Overview
Microorganisms are a heterogeneous group of distinct classes of living beings. Based on cellular organisation and biochemistry, the kingdom Protista is divided into prokaryotes and eukaryotes. Bacteria and blue-green algae are prokaryotes; fungi, algae, slime moulds and protozoa are eukaryotes.
Bacteria are prokaryotic microorganisms that do not contain chlorophyll. They are unicellular and do not show true branching (except higher bacteria like actinomycetales). This foundational lesson covers bacterial morphology, classification, structure, and growth โ the essential knowledge for all clinical microbiology work that follows.
Learning Objectives
- Describe the structure of prokaryotic and eukaryotic cells and their key differences
- Explain the units of measurement used in bacteriology
- Classify bacteria based on shape and arrangement with examples
- Describe the structure and function of the bacterial cell wall
- Explain the types of microscopes used in bacteriology
- Describe the four phases of the bacterial growth curve
- Explain the factors affecting bacterial growth
Clinical Story
A patient presents with a severe sore throat. The doctor sends a throat swab to the microbiology lab. Under the microscope, the microbiologist sees chains of spherical bacteria โ streptococci. This single observation, combined with a Gram stain showing they are Gram-positive, already points to Streptococcus pyogenes before any culture result is available. Knowing bacterial morphology saves time and guides early treatment. This is why Lesson 1 matters.
Prokaryotes vs Eukaryotes
| Character | Prokaryotes (Bacteria) | Eukaryotes (Fungi, Protozoa) |
|---|---|---|
| Nucleus | Absent โ no nuclear envelope; DNA in nucleoid region | Present โ true nucleus with nuclear envelope and nucleolus |
| Membrane-bound organelles | Absent | Present โ mitochondria, chloroplasts, lysosomes |
| Chromosome (DNA) | Single circular chromosome in cytoplasm; histone-like proteins | Multiple linear chromosomes with histone proteins |
| Ribosomes | 70S โ free in cytoplasm | 80S โ free and on rough ER; 70S in mitochondria |
| Cell Wall | Peptidoglycan (Eubacteria); pseudomurein (Archaea) | Cellulose (plants); chitin (fungi); absent in animal cells |
| Mitochondria | Absent | Present |
| Mitotic division | Absent โ binary fission | Present |
"No nucleus, No organelles, 70S ribosomes, peptidoglycan cell wall." These four facts distinguish prokaryotes from all eukaryotes. Peptidoglycan is the unique target of antibiotics like penicillin โ eukaryotic cells do not have it, which is why penicillin is selectively toxic to bacteria.
Size & Microscopy
Units of Measurement in Bacteriology
Types of Microscopes
Simple microscope: single lens (e.g. hand lens). Compound microscope: two or more lenses in series โ the image formed by the first lens is further magnified by another. Bacteria may be examined either in the living state (wet films, hanging drops โ shows shape, arrangement, motility) or after fixation and staining. Live bacteria lack contrast, so staining is usually required.
Imposes contrast by exploiting differences in refractive index between bacteria and surrounding medium. Makes internal cell structures visible without staining. Retardation of light rays passing through the object creates phase difference, producing contrast.
Uses reflected light instead of transmitted light. Bacteria appear bright against a dark background. Very useful for visualising spirochetes (e.g. Treponema pallidum) which are too slender to be seen under ordinary illumination.
Uses beams of electrons instead of light. The object scatters electrons and produces an image focused on a fluorescent viewing screen. Objects must be examined in a vacuum (gas molecules scatter electrons). Provides ultra-high magnification for detailed bacterial ultrastructure.
Staining Techniques
Live bacteria show poor contrast under the light microscope. Staining produces colour contrast to reveal morphology. Bacteria have an affinity for basic dyes due to the acidic nature of their protoplasm.
| Stain Type | Principle / Dye | Use |
|---|---|---|
| Simple Stain | Methylene blue or basic fuchsin | Colour contrast only โ all bacteria stain the same colour |
| Negative Stain | Indian ink or nigrosin | Coloured background; bacteria appear unstained. Used for spirochetes and capsule demonstration |
| Impregnation | Silver impregnation | Renders very thin structures (spirochetes, flagella) visible by coating with silver |
| Gram Stain | Crystal violet + iodine + decolouriser + safranin | Differential โ Gram +ve (purple) vs Gram โve (pink). Most important routine stain |
| Acid Fast Stain (ZN) | Carbol fuchsin + acid decolouriser | Mycobacterium tuberculosis โ resists decolourisation with acids (acid-fast bacilli) |
Thick peptidoglycan layer. Teichoic acids intertwined within peptidoglycan โ major surface antigen determinants. No outer membrane. Simpler chemical structure with acidic protoplasm. Retains crystal violetโiodine complex during decolourisation โ appears purple.
