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MICROORGANISMS
Microorganisms are defined as single-celled organisms that are invisible to the naked eye. They are also known as microorganisms or microscopic organisms as they could only be seen under a microscope. They make up almost 60% of the earth’s living matter. The term “microbes” is used to describe several different life forms with different sizes and characteristics. Microbes are classified into bacteria, fungi, protists, viruses and archaea. Microbes can be useful as well as harmful. Certain microbes cause severe infections and diseases and can also spoil food and other materials. While others play an important role in maintaining environmental balance. Pathogens: microorganisms that have the potential to cause infectious diseases in its host. BACTERIA Characteristics of a bacterial cell
- Bacteria are unicellular (single-celled) organisms belonging to the prokaryotic group where the organisms lack a few organelles and a true nucleus.
- They are present in different shape, size and arrangement.
- The bacterial cell lacks nucleus and membrane-bound cell organelles.
- Bacterial DNA is found in the cytoplasm and not packaged to form chromatin as in eukaryotic cell.
- Bacteria cell is 10 times smaller than the human cell.
- The diameter of a bacteria cell is ~1μm (10-6 m)
- The outer covering of a bacteria cell is the cell wall, which is rigid and provides structural integrity.
- The bacteria cell wall is made up of peptidoglycan or murein.
- Different shapes of bacteria cell are the characteristic feature of a bacteria species.
- Bacteria cells may contain external appendages like cilia, flagella, etc.
- Bacteria can be photoautotrophs, chemoautotrophs or parasites. STRUCTURE OF A BACTERIAL CELL Capsule : Made up of complex polysaccharides, it forms the outermost covering of the bacterial cell. Capsules are an important structural component that keeps the bacterium from drying out, also protecting them from being engulfed by larger microbes. It provides the bacterial cell the ability to cause disease. Cell Wall : It is a rigid covering composed of peptidoglycan, a polymer of protein and sugar molecules that provides shape and structural support to the cell. Based on their staining properties and composition of cell wall, bacteria can be classified into gram-positive and gram-negative group. Cytoplasmic Membrane : It is a semi-permeable membrane composed of lipids and proteins that separates the inside of the cell from its external environment. The cytoplasmic membrane carries out many necessary cellular functions such as energy generation, protein secretion, cell division, and transport of nutrients across the cell. Cytoplasm : Present below the cytoplasmic membrane and distributed throughout the cell, it is a gel-like matrix mainly composed of water along with dissolved salts and minerals. Along with some cellular parts and components such as nucleoid, ribosomes, and inclusion bodies, the cytoplasm of a bacterial cell also contains a circular (or sometimes linear) auxiliary DNA molecule called a plasmid. This extra-chromosomal piece of DNA participates in cell division and confers the bacterial cell with properties such as antibiotic resistance and the ability to infect other cells known as pathogenicity.
over its surface. The flagella beat in a propeller-like motion to help the bacterium move toward nutrients and light, as well as away from toxic chemicals. The presence of flagella, their number, and arrangement on their cell surface also forms a basis of classification in bacteria. Pili : They are short hair-like projections emerging from outside the cell. These outgrowths assist the bacteria in attaching to other cells and surfaces, such as teeth, intestines, and rocks. Bacteria use specialized pili, called sex pili, for reproduction CLASSIFICATION OF BACTERIA ON THE BASIS OF MORPHOLOGY / SHAPE Cocci : These bacteria are oval, round, or spherical in shape (e.g., Streptococcus aureus ). Bacilli : These bacteria are rod-shaped (e.g., Escherichia coli ). Spirilla : These bacteria are twisted and helical in appearance (e.g., Helicobacter pylori ). Vibrio : These bacteria have a single bend or curve in shape. They appear like a comma (e.g., Vibrio cholerae ).
