These notes are highlighted from our A-level Biology exam focus textbook, for complete study materials on this course, refer to the textbook. For effective preparation, practice past questions on this course as much as possible.
Topic 20: History of the Discovery of Microorganisms
Microorganisms are living things that are too small to be seen with the naked eye. The history of how humans discovered and came to understand these tiny organisms is a fascinating journey that spans several centuries.
For a long time people believed in a theory called spontaneous generation. This theory claimed that living organisms could arise on their own from non-living matter. For example people believed that maggots appeared spontaneously from rotting meat and that mice came from grain. This idea was widely accepted for many years until scientists began to question and test it through experiments.
The conflict over spontaneous generation was eventually settled through careful scientific experimentation. The most famous experiment was conducted by Louis Pasteur who used swan-neck flasks to show that broth only became contaminated when microorganisms from the air were allowed to enter. This effectively disproved spontaneous generation and established that life only comes from pre-existing life, a principle known as biogenesis.
The period between 1860 and 1910 is known as the golden era of microbiology. This was a time of rapid discovery and progress in the field. Scientists like Louis Pasteur and Robert Koch made groundbreaking contributions that transformed medicine and our understanding of disease. Robert Koch developed a set of rules called Koch's Postulates which are used to prove that a specific microorganism causes a specific disease. This gave rise to the germ theory of disease which states that many diseases are caused by microorganisms entering and multiplying within the body.
The discovery of viruses came later when scientists realized that some diseases were caused by agents even smaller than bacteria that could pass through filters that trapped bacteria. These agents were eventually identified as viruses, which are not considered fully living organisms because they cannot reproduce on their own without a host cell.
In the twentieth century microbiology expanded rapidly with advances in technology. Scientists developed better microscopes, discovered antibiotics, learned how to grow microorganisms in laboratories, and began to understand the molecular basis of microbial life. This led to the development of vaccines, improved food preservation methods, and new medical treatments.
In practical class students are introduced to basic microbial laboratory equipment such as the microscope, autoclave, incubator, petri dishes, inoculation loops, and bunsen burners. Students learn the principles of how each piece of equipment works and practice making drawings of what they observe.
Topic 21: Types and Taxonomic Groupings of Microorganisms
Microorganisms are an incredibly diverse group of living things and they are classified into several major categories based on their structure, size, and characteristics.
Bacteria are single-celled prokaryotic organisms, meaning they do not have a true nucleus. They vary greatly in size and shape. The main shapes are cocci which are spherical, bacilli which are rod-shaped, and spirilla which are spiral-shaped. Bacteria can also be motile, meaning they can move using structures called flagella, or non-motile. They are classified using various methods including their shape, staining characteristics, and the way they obtain energy.
Fungi include yeasts and moulds. Yeasts are single-celled fungi that reproduce by budding. Moulds are multicellular fungi that grow as thread-like structures called hyphae, which together form a mass called mycelium. Fungi are classified based on their size, shape, and reproductive structures.
Protozoa are single-celled eukaryotic organisms, meaning they have a true nucleus. They live in water and moist environments and some are parasites that cause disease in humans and animals. Some protozoa are motile and move using structures like cilia or flagella while others are non-motile. They obtain nutrition in different ways, with some engulfing food particles and others absorbing nutrients from their surroundings.
Viruses are the smallest of all microorganisms and they are unique because they are not considered fully living. They consist of a core of genetic material, either DNA or RNA, surrounded by a protein coat called a capsid. Bacteriophages are viruses that specifically infect bacteria. Viroids are even simpler than viruses and consist only of a small piece of RNA without a protein coat. Prions are misfolded proteins that can cause other proteins to misfold and are responsible for diseases like mad cow disease.
Algae are photosynthetic microorganisms found mainly in aquatic environments. They vary greatly in size from microscopic single cells to large seaweeds. Diatoms are a type of algae with intricate glass-like cell walls. Lichens are a special combination of algae and fungi living together in a mutualistic relationship. Algae can reproduce both sexually and asexually depending on the species and environmental conditions.
