Bacterial Taxonomy, Specialised Eubacteria, Archaea, and Microbial Control

This guide provides analysis of the classification, molecular diversity, physiology, pathogenicity, and control of prokaryotic organisms, covering bacterial taxonomy, key clinical and environmental bacterial groups, the biochemistry and ecology of Archaea, bacterial toxins, physical and chemical growth control, and the molecular mechanisms of antibiotics.

1. Bacterial Taxonomy and the Species Concept

1.1 Understanding Bacterial Taxonomy

Taxonomy is the science of classification, identification, and nomenclature. It forms the structural framework by which organisms are systematically organised into hierarchical categories based on mutual similarities, evolutionary relationships, or genetic characteristics:

• Classification: The orderly arrangement of organisms into groups (taxa) based on shared properties.
• Identification: The process of determining that a particular isolate belongs to a recognised taxon.
• Nomenclature: The assignment of scientific names to taxonomic groups according to international rules.

1.2 Taxonomic Hierarchy

The classification of prokaryotes follows a structured taxonomic hierarchy, moving from the most inclusive group to the individual species. The highest taxonomic rank is the Domain. All prokaryotic organisms are placed within two domains: Bacteria and Archaea.

The table below illustrates the taxonomic hierarchy using Legionella pneumophila (the causative agent of Legionnaires’ disease) as a representative example:

1.3 Defining “Strain” in Bacteriology

According to the first edition of Bergey’s Manual of Systematic Bacteriology, a strain is made up of the descendants of a single isolation in pure culture and is usually made up of a succession of cultures ultimately derived from an initial single colony. In bacteriology, the strain acts as the basic operational unit. Strains within the same species can be distinguished from other isolates of the same genus and species by specific phenotypic characteristics or genotypic characteristics, or both:

• Biovars (Biotypes): Strains characterised by biochemical or physiological differences.
• Morphovars (Morphotypes): Strains with distinct morphological features.
• Serovars (Serotypes): Strains distinguished by specific antigenic characteristics.
• Pathovars (Pathotypes): Strains characterised by their pathogenic virulence or host specificity.

1.4 The Species Concept: Eukaryotes vs. Bacteria

The basic taxonomic unit is the species. However, the definition of species differs fundamentally between eukaryotic and prokaryotic biology:

• Eukaryotic Species: Defined primarily via the biological species concept, stressing the ability of similar organisms to reproduce sexually with the formation of a diploid zygote and to produce fertile offspring.
• Bacterial Species: Because bacteria do not undergo sexual reproduction (reproducing instead by asexual binary fission) and are subject to frequent horizontal gene transfer, the biological species concept cannot be applied. Instead, other phenotypic, biochemical, and genotypic criteria must be used. Around 4,000 bacterial species have been described in historical literature, a number that has grown exponentially with molecular genetics.

1.5 Taxonomic Classification Systems

Historically, taxonomy relied heavily on the phenetic system, which is based on measuring overall similarity and based on all available characters without any weighting. Because of the lack of a fossil record in bacteria and the limitations of sexual isolation, classification based on the phenetic system has always been considered limited.

Modern taxonomy relies on phylogenetic classification, which arranges organisms based on evolutionary relationships. Comparing genetic materials and gene products has successfully overcome the historical lack of a bacterial fossil record, allowing the total DNA of one bacterium to be compared with that of any other bacterium.

1.6 Five Parameters for Determining DNA Relatedness

Five major molecular factors are utilised to determine DNA relatedness and genetic similarity among prokaryotes:

• Genome Size: Comparing the overall size of the genome in base pairs or mass.
• Guanine-plus-Cytosine (G+C) Content: The G+C content in bacterial DNA ranges from about 25 to 75 percent. This percentage is specific, but not exclusive, for a species; two strains with similar G+C content may or may not belong to the same species. However, if the G+C contents are very different, the strains cannot be members of the same species.
• DNA-DNA Hybridisation: Determined by allowing single-stranded DNA from one strain to reassociate with complementary single-stranded DNA from a second strain to form a double-stranded DNA molecule. This is a specific, temperature-dependent reaction.
• Thermal Stability of Related DNA Sequences ($\Delta T_m$): Measuring the temperature at which hybridised DNA strands denature. A lower thermal stability indicates greater mismatching between the hybridised strands.
• DNA Relatedness under Supraoptimal Conditions: Testing hybridisation kinetics under stringent conditions (such as elevated temperatures) to filter out weak, non-specific cross-hybridisation.

