Microbiology and Disease

Grade 9 · Science · Study Guide

Understand the major types of microorganisms, how pathogens cause disease, and how the immune system and modern medicine defend against infection.

Microbiology is the study of microorganisms — living things too small to see with the naked eye. The major categories include bacteria (prokaryotes), viruses (non-living infectious particles), fungi, protozoa, and helminths (parasitic worms). Not all microorganisms are harmful; many are beneficial, like the gut microbiome and bacteria used in fermentation.

Bacteria cause disease by producing toxins, invading host cells, or triggering damaging immune responses. Examples include Streptococcus (strep throat), Mycobacterium tuberculosis (tuberculosis), and Vibrio cholerae (cholera). Antibiotics target bacterial structures (cell walls, ribosomes) and can treat bacterial infections. Antibiotic resistance arises when bacteria evolve mechanisms to survive antibiotic treatment — a growing global health crisis.

Viruses are not cells; they are packets of nucleic acid (DNA or RNA) wrapped in a protein coat (capsid). They replicate by injecting their genetic material into host cells and hijacking cellular machinery. Examples include influenza, HIV, SARS-CoV-2, and the varicella-zoster virus (chickenpox). Antibiotics do not work against viruses; antiviral drugs or vaccines are used instead.

The immune system defends against pathogens through non-specific defenses (skin, mucus, fever, phagocytes) and specific immunity (B cells producing antibodies; T cells killing infected cells). Vaccines stimulate specific immunity by exposing the immune system to harmless antigens, training it to respond rapidly if the real pathogen appears.

Key terms

Pathogen: Any microorganism or agent capable of causing disease in a host, including bacteria, viruses, fungi, and protozoa.
Antibiotic: A chemical substance that kills or inhibits the growth of bacteria; ineffective against viral infections.
Virus: A non-cellular infectious particle consisting of nucleic acid (DNA or RNA) enclosed in a protein capsid; replicates inside host cells.
Immune Response: The body's defense reaction to pathogens, including non-specific (innate) defenses and specific (adaptive) immunity using B and T cells.
Antibiotic Resistance: The ability of bacteria to survive antibiotic treatment due to genetic mutations or acquired resistance genes; a major public health concern.
Vaccine: A biological preparation that stimulates immunity against a specific pathogen by exposing the immune system to harmless antigens.

Flashcards

Bacteria: Unicellular prokaryotes; can be beneficial (gut flora) or pathogenic; treated with antibiotics.
Virus: Non-living infectious particle (DNA or RNA + capsid); requires host cell machinery to replicate; not treated by antibiotics.
Protozoa: Unicellular eukaryotic organisms; some are parasitic pathogens (e.g., Plasmodium causing malaria, Giardia).
Antibody: Y-shaped protein produced by B cells that binds specifically to an antigen to neutralize or mark pathogens for destruction.
Antigen: A foreign molecule (often on a pathogen's surface) that triggers an immune response and is recognized by specific antibodies.
Phagocyte: A white blood cell (like a neutrophil or macrophage) that engulfs and destroys pathogens by phagocytosis.
Koch's Postulates: Four criteria established by Robert Koch to prove a specific microorganism causes a specific disease.
Epidemic vs. Pandemic: Epidemic = disease outbreak in a specific region; Pandemic = worldwide spread of an infectious disease.

Questions

Why are antibiotics ineffective against viral infections like the flu?
1. Viruses are too large for antibiotics to affect
2. Viruses lack cell walls and cellular machinery that antibiotics target
3. Viruses destroy antibiotics before they can act
4. Antibiotics only work inside cells, not in the bloodstream
Show answer
Answer: Viruses lack cell walls and cellular machinery that antibiotics target
Antibiotics target structures unique to bacteria such as peptidoglycan cell walls, bacterial ribosomes (70S), or metabolic pathways absent in human cells. Viruses have none of these — they are not cells — so antibiotics have no target.
Vaccines work by introducing a weakened or harmless form of an antigen to train the immune system to recognize and fight the real pathogen.
Show answer
Answer: True
Vaccines introduce antigens (weakened pathogens, inactivated toxins, or specific proteins) without causing disease. The immune system forms memory B and T cells so it can mount a faster, stronger response to future real infections.
The protein coat surrounding a virus's genetic material is called a ___.
Show answer
Answer: capsid
The capsid is the protein shell surrounding a virus's genetic material (DNA or RNA). It protects the genome and helps the virus attach to and enter host cells.
Which immune cells produce antibodies to neutralize specific pathogens?
1. T cells
2. Red blood cells
3. B cells
4. Neutrophils
Show answer
Answer: B cells
B cells (B lymphocytes) are the antibody-producing cells of the adaptive immune system. When activated by an antigen, they differentiate into plasma cells that secrete large quantities of specific antibodies.
Antibiotic resistance is primarily caused by individual patients not completing their full course of prescribed antibiotics.
Show answer
Answer: False
Antibiotic resistance is fundamentally a consequence of natural selection acting on bacterial populations. Bacteria with resistance mutations survive antibiotic exposure and reproduce. Incomplete courses contribute but the root cause is evolution of resistance genes in bacterial populations.

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