Microbiology is the study of microscopic organisms, including bacteria, viruses, fungi, and protists. This cheat sheet focuses on bacteria because they are important in health, disease, ecosystems, food production, and biotechnology. Grade 10 and 11 biology students need a clear reference for comparing cell types, identifying bacterial structures, and understanding how bacteria grow and spread.
It also supports safe lab work and careful interpretation of microbiology data.
Bacteria are prokaryotic cells, meaning they lack a nucleus and membrane-bound organelles. Their main structures include a cell wall, plasma membrane, cytoplasm, ribosomes, DNA in a nucleoid, and sometimes plasmids, capsules, pili, or flagella. Important formulas include magnification = image size / actual size and number after n divisions = starting number x 2^n.
Bacterial classification often uses shape, arrangement, Gram stain result, oxygen use, and method of obtaining energy.
Key Facts
- Bacteria are prokaryotes, so their DNA is in a nucleoid region instead of inside a nucleus.
- Magnification is calculated using magnification = image size / actual size, with both sizes in the same units.
- During binary fission, one bacterial cell divides into two genetically identical cells under favorable conditions.
- The number of bacteria after n divisions is calculated by final number = starting number x 2^n.
- Population growth rate over a time interval can be estimated by growth rate = change in population / change in time.
- Gram-positive bacteria stain purple because they have a thick peptidoglycan cell wall, while Gram-negative bacteria stain pink because they have a thin peptidoglycan layer and an outer membrane.
- Antibiotics treat bacterial infections by targeting bacterial structures or processes, such as cell wall synthesis or protein synthesis.
- Aseptic technique reduces contamination by keeping cultures, tools, hands, and work surfaces as free from unwanted microbes as possible.
Vocabulary
- Prokaryote
- A cell type that lacks a nucleus and membrane-bound organelles, such as a bacterial cell.
- Nucleoid
- The region of a prokaryotic cell where the main circular DNA molecule is located.
- Plasmid
- A small circular DNA molecule in bacteria that often carries extra genes, such as antibiotic resistance genes.
- Binary fission
- The asexual process in which one bacterial cell copies its DNA and divides into two cells.
- Gram stain
- A staining method used to classify bacteria as Gram-positive or Gram-negative based on cell wall structure.
- Antibiotic resistance
- The ability of bacteria to survive exposure to an antibiotic that would normally kill them or stop their growth.
Common Mistakes to Avoid
- Confusing bacteria with viruses is wrong because bacteria are living cells with ribosomes and membranes, while viruses are noncellular particles that must use host cells to reproduce.
- Forgetting to use the same units in magnification calculations is wrong because magnification = image size / actual size only works when both measurements are in matching units.
- Assuming all bacteria cause disease is wrong because many bacteria are harmless or helpful in digestion, decomposition, nitrogen cycling, and food production.
- Thinking antibiotics work on viruses is wrong because antibiotics target bacterial structures or processes that viruses do not have.
- Calling Gram-negative bacteria weaker because they stain pink is wrong because Gram-negative bacteria often have an outer membrane that can make them harder to treat.
Practice Questions
- 1 A bacterial cell is 2 micrometers long, and its image is 40 millimeters long. What is the magnification?
- 2 A culture starts with 50 bacterial cells. If the cells divide 6 times by binary fission, how many cells are present?
- 3 A bacterial population increases from 2,000 cells to 18,000 cells in 4 hours. What is the average growth rate in cells per hour?
- 4 Explain why using the full course of an antibiotic helps reduce the chance of antibiotic resistance developing in a bacterial population.
Understanding Microbiology & Bacteria
A bacterial cell survives because its outer layers control what enters and leaves. The cell membrane is not just a bag around the cell. It uses protein channels and pumps to take in nutrients, remove wastes, and keep the internal conditions stable.
Many bacteria release enzymes outside the cell. These enzymes break large food molecules into smaller pieces that can cross the membrane. The cell wall gives shape and resists bursting when water moves inward.
This is why drugs that damage wall building can be effective against growing bacteria. The extra outer membrane of many Gram-negative species can make some medicines harder to enter.
Bacterial populations do not grow at the same speed forever. In fresh nutrient liquid, cells first adjust to the new conditions. They may make enzymes needed for the available food.
This is followed by rapid repeated division if temperature, water, nutrients, and acidity are suitable. Later, food becomes limited and waste products build up. The number of living cells may then level off or fall.
A visible colony on an agar plate usually began from one living cell or a small clump of cells. Counting colonies gives an estimate of viable cells, not every cell present. Dead cells and cells that cannot grow under the chosen conditions will not form colonies.
Disease depends on more than the presence of a bacterium. A species must reach the right body site, attach to tissues, obtain nutrients, and avoid removal by the immune system. Some cells have capsules that make engulfment by immune cells more difficult.
Others produce toxins or form biofilms. A biofilm is a community of microbes held in a sticky layer. Biofilms can grow on teeth, wounds, catheters, and pipes.
They are often harder to remove because the outer cells can protect cells deeper inside. Antibiotic resistance spreads especially quickly when resistant cells survive treatment and reproduce. Resistance genes can move between bacteria through direct cell contact, uptake of free DNA, or viruses that infect bacteria.
Good microbiology work separates real results from contamination. A control plate without an added sample can show whether the equipment or medium was already contaminated. A comparison plate can test the effect of one changed factor, such as temperature or an antibiotic.
Labels need the sample name, date, conditions, and a clear marking on the base of the plate. When using a microscope, high magnification does not automatically give a clearer image. Resolution is the ability to distinguish two nearby points.
Correct focusing, staining, lighting, and clean slides matter as much as magnification. In school laboratories, cultures should stay closed when possible, work areas should be disinfected, and hands should be washed after handling materials.