In practical class students learn and practice aseptic techniques, which are methods used to prevent contamination of microbial cultures. These techniques include sterilizing equipment, working near a flame, and handling cultures carefully to prevent unwanted microorganisms from entering.
Topic 22: Structure, Morphology and Characteristics of Microorganisms
Understanding the structure of microorganisms is essential for identifying them and understanding how they function and cause disease.
A bacterial cell has several distinct structures. The capsule is a slimy outer layer that protects the bacterium from the immune system of the host and from drying out. Flagella are long whip-like structures that some bacteria use for movement. Pili and fimbriae are short hair-like projections on the surface of bacteria that help them attach to surfaces and to other cells. The cell wall provides shape and protection to the bacterium. The plasma membrane controls what enters and leaves the cell. The cytoplasm is the fluid inside the cell where all metabolic activities take place. Unlike eukaryotic cells, bacteria do not have a membrane-bound nucleus and their genetic material floats freely in the cytoplasm.
Fungal cells also have a cell wall but it is made of a substance called chitin, which is different from the material found in bacterial cell walls. Fungal cells have a true nucleus and other membrane-bound organelles just like plant and animal cells.
When bacteria are grown in the laboratory they show characteristic growth patterns called cultural characteristics. On solid media such as agar plates, bacterial colonies can vary in size, shape, colour, texture, and edge appearance. On liquid media bacteria can make the liquid cloudy, form a layer on the surface, or settle at the bottom. These cultural characteristics help microbiologists identify different types of bacteria.
Moulds and yeasts also show characteristic growth patterns in the laboratory. Moulds grow as fuzzy or powdery colonies on solid media and form networks of hyphae. Yeasts form smooth creamy colonies similar in appearance to bacterial colonies. Their hyphal and mycelial structures are important features used in their identification.
Bacteria and fungi also have biochemical characteristics that help in their identification. These include the types of enzymes they produce, the substrates they can digest, and the waste products they release during metabolism.
Viruses have a much simpler structure compared to bacteria and fungi. They consist of genetic material surrounded by a protein coat. Some viruses also have an additional outer envelope made from the membrane of the host cell they previously infected. The shape and structure of the protein coat varies between different types of viruses and is used in their classification.
Microorganisms reproduce in different ways. Bacteria reproduce mainly by binary fission, which is a form of asexual reproduction where one cell divides into two identical cells. Under ideal conditions bacteria can divide very rapidly. The growth of a bacterial population follows a pattern called the microbial growth curve which has four phases. The lag phase is when bacteria adjust to their new environment. The log phase is when bacteria grow and divide rapidly. The stationary phase is when the growth rate equals the death rate. The death phase is when bacteria die faster than new ones are produced due to nutrient depletion and accumulation of waste products.
In practical class students cultivate and identify bacteria from soil, water, and decomposing food to observe these characteristics firsthand.
Topic 23: Microbial Ecology
Microbial ecology studies how microorganisms interact with each other and with their environment. These interactions can have significant effects on health, agriculture, and the environment.
Predation occurs when one microorganism consumes another. Some protozoa for example feed on bacteria as their source of nutrition. Competition occurs when two or more microorganisms compete for the same limited resources such as nutrients or space. The organism that is better adapted to the conditions will eventually dominate.
Synergism is a relationship where two or more microorganisms work together and both benefit from the association even though they are not dependent on each other. This is different from mutualism where the organisms are fully dependent on each other for survival.
Commensalism is a relationship where one microorganism benefits while the other is neither helped nor harmed. Many bacteria that live on the human skin or in the gut exist in a commensal relationship with the human body.
Infectious diseases occur when pathogenic microorganisms enter the body, multiply, and cause harm to the host. Different microorganisms cause different diseases. Bacteria cause diseases like tuberculosis, cholera, and typhoid. Viruses cause diseases like malaria, HIV, influenza, and COVID-19. Fungi cause infections like ringworm and candidiasis. Protozoa cause diseases like malaria and sleeping sickness.
