Microbiology Foundations: Bacterial Growth, Genetics, and Structure

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1. Bacterial Growth & Nutritional Requirements

A. Nutrient Requirements (The "Raw Materials")

Just like human cells, bacterial cells are highly complex and require specific building blocks to construct their membranes, DNA, and proteins.

• Water: Essential for all biochemical reactions. It is the universal solvent in which all intracellular metabolic processes occur.
• Carbon Source (C): The backbone of all living molecules (carbohydrates, lipids, proteins). Bacteria are often classified by how they get carbon (e.g., heterotrophs get it from organic compounds like glucose; autotrophs get it from CO₂).
• Nitrogen Source (N): Crucial for building amino acids (which make up proteins) and nucleic acids (which make up DNA/RNA).
• Inorganic Salts, Sulfur (S), & Phosphorus (P):
  • Phosphorus is needed to synthesize ATP (energy currency) and the phospholipid bilayer of the cell membrane.
  • Sulfur is strictly needed for certain sulfur-containing amino acids (like cysteine and methionine) which hold proteins together via disulfide bonds.
• Growth Factors: Essential vitamins and amino acids that the bacteria cannot synthesize on their own. If the environment lacks these, the bacteria cannot survive.

B. Environmental Factors (The "Factory Conditions")

• Temperature: Most human pathogens grow best at 37°C (normal human body temperature). These are known as mesophiles. (Clinical Note: This is why the human body generates a fever—it raises the temperature above 37°C to make the environment uncomfortably hot and hostile for the invading bacteria!)
• Gas (Oxygen): Determines if they can breathe in air or if air is toxic to them (detailed in the next section).
• pH: Most bacteria prefer a neutral pH (around 7.0), though some have adapted to survive extreme acid.
  Example: Helicobacter pylori in the stomach survives the highly acidic gastric juice (pH ~2.0) by secreting an enzyme called urease, which creates a neutralizing "cloud" of ammonia around the bacteria.
• Osmotic Pressure: Salt and sugar concentrations in the environment.
  Clinical Example: High salt environments usually pull water out of bacteria, killing them (which is why curing meat with salt prevents rotting). However, Staphylococcus aureus is a "halophile" (salt-lover) and can easily survive on the salty surface of human skin, making it a major cause of surgical wound infections.

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2. Oxygen Requirements: Aerobic vs. Anaerobic Bacteria

Oxygen is highly reactive. When metabolized, it creates deadly byproducts called Reactive Oxygen Species (ROS), such as superoxide radicals (O₂^-) and hydrogen peroxide (H₂O₂). To survive in oxygen, a bacteria MUST have specific enzyme "shields" (Catalase and Superoxide Dismutase - SOD) to neutralize these toxins.

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3. The Bacterial Growth Curve

When bacteria invade a host or are put in a culture tube, they follow a predictable, 4-stage life cycle. Exam Trap: Know exactly what happens in the Log phase vs. Stationary phase!

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4. Fastidious Bacteria (The "Picky Eaters")

Definition: Fastidious microorganisms are extremely difficult to grow in the laboratory because they have highly complex or restricted nutritional/environmental requirements (specific temp, pH, O₂, special nutrients). They will simply die if these stringent needs aren't met.

Exam Tip: Memorize these classic examples. If you see them on a test, know they require special agars (like Chocolate agar) to grow!

• Neisseria gonorrhoeae (Causes Gonorrhea). Requires highly specific "Thayer-Martin" agar (chocolate agar with antibiotics added to kill competing bacteria).
• Haemophilus influenzae (Causes respiratory infections/meningitis). Requires Chocolate Agar, which contains heated, lysed red blood cells that release strict growth factors: Factor X (hemin) and Factor V (NAD).
• Treponema pallidum (Causes Syphilis - actually so fastidious it can't be grown on standard lab media at all! It must be grown in animal testicles).
• Legionella pneumophila (Causes Legionnaires' disease).
• Bordetella pertussis (Causes Whooping cough).
• Campylobacter jejuni (Requires microaerophilic conditions).
• Helicobacter pylori (Requires microaerophilic and acidic adaptations).
• Brucella species.
• Francisella tularensis.
• Bartonella henselae (Cat scratch disease).
• Mycoplasma pneumoniae & A. pleuropneumoniae.

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5. Bacterial Cell Division & Generation Time

Prokaryotic cells divide by Binary Fission. One cell elongates, duplicates its DNA, a cross-wall forms, and it splits exactly into two identical daughter cells (One into two, two into four, four into eight). Because of this, cell growth is mathematically exponential.

Generation Time (Doubling Time): The time it takes for a bacterial population to double in number. This varies wildly among species and has huge health consequences.

Specific Dividing Times to Know:

• Escherichia coli: Very fast! ~52.0 to 86.6 mins. (Clinical translation: A patient with an E. coli UTI can develop overwhelming, life-threatening sepsis overnight because the bacteria duplicate so rapidly).
• Proteus vulgaris: 28.2 mins.
• Enterococcus faecalis: 25.9 mins.
• Bacillus cereus: 49.0 mins.
• Fungi/Yeasts (Saccharomyces cerevisiae): ~99 - 107 mins.