CLASSIFICATION OF BACTERIA BASED ON MODE OF RESPIRATION
Bacteria can be classified into different groups depending on how they use oxygen or other molecules to carry out respiration. Aerobic Respiration : Bacteria use oxygen to break down glucose, releasing more energy (ATP). This is a highly efficient process. Final products are carbon dioxide and water. This type of respiration occurs in obligate aerobes, facultative anaerobes (when oxygen is available), and microaerophiles. Anaerobic Respiration : In this type of respiration, no oxygen is used; instead, other molecules like nitrate, sulphate, or carbon dioxide act as electron acceptors. It produces less ATP compared to aerobic respiration. It occurs in obligate anaerobes and facultative anaerobes (when oxygen is absent). Difference Type of Bacteria Oxygen Requirement Respiration Type Examples Obligate Aerobes These bacteria require oxygen to survive. They use oxygen to produce energy through a process called aerobic respiration, in which glucose is completely broken down in the presence of oxygen, releasing carbon dioxide, Anaerobic Mycobacterium tuberculosis
Aerotolerant Anaerobes These bacteria do not use oxygen for respiration, but unlike obligate anaerobes, they are not harmed by its presence. They rely entirely on anaerobic pathways (like fermentation) to generate energy but can live in oxygen- rich environments. Anaerobic Lactobacillus CLASSIFICATION OF BACTERIA ON THE BASIS OF CELL WALL Difference Features Gram-Positive bacteria Gram-Negative bacteria Gram-positive bacteria are a type of bacteria. They have thicker cell walls and lack an outer membrane. Gram-negative bacteria are a type of bacteria. Their cell wall comprises of a thin layer of peptidoglycan, surrounded by an outer membrane made up of lipopolysaccharides and phospholipids and a cytoplasmic inner membrane. Cell wall The thickness of the cell wall is 20 to 80 nanometres The thickness of the cell wall is 2 to 8 nanometres
Peptidoglycan Layer They have a thick multilayered peptidoglycan layer They have a thin layer/single- layered peptidoglycan layer Teichoic and lipoteichoic acids Teichoic and lipoteichoic acids are present. They are embedded in the cell wall. Teichoic and lipoteichoic acids are absent. Outer membrane The outer membrane is absent The outer membrane is present Lipopolysaccharides (LPS) Lipopolysaccharide is absent Lipopolysaccharide is present in outer membrane Porins Absent Present in the outer membrane Gram staining Distinctive purple appearance after gram staining Pink/red colour after gram staining Resistance to Antibiotic More susceptible towards antibiotics like penicillin and lysozyme More resistant towards antibiotics due to outer membrane Reason : Gram-negative bacteria have a more complex structure with an outer membrane, making them more resistant to antibiotics and certain environmental stresses. The lipopolysaccharides (LPS) in the outer membrane can act as endotoxins, causing strong immune response
CLASSIFICATION OF BACTERIA ON THE BASIS OF NUTRITIONAL
REQUIREMENTS
Bacteria can be classified as autotrophic and heterotrophic bacteria based on how they obtain their energy and nutrients requirements. Difference Autotrophic bacteria Heterotrophic bacteria Definition Autotrophic bacteria are self- sufficient. They can synthesize their own food from simple inorganic substances like carbon dioxide (CO₂) and water. Heterotrophic bacteria cannot produce their own food. They rely on organic compounds, obtained from other organisms, as their source of carbon and energy. Source of Carbon They use inorganic carbon (e.g., CO₂) as their carbon source. They use organic carbon (e.g., sugars, fats, proteins) from external sources. Energy source
- Photoautotrophs : Use light energy (photosynthesis).
- Chemoautotrophs : Use energy from chemical reactions (chemosynthesis). They obtain energy by breaking down organic matter from other organisms, through respiration or fermentation.
Examples (^) • Photoautotrophs : Cyanobacteria (use light to make food).
- Chemoautotrophs : Nitrosomonas (use chemical reactions like nitrification). - Saprophytic bacteria : Escherichia coli (feed on decaying matter). - Parasitic bacteria : Mycobacterium tuberculosis (cause diseases by living inside hosts). Role in the environment
- They play a crucial role in carbon fixation and oxygen production (especially photoautotrophs).
- They support nutrient cycling (especially chemoautotrophs like nitrifying bacteria).
- They play an important role in decomposing organic matter and recycling nutrients.
- Important in symbiotic relationships (e.g., bacteria in the human gut).
