Bacteria, Viruses, and the Immune System: A Deep Dive
Bacteria: Reproduction and Genetics
Asexual Reproduction
Bacteria, as primitive organisms, primarily reproduce asexually through fission (simple division or bipartition). The original cell divides, resulting in two identical daughter cells. This process lacks variability.
Sporulation
A more evolved form of asexual reproduction is sporulation. Under negative environmental conditions (changes in humidity, temperature, or nutrient scarcity), bacteria form spores—resistant structures that remain dormant. When conditions improve, the spore’s metabolic activity resumes, and it breaks open (exospore). If the covering doesn’t break, and activity doesn’t resume, it’s known as an endospore. Sporulation is also observed in mosses, fungi, some algae, and ferns, enabling wider geographic distribution. However, it doesn’t introduce variability.
Sexual Reproduction: Conjugation
Conjugation is a primitive form of sexual reproduction involving the exchange of DNA segments between donor and recipient bacteria. This exchange enhances antibiotic resistance.
During conjugation, bacteria join, forming a conjugation bridge (pili). The donor cell transfers extrachromosomal genes (plasmids) to the recipient, introducing new genetic material. Chromosomal conjugation, where genes from a chromosome with incorporated plasmids are transferred, also leads to genetic variability in the recipient bacterium.
Bipartition
Sporulation
Plasmids
Anthrax
Anthrax is an aggressive bacterial disease that has caused deaths and been used as a biological weapon. Herbivores can act as vectors, spreading the pathogenic bacteria into the soil (reservoir). Spore formation allows the bacteria to remain dormant for extended periods, thanks to their resistant shell (exine and intine), facilitating wider geographic distribution and affecting both humans and animals.
Viruses: Structure, Function, and Diseases
Viruses are macromolecules with a protein coat (capsid) containing either DNA (adenoviruses) or RNA (retroviruses). They are obligate intracellular parasites, relying on the host cell’s genetic material for their metabolism.
Viruses exist in various crystalline forms and represent a link between living and non-living matter. Because they are not cells, they are not classified into any kingdom and lack scientific names. They are designated as parasites of plants, bacteria, animals, and humans, causing various diseases.
Antibiotics are ineffective against viruses, which reproduce rapidly. This rapid reproduction makes controlling viral diseases challenging, as seen with HIV’s progression to AIDS.
Viruses’ high aggressiveness makes them suitable biological weapons. They cause numerous diseases, including:
- Influenza
- Common cold
- Measles
- Herpes
- Warts
- AIDS
- Infectious hepatitis
- Mononucleosis
- Rubella
- Chickenpox (varicella)
- Rabies (hydrophobia)
- Mumps
- Poliomyelitis
- Smallpox
- Human influenza
- Condyloma
- Hantavirus
- Avian flu
Hantavirus has caused significant mortality in Chile in recent years.
Bacteriophages: Viral Parasites of Bacteria
Bacteriophages are viruses that parasitize bacteria. They can follow a lytic pathway, destroying the bacteria and producing new viral particles, or a lysogenic pathway, where the viral genetic material integrates into the host’s DNA.
Lytic Cycle
In the lytic cycle, viral genes are transcribed and translated, leading to the assembly of new viral particles and the lysis (breakdown) of the infected cell.
Lysogenic Cycle
In the lysogenic cycle, viral genetic material integrates into the host cell’s DNA, with minimal viral gene expression. However, the virus retains the potential for future replication.
Viral Replication Stages
- Penetration: The virus attaches to the host cell. Viral surface proteins bind to host cell receptors, stimulating endocytosis. Viral genetic material is then released into the cytoplasm.
- Duplication: The genetic material is copied multiple times.
- Transcription: Viral genetic material is used as a template to create mRNA.
- Protein Synthesis: In the host cytoplasm, mRNA directs the synthesis of viral proteins.
- Assembly: Viral enzymes, genetic material, and coat proteins assemble.
- Liberation: Viruses leave the cell through budding or cell rupture.
Symptoms of Viral Diseases
- Fever
- Headache
- Chills
- Muscle pain
- Skin inflammation
- Nausea
- Vomiting
- Rhinorrhea
- Diarrhea
Antigens and the Immune Response
Antigens are specific proteins on the surface (or inside the capsid) of pathogenic organisms. Their presence on red blood cells determines blood type. During infection, viral or bacterial antigens appear in blood plasma, serving as diagnostic markers. For example, AIDS is detected via the ELISA test.
A symptomatic host develops a lytic pathway, while an asymptomatic host develops a lysogenic pathway.
Human Immunodeficiency Virus (HIV)
AIDS symptoms arise from the impact of HIV in its final stage. HIV is a global pandemic, transmitted through various means (see table below).
The origin of HIV is debated. One theory suggests genetic manipulation in a laboratory; another points to the African green monkey, with transmission possibly occurring through bestiality or consumption of contaminated meat.
HIV is a retrovirus that cannot be eradicated due to its rapid mutation rate. The ELISA test detects the presence of HIV antigens. Individuals with the antigen are considered HIV+ and may develop AIDS within 10 years.