1.7 Molecular Standards for Species Definition

To establish a clear phylogenetic definition of a bacterial species (often referred to as a genospecies or genomic species), bacteriologists have defined standard thresholds:

• DNA-DNA Hybridisation: Two strains of the same species must have a similar mole percent G+C content and must exhibit 70% or greater DNA-DNA hybridisation.
• Thermal Stability ($\Delta T_m$): There must be 5°C or less $\Delta T_m$ (the difference in melting temperature between homoduplex and heteroduplex DNA molecules). Both values must be considered together.
• 16S rRNA Similarity: According to Stackebrandt and Goebel (1994), the genetic definition of 70% relatedness with 5% or less divergence correlates with 16S rRNA sequence similarity. A 16S rRNA sequence similarity of less than 97% between strains indicates that they represent different species, but at 97% or higher sequence similarity, DNA-DNA hybridisation must be used to determine whether strains belong to different species.

2. General Features of Key Bacterial Groups

2.1 Rickettsias and Chlamydiae (Obligate Intracellular Pathogens)

Both rickettsias and chlamydiae are Gram-negative, non-motile, obligate intracellular parasites that must grow and reproduce within host cells. However, they differ in their vectors, clinical manifestations, and metabolic capabilities.

Rickettsias

• Named after their discoverer, the American pathologist Harold Taylor Ricketts, who died of typhus while investigating the disease.
• They live in vertebrate hosts and are transmitted by blood-sucking arthropods such as fleas, ticks, lice, and mites.
• They are causative agents of several severe human diseases:
  • Typhus fever: Caused by Rickettsia typhi (endemic typhus) and Rickettsia prowazekii (epidemic typhus).
  • Rocky Mountain spotted fever: Caused by Rickettsia rickettsii.
  • Q fever: Caused by Coxiella burnetii (which is highly resistant to heat and desiccation).

Chlamydiae

• Classified within the Phylum Chlamydiae, which comprises one class, one order, four families, and six genera, with the genus Chlamydia being the best studied.
• Originally thought to be viruses because of their dependence on the host cell to supply them with ATP and other intermediates (making them “energy parasites”). However, they possess a true cell wall, contain DNA, RNA, and ribosomes, and are thus classified as true bacteria.
• Reproduce within the cytoplasmic vacuoles of host cells via a unique biphasic life cycle involving two distinct morphological forms:
  • Elementary Bodies (EBs): Small, rigid, infectious particles adapted for extracellular survival.
  • Reticulate Bodies (RBs): Larger, fragile, non-infectious reproductive forms adapted for active intracellular metabolism.

Furthermore, the biological differences between typical bacteria, obligate intracellular bacteria (Rickettsias/Chlamydiae), and viruses are summarized below:

2.2 Actinomycetes

Actinomycetes are aerobic, Gram-positive, mold-like bacteria that form highly branched, septate hyphae resembling fungal mycelia and reproduce via asexual spores:

• Spores: These thin-walled asexual spores are called conidiospores or conidia (located at the tip of the hyphae) and sporangiospores (located inside a specialized sac called a sporangium).
• Taxonomy & Motility: Classification is primarily based on properties like conidia arrangement, the presence or absence of a sporangium, and cell wall type. Most actinomycetes are non-motile; when motility is present, it is confined to flagellated spores only.
• Genus Streptomyces: This is the largest genus of actinomycetes. Members of this genus are strict aerobes and mostly non-pathogenic saprophytes that bear chains of non-motile conidia. Their natural habitat is the soil; the characteristic earthy odor of moist soil is largely due to their production of volatile organic substances such as geosmin.
• Antibiotic Powerhouses: Streptomyces are world-renowned for synthesizing a vast array of clinically invaluable antibiotics, including:
  • Amphotericin B and Nystatin (antifungals)
  • Chloramphenicol, Erythromycin, Neomycin, Streptomycin, Tetracycline, and Gentamicin (antibacterials)

2.3 Spirochetes

Spirochetes are a phylum of Gram-negative, chemoheterotrophic, slender, flexible, helical bacteria. They exhibit unique creeping or crawling movements and lack external rotating flagella:

• The Axial Filament: Spirochete motility is driven by an unusual morphological structure called the axial filament (comprising axial fibrils or periplasmic flagella).
• Structural Layout: The spirochete body consists of an inner protoplasmic cylinder containing the cytoplasm and nucleoid, bounded by a plasma membrane and a Gram-negative cell wall. Two or more (up to a hundred) periplasmic flagella extend from both ends of the cylinder, often overlapping in the middle. The entire complex of periplasmic flagella lies inside a flexible outer membrane.
• Pathogenic Species: Representative spirochetes include Treponema pallidum (the causative agent of syphilis) and Borrelia burgdorferi (the causative agent of Lyme disease, which possesses 7 to 11 flagella attached near each end of the protoplasmic cylinder).

