the microbial world

🎯 Learning Objectives

At the end of this topic, you must be able to:

• Differentiate prokaryotic from eukaryotic cells; and

• Characterize the different types of microorganisms

🔬 About | Overview | Types | Eukaryotes | Prokaryotes | Acellular

• All organisms are composed of cells, the basic fundamental unit of life. The cells of microorganisms are categorized into two – prokaryotes and eukaryotes.

• Eukaryotic cells contain membrane-bound organelles, including a nucleus. Eukaryotes can be single-celled or multi-celled, such as fungi, parasites, and algae.

• Bacteria are an example of prokaryotes. Prokaryotic cells do not contain a nucleus or any other membrane-bound organelle. Prokaryotes include two groups: bacteria and another group called archaea.

• Viruses are acellular. They are neither prokaryotic nor eukaryotic.

🧾 Types of Microorganisms

• Eubacteria

• Archaea

• Fungi

• Protozoa

• Algae

• Helminths

• Virus

• Prions

🧪 Eubacteria

• The bacteria of interest in medicine

• Cell walls largely composed of carbohydrate + protein complex called peptidoglycan

• Reproduce by dividing into two equal cells (called binary fission)

• Many bacteria can “swim” using moving appendages called flagella

🌋 Archaea

• Have no medical importance

• Found in extreme environments, divided into three groups:

  • Methanogens → produce methane as waste product from respiration

  • Extreme halophiles → live in salty environments

  • Extreme thermophiles → live in hot sulfurous water

• Not known to cause disease in humans

🍄 Fungi

• May be unicellular or multicellular

• Large multicellular fungi (mushrooms) may look like plants, but cannot carry out photosynthesis

• True fungi → cell walls composed of chitin

• Human fungal diseases (mycoses) classified by infection location on the body

• Divided into two: Molds & Yeasts

🦠 Protozoa

• Unicellular eukaryotic microbes

• Move via:

  • Pseudopods (extensions of their cytoplasm or false feet)

  • Flagella (long)

  • Cilia (shorter appendages)

• Have a variety of shapes; live either as free-entities or parasitic that are (absorbing/ingesting organic compounds from their environment)

• Reproduce asexually or sexually

🌱 Algae

• Photosynthetic eukaryotes (both sexual & asexual reproductive forms)

• Cell walls contain a carbohydrate called cellulose

• Found in freshwater, saltwater, soil, and with plants

• Do not cause significant disease in humans

• Beneficial → sources of food, iodine, other minerals

🪱 Helminths

• Parasitic worms (live as parasites)

• Eukaryotic organisms with complex body organization

• They are parasitic in the sense that they absorb nutrients, fluids, or tissues of their host

• Three groups:

  • Tapeworms (cestodes)

  • Flukes (trematodes)

  • Roundworms (nematodes)

🧬 Virus

• Acellular, obligate intracellular parasites

• Reproduce only by using host cellular machinery

• Structure:

  • Core → made up of only one type of nucleic acid, either DNA or RNA

  • The core is surrounded by a Protein coat

  • Sometimes → the coat is encased by an additional layer called envelope (lipid membrane)

🧩 Prions

• Infectious protein particle/misfolded proteins

• Responsible for fatal neurodegenerative diseases of animals (TSE: transmissible spongiform encephalitis)in humans

• Cause remains unknown

• Examples:

  • BSE (Bovine Spongiform Encephalopathy or Mad Cow Disease) in cattle

  • Scrapie in sheep/goats

  • CJD (Creutzfeldt-Jakob Disease) in humans

🔬 Cellular Structures and Functions

🎯 Learning Objectives

At the end of this topic, you must be able to:

• Distinguish among the general shapes of bacteria (with examples)

• Discuss the different parts of a bacterial cell and its functions

🧱 Bacterial Cell Structures

🔹 Structures External to the Cell Wall

• Glycocalyx

• Flagella

• Axial Filaments

• Pili/Fimbriae

🔹 Structures Internal to the Cell Wall

• Plasma Membrane

• Cytoplasm

• Nucleoid

• Ribosome

• Plasmid

• Inclusions

• Endospores

🦠 Bacterial Shape

(Examples provided in figures: Streptococcus, Clostridium, Salmonella, Staphylococcus, Vibrio, etc.)

🧃 Glycocalyx

• Aka Sugar Coat

• Viscous, sticky polymer external to cell wall (made of polysaccharide, polypeptide, or both)

• Capsule = organized & firmly attached

• Slime layer = loosely attached

🌀 Flagella

• Long filamentous appendages for motility

• Types:

  • Atrichous → no flagella

  • Peritrichous → all over

  • Monotrichous → single

  • Lophotrichous → tuft at one pole

  • Amphitrichous → both poles

🧵 Axial Filaments

• Aka Endoflagella

• Similar to flagella, but rotate to produce spiral motion (in spirochetes)

🧲 Pili / Fimbriae

• Hair-like appendages (shorter, straighter, thinner than flagella)

• Function: attachment & DNA transfer (not motility)

🧱 Cell Wall

• Defines bacterial shape

• Protects from osmotic shock

• If destroyed → cell dies

• Made of peptidoglycan (murein)

• genus Mycoplasma → no cell wall (Gram stain resistant)

🟦 Gram Positive Cell Wall

• 40 layers thick

• 90% peptidoglycan, 10% teichoic acid

• Teichoic acids: consist primarily of an alcohol (such as glycerol or ribitol) and phosphate; regulate cation movement, help in growth, prevent extensive wall breakdown & lysis

• Two types:

  • Lipoteichoic acid → linked to plasma membrane

  • Wall teichoic acid → linked to peptidoglycan layer

🟥 Gram Negative Cell Wall

• Thin (1 layer only)

• No teichoic acid, but with outer envelope

• Envelope layers:

  • Lipopolysaccharide - toxic & antigenic (LPS → Lipid A toxic portion → when released in circulation may cause diarrhea, fever, shock, dilation of blood vessels, blood clotting)

  • Phospholipid layer

  • Periplasmic space (contains enzyme that inactivates antibiotics)

🧩 Plasma Membrane

• Selective barrier for material exchange

• Contains phospholipids → selective permeability

• Contains enzymes → breakdown nutrients, produce ATP

💧 Cytoplasm

• 80% water, thick, semitransparent, elastic

• Contains: nucleoid, ribosomes, inclusions

🧬 Nucleoid

• Circular, double-stranded DNA (bacterial chromosome)

• Carries genetic information

🔹 Ribosome

• Site of protein synthesis

• Subunits: 30S + 50S

• Target of many antibiotics

🧾 Plasmids

• Extrachromosomal DNA, independent replication

• Genes: antibiotic resistance, toxin production, enzyme synthesis

• Used in biotechnology

📦 Inclusions

• Reserve deposits of nutrients (stored when abundant, used when scarce)

🌱 Endospores

• Resting cells (produced by Bacillus & Clostridium)

• Resistant to heat, drying, chemicals, radiation

• Formed during hostile conditions → sporulation

• Return to vegetative state → germination

🧪 Figure: Bacterial Cell Diagram

• Capsule / slime layer

• Cell wall

• Plasma membrane

• Cytoplasm

• Chromosome

• Plasmid

• Ribosome

• Inclusions

• Pili (fimbriae)

• Flagellum