15