microbiology

• Medical Microbiology


  A branch of medicine that deals with the study of microorganisms such as bacteria, viruses, parasites or fungi; which produce disease in humans, called as infectious diseases
• Bacteria


  • Unicellular prokaryotic organisms with diverse shapes, sizes, and metabolic capabilities
  • They inhabit virtually every environment on Earth, from deep-sea vents to polar ice caps
• Viruses


  • Non-cellular entities consisting of genetic material (DNA or RNA) enclosed in a protein coat
  • Obligate intracellular parasites, they rely on host cells for replication and cause diseases such as the common cold, influenza, and HIV/AIDS
• Fungi


  • Eukaryotic organisms encompassing yeasts, molds, and mushrooms
  • They exhibit diverse morphologies and play essential roles as decomposers, pathogens, and symbionts
• Parasites


  • Protozoa: Unicellular eukaryotic organisms exhibiting remarkable morphological diversity. They inhabit diverse habitats, ranging from freshwater ponds to the human intestine, and include parasites responsible for diseases like malaria, amoebiasis, and giardiasis
  • Helminths: Multicellular eukaryotic organisms comprising parasitic worms (nematodes, trematodes, and cestodes). They infect humans and animals, causing diseases such as schistosomiasis, soil-transmitted helminthiasis, and filariasis
• Commensalism


  1. Microorganisms coexist with humans without harm, offering benefits

  2. Gut microbiota aids in digestion, nutrient absorption, and immune modulation
• Pathogenesis


  1. Pathogenic microorganisms cause diseases through possession of virulence factors that enable colonization of host tissues, evading immune defenses, and toxin production

  2. Infections outcomes vary from mild localized infections to severe systemic diseases
• Host-Microbe Interactions


  The host's immune system defends against microbial invaders through innate and adaptive immune responses involving recognition of pathogens, neutralization, elimination, and establishment of immunological memory
• Antonie Philips van Leeuwenhoek (1676)


  First scientist to observe bacteria and other microorganisms - using single-lens microscope. Named small organisms as 'Little animalcules'
• Edward Jenner (1796)


  Developed first vaccine - smallpox vaccine
• Louis Pasteur (1822–1895)


  Father of microbiology. Proposed principles of fermentation, introduced sterilization techniques, described pasteurization of milk, and developed vaccines against anthrax, fowl cholera and rabies
• Joseph Lister (1867)


  Father of antiseptic surgery. Observed that postoperative infections were reduced by using diluted carbolic acid to sterilize the instruments and to clean wounds during surgery
• Robert Koch (1843–1910)


  First to use agar (solidifying agent) for preparation of solid media, introduced concept of pure culture, discovered anthrax bacilli, tubercle bacilli and cholera bacilli, and introduced staining techniques using aniline dye
• Koch's postulates


  1. Microorganism constantly associated with the lesions of the disease

  2. Isolate the organism in pure culture

  3. Same disease must result when isolated microorganism is inoculated into a suitable laboratory animal

  4. Possible to re-isolate the organism in pure culture

  5. Antibody to the causative organism should be demonstrable in the patient's serum
• Size of Microorganisms


  • Bacteria- 0.2–1.5 μm in diameter; 3–5 μm in length
  • Viruses- 20–300 nm in diameter
  • Parasites- Protozoans measured in µm; helminths - few mm to meters
  • Fungi- Grow as hyphae - thread-like structures 2–10 µm in diameter and up to several cm in length
• Shape of Bacteria


  • Cocci (meaning berry) - oval or spherical cells
  • Bacilli or rods - rod shaped
• Arrangement of Cocci and Bacilli

  • Cocci - arranged in groups (clusters), pair or chains
  • Bacilli - arranged in chain, pair, and some bacilli are curved, comma shaped, or cuneiform shaped
• Gram Staining of Bacteria

  • Gram-positive cocci : Round shape Violet color
  • Gram-negative cocci : Rod shape pink color
  • Gram-positive bacilli : Rod shape violet color
  • Gram-negative bacilli : Rod shape Pink color
• Bacterial Cell Wall