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6. Morphology: Size and Shape

Size

Bacteria generally range from 0.1 to 5 µm in diameter. They are much smaller than human Eukaryotic cells, but significantly larger than viruses. You need a Light Microscope to see bacteria, but a high-powered Electron Microscope to see viruses.

• Haemophilus influenzae: 0.25 × 1.2 µm (Very small)
• Escherichia coli: 1.3 × 3 µm (Average)
• Cyanobacteria: 5 × 40 µm (Giant for a bacteria)

Shapes & Arrangements

Pathologists use these shapes to instantly narrow down the cause of an infection.

• Cocci (Spheres):
  • Diplococci: Pairs. (e.g., Streptococcus pneumoniae, and Neisseria gonorrhoeae which is famously a Gram-negative diplococci).
  • Streptococci: Chains. (Looks like a string of pearls under the microscope).
  • Staphylococci: Grape-like clusters. (e.g., Staphylococcus aureus. If a doctor sees Gram-positive clusters on a blood culture, they immediately suspect Staph!).
  • Tetrads (groups of 4) & Sarcina (3D cubes of 8).
• Bacilli (Rods): Coccobacillus (plump, oval rod), Chain of bacilli (Bacillus anthracis), Flagellate rods (Salmonella typhi), Spore-formers (Clostridium botulinum).
• Others:
  • Vibrios: Comma-shaped (Vibrio cholerae).
  • Spirilla / Spirochaetes: Corkscrew shaped (Helicobacter pylori, Treponema pallidum).
  • Filamentous: Long, branching threads resembling fungal hyphae. Examples include Mycobacteria (visible on ZN/Ziehl-Neelsen Acid-Fast stain), Actinomyces, and Nocardia.

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7. Eukaryotic vs. Prokaryotic Cell Comparison

Exam Trap: You must know the absolute differences. This is the entire foundation of Selective Toxicity in pharmacology! We want drugs that kill bacteria (prokaryotes) without harming human host cells (eukaryotes).

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8. Bacterial Genetics: DNA, Transcription, & Translation

A. Bacterial DNA

Most bacteria have a haploid genome (only one copy of their genes, meaning any mutation shows up immediately, with no backup copy to hide a lethal recessive trait). The genome is a single chromosome consisting of a circular, double-stranded DNA molecule.

Plasmids: Extra, small circular DNA pieces are also often present. Plasmids are not essential for basic life, but they carry "superpowers" like antibiotic resistance genes or toxin genes. Bacteria can pass these plasmids to each other via conjugation (like sharing a flash drive of data).

Exceptions to the Rule (Exam Favorites!):

• Linear chromosomes exist in Gram-positive Borrelia and Streptomyces.
• Agrobacterium tumefaciens (Gram-negative) has one linear AND one circular chromosome!

B. The Central Dogma & RNA Processing

DNA replicates → DNA is transcribed into mRNA → mRNA is translated by ribosomes into Protein.

Crucial Difference: In Eukaryotes (humans), DNA has "junk" sequences called introns that must be spliced (cut) out, leaving only exons. Bacteria generally do NOT have introns and do not require RNA splicing. Their mRNA is ready to be translated immediately, allowing them to adapt to new environments at lightning speed.

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9. The Cell Envelope: Cytoplasmic Membrane & Cell Wall

The envelope is everything surrounding the cytoplasm. It consists of the Cell (Plasma) Membrane and the Cell Wall.

A. Cytoplasmic / Cell / Plasma Membrane

It is a Phospholipid bi-layer (hydrophilic heads facing out, hydrophobic tails facing in). Because bacteria lack internal organelles, this thin outer membrane has to do 5 crucial jobs:

1. Selective permeability barrier: Keeps nutrients in, keeps toxins out.
2. Electron transport and oxidative phosphorylation: Since bacteria have NO mitochondria, the cytochromes and dehydrogenase enzymes for the respiratory chain (ATP making) are embedded directly in the cell membrane!
3. Excretion: Pumps out hydrolytic enzymes and pathogenic toxins into the host body.
4. Biosynthetic function: Contains the enzymes that build the cell wall.
5. Chemotactic systems: Receptors that bind attractants (food) and repellants. Example: E. coli has 20 different chemoreceptors on its membrane to navigate its environment!

*Note: Antibacterial agents like Polymyxins and Ionophores specifically destroy the bacterial cell membrane, causing the cell's contents to leak out and die.

Exceptions: Archaebacteria lack peptidoglycan. Some eukaryotic cells have walls made of cellulose (plants) or chitin (fungi).

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10. Gram-Positive vs. Gram-Negative Envelopes

This structural difference is why Gram staining works (Gram-positives trap the purple crystal violet dye in their thick walls, while Gram-negatives lose it and stain pink), and it fundamentally decides what antibiotics a doctor will prescribe.

