Comprehensive Study Notes on Microbiology, Bacterial Genetics, and Clinical Protocols

Characteristics of the Microbial World

Microorganisms are defined by several key characteristics often summarized by the acronym MICRO. The "M" signifies their microscopic size, meaning they are invisible to the naked eye. "I" refers to their status as independent units, where each cell functions independently. "C" indicates they possess a complex-less or simple structure. "R" denotes their rapid growth and quick division rates. Finally, "O" represents their omnipresence, as they are found everywhere. These organisms are so small that they can only be observed using a microscope and they constitute approximately 60% of all living matter on Earth.

Microorganisms, also referred to as microbes, fall under the scientific study of microbiology. This category includes a diverse range of organisms such as bacteria, fungi, archaea, and protists. Notably, viruses and prions are excluded from this classification because they are categorized as non-living entities.

General Characteristics of Bacteria

Bacteria represent the smallest living beings. The smallest among them exist at the limit of the resolution of the best light microscopes, measuring approximately $0.15 \times 10^{-6}\,m$ to $0.2 \times 10^{-6}\,m$. Conversely, the largest bacteria, such as specific purple and sulfur bacteria, can reach sizes of several dozen micrometers. In terms of structure, bacterial cells differ significantly from eukaryotic cells, primarily because they are prokaryotes, meaning they lack a cell nucleus. Their genetic material is organized as a nucleoid or genophore, which consists of a long, double-stranded, circularly closed DNA molecule containing individual genes and very few non-coding sequences.

Bacteria also contain plasmids, which are extra-chromosomal genetic factors. Plasmids are autonomous replicons, possessing their own replication systems independent of the chromosome. They are responsible for traits such as antibiotic resistance and the synthesis of bacteriocins. Some plasmids, known as episomes (e.g., the F factor), can recombine with the bacterial genome and become an integral part of it. Plasmids vary in size: small plasmids are below 25,000 base pairs, while large plasmids exceed 25,000 base pairs. Based on their ability for auto-transfer, they are classified as conjugative (1-3 copies per cell, e.g., R, F, Col B, Col V plasmids) or non-conjugative (10-100 copies per cell, e.g., Col E 1, Col E 2 plasmids), which typically infect cells via mobilization, transformation, or transduction.

Bacterial cells use ribosomes for protein biosynthesis and possess the ability to store reserve substances. Morphologically, bacteria appear in several basic forms: spherical or ovoid (coccus), cylindrical (bacterium or bacillus), and spiral-shaped cylinders (vibrio or spirillum). Spherical bacteria can also form characteristic arrangements. Some bacteria can produce specialized resting forms known as spores, which include cysts, endospores, and exospores. These spores have a unique physicochemical structure providing resistance to high temperatures, dehydration, chemicals, radiation, and other physical factors, making them far more resilient than vegetative forms. This resistance is a critical consideration in food preservation methods.

Bacterial Classification and Cell Wall Structure

Bacteria are classified based on their relationship with oxygen into aerobes, microaerophiles, and anaerobes. Based on temperature growth requirements (minimum, optimal, and maximum), they are categorized as psychrophilic (cold-loving), mesophilic (growing at moderate temperatures), and thermophilic (heat-loving).

Classification is also based on cell wall structure using the Gram stain method. Gram-positive and Gram-negative terms define these categories. Most bacterial cell walls contain peptidoglycan (murein), a large polymeric molecule unique to bacteria composed of sugars and amino acids. Gram-positive bacteria have a dense, mesh-like layer of peptidoglycan. In contrast, Gram-negative bacteria have a thin layer of peptidoglycan but possess an outer membrane containing lipopolysaccharides (LPS), which is absent in Gram-positive bacteria.