                            Spirochete Cross-Section

         ┌──────────────────────── Outer Membrane ────────────────────────┐
         │   ┌────────────────────── Cell Wall ──────────────────────┐    │
         │   │                                                       │    │
         │   │   ═══════════════ Periplasmic ═══════════════         │    │
         │   │                  Flagella (Axial Fibrils)             │    │
         │   │                                                       │    │
         │   │   ┌────────────── Protoplasmic Cylinder ──────────┐   │    │
         │   │   │                                               │   │    │
         │   │   │              Cytoplasm & Nucleoid             │   │    │
         │   │   │                                               │   │    │
         │   │   └───────────────────────────────────────────────┘   │    │
         │   └───────────────────────────────────────────────────────┘    │
         └────────────────────────────────────────────────────────────────┘
  

2.4 Mycoplasmas

Mycoplasmas are the smallest and simplest self-reproducing Gram-negative bacteria:

• Lack of Cell Wall: They completely lack cell walls and are thus placed in a separate class called Molllicutes (from the Latin mollis meaning “soft”, and cutis meaning “skin”).
• Historical Name: Formerly referred to as Pleuropneumonia-like organisms (PPLO) because the first strains were isolated from cattle suffering from pleuropneumonia.
• Morphology & Cultivation: Because they lack a rigid wall, they are highly pleomorphic (varying in shape) and mostly non-motile. They have a general chemoorganoheterotrophic metabolism and require cholesterol in their growth media. When grown on agar, they form highly characteristic “fried-egg” colonies with a dense, dark central region penetrating the agar, surrounded by a lighter, circular peripheral zone.

2.5 Cyanobacteria (Blue-Green Algae)

Cyanobacteria are ubiquitous Gram-negative, oxygenic photosynthetic, and obligate photolithoautotrophic bacteria:

• Photosynthetic Apparatus: They perform oxygenic photosynthesis using Chlorophyll a (and in very few cases, Chlorophyll b and Chlorophyll d), carotenoids, and accessory water-soluble pigments called phycobilins (including phycocyanin which provides a blue-green colour, and phycoerythrin which provides a reddish colour).
• Thylakoid Membranes: Photosynthesis and respiration are localized on an internal system of thylakoid membranes. An exception is the unique genus Gloeobacter violaceus, which lacks thylakoids entirely; its light reactions occur directly on the plasma membrane.
• Inclusions: The cyanobacterial cytosol contains specialized inclusions such as carboxysomes (concentrating RuBisCO), glycogen granules, cyanophycin granules (nitrogen storage), polyphosphate bodies, lipid bodies, and polyhydroxybutyrate granules.
• Reproduction & Spores: They reproduce via binary fission, budding, or multiple fission. In filamentous forms (termed trichomes), fragmentation of filaments occurs to form motile hormogonia. During unfavourable environmental periods, they form thick-walled resting spores called akinetes.
• Incomplete TCA Cycle: Cyanobacteria lack a fully functional citric acid cycle because they lack the enzyme $\alpha$-ketoglutarate dehydrogenase.
• Nitrogen Fixation & Heterocysts: Many nitrogen-fixing cyanobacteria possess specialised, thick-walled, non-photosynthetic cells called heterocysts. Heterocysts lack Photosystem II (preventing oxygen generation) and express the oxygen-sensitive enzyme nitrogenase to reduce atmospheric nitrogen ($N_2$) into ammonia ($NH_3$) in an anaerobic microenvironment.

3. Archaebacteria (Archaea) Biochemistry and Diversity

Members of the Domain Archaea are distinguished from Eubacteria by several ancient biochemical features, most notably their unique oligonucleotide signature sequences in 16S rRNA. While superficially similar to bacteria in size and morphology (lacking membrane-bound nuclei, having 70S ribosomes, and possessing circular chromosomes), their molecular machinery for DNA replication, transcription, and translation is closely aligned with Eukaryotes.

