Introduction to Microbiology & Bacteria
The foundation of pharmaceutical microbiology. Understanding how bacteria are structured, how they grow, and how they are counted is prerequisite for understanding sterilization, disinfection, and sterility testing – the most important applied topics in this subject.
Syllabus & Topics
- 1History of Microbiology: Antoni van Leeuwenhoek (discovered microorganisms, 1676), Louis Pasteur (germ theory, pasteurization, vaccination), Robert Koch (Koch’s Postulates, TB causative agent), Joseph Lister (antiseptic surgery).
- 2Branches of Microbiology: Bacteriology, Virology, Mycology, Parasitology, Immunology, Environmental Microbiology, Pharmaceutical Microbiology.
- 3Scope and importance of Pharmaceutical Microbiology.
- 4Prokaryotes vs Eukaryotes: Key differences in cell structure (no membrane-bound nucleus in prokaryotes, no organelles), size (1-10 µm vs 10-100 µm), reproduction (binary fission vs mitosis/meiosis).
- 5Ultrastructure of Bacteria: Cell wall (Peptidoglycan in Gram+ve – thick; Gram-ve – thin + outer membrane), Cell membrane, Cytoplasm, Ribosomes (70S), Nucleoid (DNA), Plasmids, Flagella, Pili/Fimbriae, Capsule, Endospores.
- 6Morphological Classification of Bacteria: Cocci (Spherical: Diplococci, Streptococci, Staphylococci, Tetracocci, Sarcinae), Bacilli (Rod-shaped: Diplobacilli, Streptobacilli), Spirilla (Spirochetes, Vibrio, Spirilla).
- 7Nutritional Requirements: Carbon source (autotrophs vs heterotrophs), Nitrogen source, Minerals, Growth factors (vitamins, amino acids), Water.
- 8Raw Materials for Culture Media: Peptone, Meat extract, Agar, NaCl. Types: nutrient broth, nutrient agar, selective media, differential media.
- 9Physical Parameters for Growth: Temperature (Psychrophiles <20°C, Mesophiles 20-45°C, Thermophiles >45°C), pH (optimum ~7.0), Oxygen (Aerobes, Anaerobes, Facultative anaerobes), Osmotic pressure, Light.
- 10Growth Curve of Bacteria: (1) Lag Phase: metabolic adjustment, no cell division. (2) Log/Exponential Phase: maximum growth rate, cell doubling. (3) Stationary Phase: growth rate = death rate, nutrient depletion. (4) Decline/Death Phase: death rate > growth rate.
- 11Isolation Methods: Streak plate method, Pour plate method, Spread plate method, Serial dilution.
- 12Preservation of Pure Cultures: Refrigeration (short-term), Glycerol stocks at -80°C, Lyophilization (freeze-drying – long-term), Oil overlay of cultures.
- 13Cultivation of Anaerobes: Anaerobic jars (GasPak system), Anaerobic chambers/glove boxes, Thioglycollate broth.
- 14Quantitative Measurement of Bacterial Growth: (a) Total Count: Counting chamber (Haemocytometer), Turbidimetry/Spectrophotometry (OD600). (b) Viable Count: Colony forming units (CFU) – pour plate, spread plate method. Serial dilution technique.
- 15Microscopy: (1) Brightfield microscopy – routine. (2) Phase contrast microscopy – live unstained cells by phase differences. (3) Dark field microscopy – bacteria appear bright on dark background (spirochetes). (4) Electron microscopy – TEM (internal ultrastructure), SEM (surface topography).
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. What are Koch’s Postulates?
Koch’s postulates are a set of criteria established by Robert Koch to determine whether a specific microorganism causes a particular disease. The four postulates state that:
(1) the microorganism must be present in all cases of the disease;
(2) it must be isolated from the diseased host and grown in pure culture;
(3) the cultured organism should reproduce the disease when introduced into a healthy host;
(4) the same microorganism must be re-isolated from the experimentally infected host and shown to be identical to the original organism.
Q2. What is the Significance of the Lag Phase in Bacterial Growth?
The lag phase is the initial stage of the bacterial growth curve in which bacteria are metabolically active but not yet dividing. During this period, cells adapt to the new environment by synthesizing essential enzymes, repairing cellular components, adjusting to nutrient availability, and increasing in size. The duration of the lag phase depends on factors such as the age of the inoculum, composition of the growth medium, and temperature. In food microbiology and pharmaceutical preservation, extending the lag phase helps delay microbial growth and spoilage.
Q3. What is the Difference Between Total Count and Viable Count?
Total count methods, such as turbidimetry or haemocytometer counting, estimate the total number of cells in a sample, including both living and dead cells. In contrast, viable count methods, such as the plate count or colony-forming unit (CFU) method, measure only living cells capable of forming colonies. Viable counts are particularly important in pharmaceutical microbiology for sterility testing and bioburden assessment of products.
Q4. What is Phase Contrast Microscopy?
Phase contrast microscopy is a technique that converts differences in phase (caused by variations in optical path length as light passes through different parts of a specimen) into differences in brightness or contrast. This allows visualization of internal structures in live, unstained, transparent microorganisms without killing or staining them, making it especially useful for studying living cells and their dynamic processes.
Q5. Why Does a Bacterium’s Cell Wall Structure Determine Gram Staining Result?
The Gram staining result depends on differences in bacterial cell wall structure. Gram-positive bacteria possess a thick peptidoglycan layer that retains the crystal violet–iodine complex even after decolorization with alcohol or acetone, appearing purple under the microscope. Gram-negative bacteria have a thin peptidoglycan layer and an outer lipid membrane; during decolorization, the alcohol dissolves the outer membrane and the crystal violet–iodine complex is washed out. These cells are then counterstained with safranin and appear pink or red.