Bacterial Genetics and Variation

The bacterial genome comprises the nucleoid, plasmids, prophages (e.g., Mu), and transposable genetic elements. Transposable elements include insertion sequences (IS), which code for integrase facilitating translocation between replicons; transposons (Tn), which contain IS at both ends and structural genes (for antibiotic resistance, heavy metal resistance, or virulence) in between; and integrons, which contain a gene cassette where structural genes can be integrated.

Genetic variation in bacteria occurs through mutation and recombination. Mutations are divided into spontaneous mutations, which occur at low frequencies (e.g., $10^{-7}$ for streptomycin resistance), and induced mutations caused by mutagens such as UV radiation, nitrous acid (III), hydroxylamine (HA), 5-Bromouracil (BU), 2-Aminopurine (AP), and Ethyl ethanesulfonate (EES). Mutations are classified by size into point mutations (transitions, transversions, insertions, and deletions) and chromosomal mutations (inversions, duplications, large deletions, and translocations).

Recombinational variation occurs via three primary mechanisms: conjugation, transformation, and transduction. Conjugation is the most frequent method, requiring direct cell-to-cell contact. It involves a donor cell (F+) containing the F factor (fertility factor) and a recipient cell (F-). The F factor codes for sex fimbriae and other surface structures. If the F factor integrates into the chromosome, the cell becomes an HFR (high-frequency recombination) cell. During HFR conjugation, a copy of the chromosomal DNA is transferred to the recipient, with the amount transferred depending on the duration of contact, usually limited by the stability of the fimbria-receptor (OmpA protein) connection which typically breaks before the 2 hours required for full transfer.

Transformation involves the uptake of free DNA released by dying bacteria. This DNA can be chromosomal or plasmid-based. While foreign DNA is often digested by restriction endonucleases, it can be integrated under certain conditions, potentially spreading virulence factors. Stages include reversible binding to the cell wall, irreversible binding to the internal membrane, entry into the cytoplasm as a single strand, and integration with the chromosomal DNA. Transduction is the transfer of DNA from one cell to another via bacterial viruses known as bacteriophages.

Extranuclear DNA

Mitochondrial and chloroplast DNA (mtDNA and cpDNA) differ from nuclear DNA. They exhibit a high copy number; since cells contain many mitochondria (and chloroplasts in plants), and each organelle has multiple DNA copies, a single cell may contain thousands of copies of this extranuclear DNA.

Pathogenicity and Virulence

Pathogenicity is the ability of a bacterium to initiate disease, which involves transmission, survival in the host, infectivity (overcoming barriers), and virulence (damaging the host). Transmission routes include the respiratory tract, digestive tract, urogenital tract, insect bites, and tissue trauma. Pathogens can also be opportunistic members of the host's own microbiome.

Colonization involves establishing a stable bacterial population on the skin or mucous membranes, facilitated by adhesion (receptors on host membranes, bacterial ligands, hydrophobicity, and pili). Invasion is the penetration of host cells and tissues, driven by proteins called invasins. Some bacteria, like Clostridium tetani and Corynebacterium diphtheriae, do not penetrate tissues but produce potent toxins. Some pathogens enter cells through phagocytosis-like processes, eventually escaping vesicles to multiply in the cytoplasm.

Virulence factors (aggressins) are often strain-dependent and regulated by environmental factors like iron levels, oxygen, and temperature. These genes can be transferred via plasmids or bacteriophages. Bacteria produce toxins, typically proteins or enzymes, categorized into endotoxins (LPS or LOS found on Gram-negative outer membranes) and exotoxins (released into the environment). Exotoxins can damage host cells, destroy tissue structures, or degrade products (e.g., mucinases, phospholipases, collagenases, hyaluronidases).

Survival Mechanisms and Antibiotic Resistance

Bacteria survive by overcoming physical and physiological barriers via toxins, enzymes, and motility (e.g., spirochetes). They avoid phagocytosis using extracellular capsules or M protein. Other strategies include antigenic mimicry, hiding inside cells, and antigenic variation through mutation or horizontal gene transfer. They may produce proteases to degrade IgA antibodies, resist serum, or form biofilms—supporting consortia of microbes protected by polysaccharides.