Immunity is the ability of the body to resist infection and disease. The immune system recognizes foreign microorganisms and produces responses to destroy them. Vaccines work by introducing a weakened or killed form of a pathogen into the body, training the immune system to recognize and fight it without causing actual disease.
Microorganisms also cause spoilage of food by breaking down the nutrients in food, producing waste products that change the taste, smell, and texture of food and making it unsafe to eat. Understanding how microorganisms spoil food has led to the development of preservation methods like refrigeration, canning, pasteurization, and the use of preservatives.
Controlling microbial activities is important in medicine, food production, and public health. Methods of control include sterilization which kills all microorganisms, disinfection which kills most harmful microorganisms on surfaces, antiseptics which are applied to living tissues to prevent infection, and antibiotics which are drugs that kill or inhibit the growth of bacteria inside the body.
Topic 24: Microbial Nucleic Acids in Information Storage and Transfer
The genetic information of all living organisms, including microorganisms, is stored in nucleic acids. Understanding how this information is stored and transferred is central to modern biology and microbiology.
DNA, which stands for deoxyribonucleic acid, is the molecule that carries the genetic instructions for the structure, function, growth, and reproduction of all living organisms. DNA is made up of smaller units called nucleotides. Each nucleotide consists of a sugar molecule called deoxyribose, a phosphate group, and one of four nitrogen bases which are adenine, thymine, guanine, and cytosine. The DNA molecule consists of two strands twisted around each other in a structure called a double helix. The two strands are held together by bonds between complementary base pairs where adenine always pairs with thymine and guanine always pairs with cytosine.
Nucleosides are formed when a nitrogen base joins to a sugar molecule. When a phosphate group is added to a nucleoside it becomes a nucleotide. Nucleotides are the building blocks of both DNA and RNA.
RNA, which stands for ribonucleic acid, is similar to DNA but has some differences. RNA is single-stranded, uses the sugar ribose instead of deoxyribose, and uses the base uracil instead of thymine. There are three main types of RNA. Messenger RNA carries the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm. Transfer RNA brings amino acids to the ribosome during protein synthesis. Ribosomal RNA forms part of the structure of the ribosome itself.
DNA replication is the process by which a cell makes an exact copy of its DNA before cell division. Several enzymes are involved in this process. Helicase unwinds and separates the two strands of the DNA double helix. DNA polymerase then reads each strand and builds a new complementary strand by adding nucleotides one at a time. The result is two identical DNA molecules each containing one original strand and one new strand.
The genetic code is the set of rules by which the sequence of nucleotides in DNA is translated into the sequence of amino acids in a protein. The code is read in groups of three nucleotides called codons. Each codon specifies a particular amino acid.
Transcription is the process by which the information in a section of DNA is copied into messenger RNA. This takes place in the nucleus. Translation is the process by which the messenger RNA is read by the ribosome and used to assemble a chain of amino acids into a protein. This takes place in the cytoplasm at the ribosomes.
Genetic material can also be transferred between prokaryotic organisms like bacteria through several processes. Transformation is when a bacterium takes up free DNA from its environment. Transduction is when a bacteriophage carries DNA from one bacterium to another. Conjugation is when two bacteria form a temporary connection and directly transfer genetic material between them.
Mutations are changes in the sequence of nucleotides in DNA. Spontaneous mutations occur naturally due to errors during DNA replication. Induced mutations are caused by external agents called mutagens such as radiation or certain chemicals. The expression of a mutation depends on where in the DNA it occurs and whether it affects a functional gene.
Microorganisms have many important biotechnological applications. In the food industry microorganisms are used in the production of bread, beer, wine, yogurt, and cheese. In environmental management microorganisms are used to break down pollutants in a process called bioremediation. In the pharmaceutical industry microorganisms are used to produce antibiotics, insulin, and other important drugs. In medicine and agriculture microorganisms are used to develop vaccines, improve soil fertility, and protect crops from disease.