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11. Lipopolysaccharides (LPS) - The Gram-Negative Weapon

LPS is found exclusively in the outer leaflet of the outer membrane of Gram-negative bacteria. It consists of 3 specific parts:

• Complex Lipid A (The Endotoxin): Made of fatty acids (caproic, lauric, myristic, palmitic, and stearic acids). Notice it does NOT contain glycerol.
  Clinical Scenario (Septic Shock): Lipid A is highly toxic. When intact inside the bacteria, it does little harm. However, if you give a patient strong antibiotics and burst open millions of Gram-negative bacteria (like E. coli or Salmonella) in the blood, massive amounts of Lipid A are released. Human macrophages detect Lipid A via Toll-like receptor 4 (TLR4), causing a massive immune overreaction (cytokine storm of TNF-alpha and IL-1) leading to severe fever, a deadly drop in blood pressure, and catastrophic Septic Shock.
• Core Polysaccharide: Similar across all Gram-negative bacteria of the same genus. Connects Lipid A to the outer chain.
• Terminal O-Polysaccharides (O-Antigen): A repeating series of sugar units sticking out into the environment. This is the major surface antigen recognized by host antibodies. Because it is highly variable, bacteria use it to evade the immune system.
  Fact: There are >2500 different antigenic types in Salmonella alone! Public health scientists use this to track outbreaks (e.g., the deadly strain of E. coli known as O157:H7 is named entirely after its specific O-Antigen and Flagellar H-antigen!).

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12. Capsules, Slime Layers, and Appendages

A. Capsules and Slime Layers (Glycocalyx)

A slimy/gummy extracellular material secreted by prokaryotes. It is almost always an extracellular polymer of highly hydrated polysaccharide.

The ONE Exam Exception: The capsule of Bacillus licheniformis (and the deadly Bacillus anthracis) is uniquely made of protein (poly-D-glutamic acid), not polysaccharide!

• Attachment: E.g., Streptococcus mutans uses its heavy slime layer to firmly stick to the smooth enamel of teeth, initiating plaque and causing dental caries (cavities). Furthermore, slime layers allow bacteria (like Staph epidermidis) to form impenetrable biofilms on hospital catheters and IV lines.
• Anti-phagocytic: The capsule acts like a "greased pig." Immune cells (macrophages and neutrophils) try to grab and eat the bacteria, but they slip right out of the immune cell's grip. This makes the bacteria highly pathogenic. (Patients without a functioning spleen, like Sickle Cell patients, are highly susceptible to encapsulated bacteria like Streptococcus pneumoniae).
• Antigenic structure: Doctors use the specific sugars of the capsule to identify (type) the bacteria and to create life-saving vaccines (like the Pneumococcal polysaccharide vaccine).

B. Bacterial Appendages

• Fimbriae: Short, fine, rigid surface structures. Enable bacteria to stick to inert surfaces or form pellicles/scums on surface liquids. Neisseria gonorrhoeae uses fimbriae to tightly anchor itself to the mucosal lining of the urethra so it doesn't get washed away by urine.
• Pili: Longer than fimbriae, usually only 1 or a few present per cell. Made of protein subunits called pillins.
  • Adherence: Grabbing onto host tissues (e.g., Uropathogenic E. coli uses special P-pili to climb up the urinary tract and cause severe kidney infections).
  • Sex Pili (F-pili): Used like a hollow grappling hook to attach a donor cell to a recipient cell during bacterial conjugation (sharing DNA/plasmids).
  • Antigenic Variation: Neisseria gonorrhoeae constantly alters the genetics of its pili proteins. By the time the host immune system creates an antibody to destroy the pili, the bacteria has already swapped out its pili for a new version, meaning the immune system can never create a lasting antibody against it!
• Flagella: Thread-like appendages composed entirely of flagellin protein arranged in a helical structure (12-13nm diameter).
  • Function: The primary organ of locomotion (swimming). Bacteria spin these like microscopic boat propellers to move toward food. Note: some bacteria lack flagella and instead glide or use internal gas vesicles to move.
  • Antigenic: Flagella are highly antigenic. In Salmonella and E. coli, this is known specifically as the H-antigen.

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13. Bacteria Endospores: The Ultimate Survival Mechanism

When environmental conditions become harsh (severe nutritional depletion, high heat, dangerous radiation), certain bacteria (mainly the Gram-positive rods like Bacillus and Clostridium) form a dormant, virtually indestructible internal "escape pod" called an endospore. The vegetative (living, eating) cell undergoes autolysis (bursts open and dies) to release the durable spore into the environment.

• Properties: They are incredibly resistant to heat, drying, radiation, acids, and chemical disinfectants. Standard boiling water will NOT kill them. (Clinical note: Standard alcohol-based hand sanitizers in hospitals DO NOT kill Clostridium difficile spores. Doctors must physically wash their hands with soap and water to wash the spores down the drain!)
• Structure: Composed of a highly dehydrated Core (containing dipicolinic acid and calcium), Cortex, tough protein Spore coat, and Exosporium.
• Classification: Pathologists look at exactly where the spore forms inside the mother cell to identify the bacteria species under a microscope (Central, Subterminal, or Terminal). (For example, C. tetani has a classic terminal spore that looks like a tennis racquet).