Microorganisms: Bacteria, Viruses, Fungi, Growth & Aseptic Technique

Bacteria are single-celled, prokaryotic organisms, meaning they lack a membrane-bound nucleus and other membrane-bound organelles. Their genetic material (DNA) is typically found in a nucleoid region and often in small, circular plasmids. They possess a cell wall (containing peptidoglycan), a cell membrane, cytoplasm, and ribosomes. Some may have a protective capsule, flagella for movement, or pili for attachment. They reproduce primarily by binary fission, an asexual process where one cell divides into two identical daughter cells. Bacteria can be autotrophic (e.g., chemosynthetic, photosynthetic) or heterotrophic (e.g., saprophytic, parasitic). They are crucial for decomposition, nitrogen fixation, food production (e.g., yoghurt), and biotechnology, but also cause diseases (e.g., tuberculosis, salmonella).

Viruses are non-cellular, obligate intracellular parasites. They are much smaller than bacteria and consist of genetic material (either DNA or RNA, but never both) enclosed within a protein coat called a capsid. Some viruses also have an outer lipid envelope derived from the host cell membrane. Viruses cannot carry out metabolic processes independently; they reproduce by hijacking the host cell's machinery to replicate their genetic material and synthesise new viral proteins, leading to the assembly of new virus particles. They cause a wide range of diseases in plants, animals, and humans (e.g., influenza, measles, HIV, COVID-19).

Fungi are eukaryotic organisms, meaning their cells have a membrane-bound nucleus and other organelles. They can be unicellular (e.g., yeast) or multicellular (e.g., moulds, mushrooms). Their cell walls are made of chitin. Multicellular fungi are composed of thread-like structures called hyphae, which collectively form a mycelium. Fungi are heterotrophic, primarily saprophytic (feeding on dead organic matter) or parasitic (feeding on living hosts). They can reproduce asexually (e.g., budding in yeast, fragmentation of hyphae, spore formation) or sexually (via spores). Fungi are important decomposers, used in food production (e.g., bread, cheese, mushrooms), and for antibiotic production (e.g., penicillin), but also cause plant and animal diseases (e.g., athlete's foot, ringworm, potato blight).

When microorganisms are grown in a closed system (batch culture) with limited nutrients, their population growth typically follows a predictable pattern, represented by a growth curve with four distinct phases: 1. Lag Phase: Little or no increase in cell number. Microorganisms are adapting to the new environment, synthesising enzymes, and repairing any damage. 2. Log (Exponential) Phase: Rapid increase in cell number. Cells are dividing at their maximum rate, doubling at regular intervals. The population is most uniform metabolically. 3. Stationary Phase: The rate of cell division equals the rate of cell death. This occurs due to nutrient depletion, accumulation of toxic waste products, or changes in pH. The total population size remains relatively constant. 4. Death (Decline) Phase: The rate of cell death exceeds the rate of cell division. The population size decreases due to continued nutrient depletion and accumulation of toxic waste.

Aseptic technique refers to a set of practices and procedures used to prevent contamination by unwanted microorganisms. It is crucial in microbiology to ensure that cultures remain pure and that the working environment and personnel are protected from pathogens. Key principles include: - Sterilisation: Eliminating all microorganisms from equipment and media (e.g., autoclaving, flaming). - Disinfection: Reducing the number of microorganisms on surfaces. - Minimising exposure: Working quickly and efficiently, keeping lids on petri dishes, working near a Bunsen flame (creates an updraft, sterilises air). - Personal hygiene: Washing hands, wearing lab coats. - Safe disposal: Sterilising contaminated waste before disposal. Aseptic technique prevents contamination of cultures, protects researchers from pathogens, and ensures reliable experimental results.