Antibiotic resistance is classified as innate (natural) or acquired. Innate resistance is a stable species trait; for example, Mycoplasma lacks a cell wall and is thus resistant to beta-lactams. Gram-negative rods are naturally resistant to glycopeptides because the molecules are too large to penetrate the outer membrane. Acquired resistance results from mutations or acquiring new genetic material via conjugation, transformation, or transduction.

Mechanisms of resistance include:

1. Enzymatic inactivation (e.g., beta-lactamases like ESBL).
2. Changes in membrane permeability.
3. Efflux pumps that actively remove the drug from the cell.
4. Modification of the target site (e.g., masking the target).
5. Protecting the target site.
6. Overproduction of the target site.
7. Bypassing the inhibited metabolic process.
8. Expressing alternative proteins (e.g., the mecA gene in MRSA producing a new penicillin-binding protein with low affinity).

Clinical Application and Antibiotic Therapy

Bacterial infections are caused by biological agents or their toxins. Therapy is classified as empirical (based on likely etiology) or targeted (based on specific microbiological results). Clinical material should ideally be collected before the first dose of antibiotics. Therapy types include de-escalation (switching to narrow-spectrum after empirical start), sequential (switching from parenteral to oral), combined (using at least two drugs with different mechanisms), and rescue or "last resort" therapy.

In pregnancy, safe antibiotics often include Ampicillin, Amoxicillin, and certain Cephalosporins. However, several are strictly prohibited or restricted. Tetracyclines (and tygecykliny) can cause yellow teeth discoloration and interfere with bone/tooth development. Aminoglycosides can damage the fetal inner ear. Sulfonamides are contraindicated in the last two weeks of pregnancy due to risks of neonatal jaundice and encephalopathy. Nitrofurantoin is linked to neonatal hemolytic anemia and birth defects if used in the first trimester. Quinolones and metronidazole should also be avoided as they increase the risk of spontaneous abortion.

Disinfection and Sterilization

Hygiene procedures are categorized into washing, disinfection, and sterilization. Washing involves the mechanical removal of visible dirt and biological material, reducing microbe counts using chemicals, mechanical action, time, and temperature. Detergents can be neutral ($pH\,5-9$), acidic ($pH < 5$), or alkaline ($pH > 9$).

Disinfection aims to remove microorganisms, specifically destroying vegetative forms but not necessarily spores. Its effectiveness depends on concentration, time, environmental pH, and chemical composition. Agents include acids, bases, oxidants (H2O2, iodine, chlorine), alcohols (ethanol, propanol), and gases (ethylene oxide). Thermal disinfection involves processes like pre-rinsing, washing at $40-60^{\circ}C$, and disinfection at $80-93^{\circ}C$. To eliminate the HBV virus, $90^{\circ}C$ must be maintained for 5 minutes or $85^{\circ}C$ for 16 minutes.

Sterilization destroys all forms, including spores, resulting in a sterile product. Methods include Koch's apparatus ($3 \times 30-60\,min$ at $100^{\circ}C$), drying ovens ($120\,min$ at $160^{\circ}C$), and autoclaves. Typical autoclave settings are $121-123^{\circ}C$ for $15-20\,min$ ($2.1\,bar$) or $134^{\circ}C$ for $3-5\,min$ ($3.04\,bar$). For prions, a special program of $134^{\circ}C$ for $18\,min$ is used. Autoclave phases include air removal (fractionated vacuum), sterilization, and drying.

Quality control of sterilization uses process indicators (external) and internal indicators like the "helix" test (thermochemical/microbiological) to ensure parameters were met inside the package. In cosmetic settings, high standards of hand hygiene (EN 1500, EN 12791), surface disinfection, and tool sterilization are necessary to prevent the spread of infections such as fungi, herpes, HPV (warts), HBV, HCV, and HIV.