Thin peptidoglycan layer (1โ2 molecules). Outer membrane present: phospholipids + proteins + lipopolysaccharide (LPS). LPS = endotoxin. No teichoic acids. Rich in lipids. More permeable during decolourisation โ loses crystal violet โ takes up safranin โ appears pink/red.
Bacterial Shapes & Arrangements
| Shape | Name | Description / Example |
|---|---|---|
| ๐ต | Cocci | Spherical or oval (from Greek: kokkos = berry). E.g. Staphylococcus, Streptococcus |
| โ | Bacilli | Rod-shaped (from Latin: baculus = rod). E.g. E. coli, Mycobacterium |
| ใฐ๏ธ | Vibrios | Comma-shaped curved rods; characteristic vibratory motility. E.g. Vibrio cholerae |
| ๐ | Spirilla | Rigid spiral forms. E.g. Spirillum serpens |
| ๐ | Spirochetes | Flexuous spiral forms (speira = coil, chaite = hair). E.g. Treponema, Leptospira |
| ๐ฟ | Actinomycetes | Branching filamentous bacteria resembling sun rays in tissue lesions |
| โ | Mycoplasmas | Cell wall deficient โ no stable morphology; round, oval or interlacing filaments |
Cocci Arrangements
Bacterial Cell Structure
Rigid; gives shape to the cell; prevents bursting due to osmotic water uptake. Composed chemically of peptidoglycan (murein) โ formed by N-acetylglucosamine and N-acetylmuramic acid alternating in chains, crosslinked by peptide chains.
Teichoic acids are embedded in Gram-positive cell walls and function as: negative charge carrier, major antigenic determinant, ion transport, anchoring, and permeability barrier.
| Feature | Gram Positive | Gram Negative |
|---|---|---|
| Peptidoglycan thickness | Thick (many layers) | Thin (1โ2 layers) |
| Teichoic acid | Present | Absent |
| Outer membrane (LPS) | Absent | Present |
| Lipid content | Low | High |
Protoplast: bacterium completely without cell wall (e.g. after lysozyme treatment). Metabolically active but cannot reproduce; easily lysed.
Spheroplast: bacterium with a damaged (not completely removed) cell wall due to antibiotics or toxic chemicals. Can revert to normal form when toxic agent removed.
Thin layer immediately beneath the cell wall in both Gram-positive and -negative bacteria. Acts as a semipermeable membrane controlling metabolite flow to and from the protoplasm. Site of respiratory enzymes (mesosomes).
Cytoplasm: colloidal system โ 80% water, 20% salts and proteins; rich in ribosomes, DNA.
Ribosomes (70S): centres of protein synthesis; slightly smaller than eukaryotic (80S) ribosomes โ the basis for selective antibiotic action.
Nucleus: no nuclear membrane or nucleolus; DNA is circular and haploid, highly coiled.
Plasmids: extra-chromosomal circular DNA; carry genes for antibiotic resistance, virulence factors.
Mesosomes: vesicular tubules formed by invagination of plasma membrane โ principal sites of respiratory enzymes; assist cell division.
Outermost layer โ usually polysaccharide. A condensed, well-defined layer closely surrounding the cell. Production depends on growth conditions.
Functions: Protects against complement; antiphagocytic (major virulence factor); prevents desiccation.
Glycocalyx: loose mesh of fibrils extending outward from cell. Slime layer: masses of polymer totally detached from cell โ helps adhesion to surfaces.
Flagella & Pili
Flagella are long, hair-like helical filaments extending from the cytoplasmic membrane to the exterior. Composed of protein flagellin (highly antigenic). Function: cell motility.
Parts: filament (external), hook (at cell surface), basal body (embedded in cell envelope).
| Arrangement | Description | Example |
|---|---|---|
| Monotrichous | Single flagellum on one side | Vibrio cholerae |
| Lophotrichous | Tuft of flagella on one side | Bartonella bacilliformis |
| Amphitrichous | Single or tuft on both ends | Spirillum serpens |
| Peritrichous | Flagella all around (lateral) | Escherichia coli, Salmonella |
Types of Motility
Pili / Fimbriae
Hair-like proteinaceous structures โ thinner, shorter and more numerous than flagella. Do not function in motility. Composed of subunit called pilin.