Triple therapy reduces disease progression but doesn’t eliminate the virus. Asymptomatic HIV+ individuals can become carriers, capable of infecting others.
HIV attacks the immune system, leading to AIDS and opportunistic infections. It specifically targets white blood cells, reducing their count significantly (from 6000-8000/ml to below 200/ml).
HIV Transmission
| Transmission Methods | Non-Transmission Methods |
|---|---|
| 1. Contaminated blood transfusion | 1. Shaking hands |
| 2. Sexual contact with an infected person | 2. Insect bites |
| 3. Sexual promiscuity | 3. Hugging |
| 4. Contaminated breast milk | 4. Kissing |
| 5. Mother-to-child transmission during pregnancy | |
| 6. Contaminated toothbrushes (gingivitis) | |
| 7. Contaminated razors | |
| 8. Tattoos with contaminated needles | |
| 9. Contaminated syringes | |
| 10. Contaminated drug paraphernalia |
White Blood Cells
Production Sites
- Lymph nodes
- Appendix
- Spleen
- Thymus
- Red bone marrow of flat bones
Characteristics
- Diapedesis (crossing capillaries)
- Positive chemotaxis
- Phagocytosis
Types affected by HIV: CD4+ helper T cells and CD8+ cytotoxic T cells.
The Immune System and Defensive Barriers
The body possesses various defense mechanisms, including mechanical barriers and chemical secretions produced by the body or specialized cells (white blood cells).
- Skin
- Mucus
- Hair and cilia
- Hair cells
- Tears
- Saliva
- Urine
- Defecation
- Vomiting
- Acidic pH of skin
- Gastric juice
- Interferons
- Fever
- Inflammation
- White blood cells
- Cough
Cilia trap germs and dust, preventing entry into the respiratory system. Mucus traps impurities and pathogens, particularly in the respiratory and gastrointestinal tracts. Hair cells, with their cilia, trap and remove foreign substances. Saliva and tears contain lysozyme, a bactericidal enzyme. Vomiting removes foreign substances from the stomach. Parietal cells in the stomach produce HCl, aiding digestion and acting as a bactericide. Urine eliminates microorganisms and pathogens. Defecation removes microorganisms from the gut. Fever inhibits microbial growth and enhances interferon’s effect. Interferons protect uninfected cells from viral diseases. Inflammation destroys germs, limits their spread, and aids tissue repair. Lymph, a fluid similar to blood without red blood cells, contains lymph nodes (white blood cell production centers). Lymphoma is an enlarged lymph node forming a tumor.
The skin acts as a physical barrier, releasing sweat to lubricate and protect against foreign agents. Sweat releases toxins and regulates body temperature. Sebaceous glands release sebum (skin oil).
White Blood Cells: The Cellular Defense
White blood cells are the primary cellular barrier, providing cellular and humoral immunity. They kill infected cells and microorganisms, and produce antibodies (agglutinins) to neutralize antigens (agglutinogens).
Leukopenia (low white blood cell count) increases susceptibility to opportunistic infections (as in AIDS). Leukemia involves many malformed white blood cells (cancer). A low white blood cell count increases susceptibility to infection. Apoptosis is programmed cell death.
White Blood Cell Features
- Phagocytosis: Engulfing pathogens via pseudopodia (false feet).
- Antibody production: Only B cells produce antibodies.
- Diapedesis: Crossing capillaries to reach infection sites.
- Positive chemotaxis: Recognizing chemical substances (antigens) to target pathogens.
White blood cells have lifespans ranging from days to years, depending on their function. They vary in size (5-20 microns in diameter) and can be classified by function, type of immunity, and nuclear appearance.
Agglutinogen = toxin or antigen
Antitoxin = agglutinin or antibody
White Blood Cells: Granular vs. Agranular
Agranular White Blood Cells
Monocytes mature into macrophages, which, along with neutrophils, phagocytose foreign substances and infected cells.
Lymphocytes
Lymphocytes are a primary defensive barrier, responsible for humoral immunity through specific proteins (released into blood plasma) that neutralize antigens.
B lymphocytes and T lymphocytes have distinct functions.
B Lymphocytes
B lymphocytes are responsible for humoral immunity, secreting specific antibodies to neutralize pathogens. They can produce:
- Antitoxins: Inhibit toxins produced by pathogens.
- Agglutinins: Clump pathogens to prevent spread.
- Precipitins: Cause precipitation, reducing pathogen dispersion.
- Opsonins: Weaken or puncture pathogen membranes, facilitating destruction by macrophages.
- Cytokins: Proteins that destroy cells.
Memory B cells produce antibodies that remain in the plasma, facilitating a faster immune response to subsequent infections.
T Lymphocytes
T lymphocytes mature in the thymus and provide cellular immunity. They kill cells by direct contact. Types of T lymphocytes include:
- CD4+ helper T cells
- CD8+ cytotoxic T cells
- Suppressor T cells
- Memory T cells
- Natural killer (NK) cells