  • Protection to cell against osmotic lysis
  • Confers rigidity - presence of peptidoglycan layer
  • Protect cell from toxic substances
  • Site of action of several antibiotics
  • Contains virulence factors (e.g. endotoxin) that contribute to pathogenicity
• Differences between gram-positive and gram negative cell wall

  • Gram-Positive Bacteria: Thick layer of peptidoglycan, Teichoic Acids present, Outer Membrane absent
  • Gram-Negative Bacteria: Thin layer of peptidoglycan, Lipopolysaccharides (endotoxin) present, Outer Membrane present
• Functions of Cell Membrane

  • Semi permeable membrane
  • Transport system -nutrient uptake, and waste excretion
  • Site for metabolic processes
  • Ribosomes - sites for protein synthesis, composed of rRNA and ribosomal proteins
• Intracytoplasmic Inclusions


  • Storage sites of nutrients/energy, formed by bacteria under nutritional deficiency conditions and disappear when the deficient nutrients are supplied
• Cell Wall Appendages


  • Capsule and Slime Layer
  • Flagella
  • Fimbriae or Pili
• Capsule and Slime Layer

  Layer of amorphous viscid material lying outside the cell wall called glycocalyx. Capsulated bacteria include Pneumococcus and Meningococcus. Functions: Bacterial virulence, Prevent cell from drying out, Protect from lysozyme and bacteriophages, Toxic to host cells, Biofilm formation
• Flagella


  • Thread-like appendages protruding from the cell wall that confer motility to the bacteria. Arrangements: Monotrichous, Lophotrichous, Peritrichous, Amphitrichous
• Fimbriae or Pili


  • Short, fine, hair-like appendages that help in bacterial adhesion. Special type called sex pilus helps in conjugation
• Bacterial Spores


  • Highly resistant resting (or dormant) stage of the bacteria formed in unfavorable environmental conditions due to depletion of exogenous nutrients. Comprise of core, cortex, coat, and exosporium layers. Sporulation is a complex process that takes about 10 hours. Germination is the transformation of dormant spores into active vegetative cells
• Shape and Position of Spores

  • A. Non-bulging, oval and terminal
  • B. Non-bulging, round, and subterminal
  • C. Non-bulging, oval and central
  • D. Bulging, round and terminal
  • E. Bulging, oval and terminal
  • F. Bulging, oval and central
• Sex pilus


  Special type of pili that helps in bacterial conjugation
• Bacterial spores


  Highly resistant resting (or dormant) stage of the bacteria, formed in unfavorable environmental conditions due to depletion of exogenous nutrients
• Structure of bacterial spore

  • Core → cortex → coat → exosporium
• Sporulation


  1. Process of formation of spores from vegetative stage of bacteria

  2. Not a method of reproduction as bacteria do not divide during sporulation

  3. Complex process that takes about 10 hours

  4. Mature spore formed is extremely resistant to heat and disinfectant
• Germination


  Transformation of dormant spores into active vegetative cells when grown in a nutrient-rich medium
• Shapes and positions of spores

  • Non-bulging, oval and terminal
  • Non-bulging, round, and subterminal
  • Non-bulging, oval and central
  • Bulging, round and terminal
  • Bulging, oval and terminal
  • Bulging, oval, and central
• Bacterial growth requirement


  • Water constitutes 80% of total bacterial cell
  • Minimum nutritional requirements include sources of carbon, nitrogen, hydrogen, oxygen and some inorganic
• Bacterial cell division


  1. Nuclear division

  2. Cytoplasmic division
• Generation time


  • Time required for a bacterium to give rise to two daughter cells under optimum condition
  • Escherichia coli and most other pathogenic bacteria - 20 minutes
  • Mycobacterium tuberculosis - 20 hours
  • Mycobacterium leprae - 20 days
• Bacterial growth curve


  1. Lag phase

  2. Log phase

  3. Stationary phase

  4. Phase of decline
• Lag phase


  • Period between inoculation and beginning of multiplication of bacteria
  • Bacteria increase in size due to accumulation of enzymes and metabolites
  • Bacteria reach their maximum size at the end of lag phase
• Log phase


  Bacteria divide exponentially so that the growth curve takes a shape of straight line