Bacterial Spores
Some bacteria form highly resistant resting structures called endospores to survive adverse conditions. Not a reproductive form and not a storage granule. Each bacterium produces only one spore. Spores are resistant to bactericidal agents, heat, desiccation and adverse physical conditions. Spore coat consists of three layers: Core โ Cortex โ Spore coat.
Bacterial Growth Curve
Bacteria divide by binary fission โ one cell splits into two daughter cells. When grown in liquid medium and bacterial counts plotted against time, a characteristic growth curve with four phases is obtained.
Bacterial Growth Curve โ Number of bacteria over time
No appreciable increase in cell number immediately after inoculation. Cells are adapting to the new environment โ synthesising enzymes, adjusting metabolism. Cell size may increase. Duration varies with species, medium, and temperature.
Cells divide rapidly and number increases exponentially with time. Cells are uniform in size and stain uniformly. Most susceptible to antibiotics at this stage โ antibiotics that target active cell wall synthesis are most effective during log phase.
Cell division stops due to nutrient depletion and accumulation of toxic metabolic products. Equilibrium between dying cells and newly formed cells โ viable count remains constant. Sporulation occurs here. Many bacteria produce exotoxins and antibiotics at this stage.
Population decreases due to cell death exceeding new cell formation. Involution forms (bizarre, irregular shapes) are common. Gram-variable staining occurs due to intracellular storage granules.
Factors Affecting Bacterial Growth
| Factor | Key Points |
|---|---|
| Temperature | Mesophiles (25โ40ยฐC): most pathogens. Psychrophiles: below 20ยฐC. Thermophiles: 55โ80ยฐC. Optimum = temperature of best growth |
| Oxygen | Obligate aerobes (need Oโ). Obligate anaerobes (die with Oโ). Facultative anaerobes (can grow with or without โ most pathogens). Microaerophilic (low Oโ best) |
| pH | Most pathogens: neutralโslightly alkaline (pH 7.2โ7.6). Each species has a pH optimum and range |
| Carbon dioxide | All bacteria need small amounts. Some (e.g. Brucella abortus) require elevated COโ for primary isolation |
| Moisture | Water essential for bacterial protoplasm. Drying is lethal โ effect varies by species |
| Light | Most bacteria grow well in the dark; sensitive to UV light |
| Osmotic effect | Bacteria tolerant to osmotic variation (cell wall protection). Hypertonic solutions cause plasmolysis |
Phototrophs = energy from sunlight ยท Chemotrophs = energy from chemical reactions ยท Autotrophs = synthesise all organic compounds ยท Heterotrophs = cannot synthesise metabolites (need them supplied). Most bacterial pathogens are chemoheterotrophs.
Interactive Quiz
5 QuestionsFlashcards
Tap to flipClinical Case Study
Ravi presents with sudden onset of profuse, painless, watery diarrhoea described as "rice-water" stools, with rapid dehydration. He recently returned from a coastal area. Stool sample sent to microbiology.
- โShape (comma/vibrio) + motility pattern (darting) together pointed to Vibrio before culture confirmed it
- โGram staining (Gram-negative) immediately narrows down the genus
- โMorphology knowledge enables rapid presumptive diagnosis before full culture results
Quick Revision
10-Minute ReviewKey Takeaways
- Bacteria are prokaryotic microorganisms โ no nucleus, no membrane-bound organelles, 70S ribosomes
- The morphological study of bacteria uses optical, phase contrast, dark field, and electron microscopes
- Staining techniques โ simple, negative, impregnation, Gram stain, acid-fast stain โ reveal bacterial structure
- Bacteria classified by shape: cocci, bacilli, vibrio, spirilla, spirochetes; and by arrangement (pairs, chains, clusters)
- Gram-positive wall: thick peptidoglycan + teichoic acids. Gram-negative: thin peptidoglycan + LPS outer membrane
- Flagella (flagellin protein) provide motility; pili provide adhesion and conjugation
- Bacterial growth curve has four phases: lag, log, stationary, and decline
- Growth factors include temperature, oxygen, pH, COโ, moisture and light