3.1 Archaeal Cell Membrane Chemistry

The archaeal plasma membrane differs fundamentally from both bacterial and eukaryotic membranes in its lipid chemistry. These differences are critical adaptations that allow Archaea to survive in extreme habitats:

• Glycerol Stereochemistry: Archaea use glycerol-1-phosphate (G1P) as the backbone for membrane lipids. In contrast, bacteria and eukaryotes use glycerol-3-phosphate (G3P). These two molecules are enantiomers, synthesized by entirely different enzymes.
• Ether Linkages: In Archaea, the hydrophobic hydrocarbon side chains are connected to the G1P backbone via ether bonds. In bacteria and eukaryotes, fatty acids are attached to G3P via ester bonds. Ether bonds are chemically much more stable and resistant to thermal and chemical degradation.
• Branched Isoprenoids: Instead of straight-chained fatty acids, Archaea utilize branched isoprenoid hydrocarbon chains derived from 5-carbon isoprene units.
• Diethers vs. Tetraethers (Monolayers):
  • Glycerol Diethers: Composed of two phytanyl chains (20-carbon branched hydrocarbons) attached to a glycerol backbone. This forms a lipid bilayer.
  • Diglycerol Tetraethers: Composed of two long biphytanyl chains (40-carbon branched hydrocarbons) spanning the entire width of the membrane, linked to glycerol at both ends. This forms a lipid monolayer.
• Monolayer Stability: Because the hydrophobic tails are covalently fused, lipid monolayers do not peel apart at high temperatures, making them highly prevalent in hyperthermophilic archaea that grow above 80°C.

            Archaeal Glycerol Diether (Phytanyl Bilayer Unit)

               H2C ─ O ─ [ Phytanyl Chain (20C Branched) ]
                │
             H ─ C ─ O ─ [ Phytanyl Chain (20C Branched) ]
                │
               H2C ─ O ─ Phosphate

            Archaeal Diglycerol Tetraether (Biphytanyl Monolayer Unit)

               H2C ─ O ─ [ Biphytanyl Chain (40C Branched) ] ─ O ─ CH2
                │                                                   │
             H ─ C ─ O ─ [ Biphytanyl Chain (40C Branched) ] ─ O ─ C ─ H
                │                                                   │
               H2C ─ O ─ Phosphate                     Phosphate ─ O ─ CH2
  

3.2 Archaeal Cell Wall Diversity

Archaeal cell walls display wide chemical diversity and never contain true peptidoglycan:

• Wall Formats: Cell walls can be composed of complex proteins, glycoproteins, or polysaccharides.
• The S-Layer: The most common cell wall type is a proteinaceous S-layer (surface-layer) composed of interlocking protein or glycoprotein subunits. This S-layer can be the outermost boundary, or separated from the plasma membrane by a pseudomurein layer.
• Pseudomurein (Pseudopeptidoglycan): Found in some methanogens. It is a heteropolysaccharide consisting of alternating $N$-acetylglucosamine (NAG) and $N$-acetyltalosaminuronic acid (NAT) (replacing the NAM found in bacteria).
• Glycosidic Linkages: The NAG-NAT sugars are linked by $\beta$-1,3-glycosidic bonds (instead of the $\beta$-1,4-glycosidic bonds of bacterial murein).
• Peptide Cross-links: Pseudomurein peptide stems contain only L-amino acids (lacking the D-amino acids found in bacterial peptidoglycan).
• Antibiotic Resistance: Because of these structural modifications, pseudomurein is completely immune to cleavage by lysozyme and is unaffected by transpeptidase-inhibiting antibiotics like penicillin.

3.3 Comparative Analysis: Bacteria vs. Archaea

The biological, genetic, and metabolic differences between the two prokaryotic domains are detailed in the comparative matrix below:

3.4 Major Physiological Groups of Archaea

Archaea occupy some of the most extreme environments on Earth and are classified into three major physiological categories:

1. Methanogens

• Metabolism: Strict (obligate) anaerobes that produce methane ($CH_4$) as an essential end-product of their energy metabolism by combining hydrogen ($H_2$) with carbon dioxide ($CO_2$) or simple organic compounds like acetate.
• Taxonomy: Comprise the largest group of Archaea. No bacterial methanogens exist.

2. Extreme Halophiles

• Habitat: Highly saline environments (salt lakes, solar salterns) requiring an absolute minimum salinity of 1.5 M NaCl for growth, and thriving up to saturation (5.5 M NaCl).
• Metabolism: Aerobic chemoorganoheterotrophs.
• Model Organism: Halobacterium salinarum. This organism performs a unique non-chlorophyll-based photosynthesis using bacteriorhodopsin (now termed archaeorhodopsin), a membrane protein bound to retinal that acts as a light-driven proton pump to generate ATP without an electron transport chain.

3. Thermophiles and Hyperthermophiles

• Habitat: Geothermally heated areas, submarine hydrothermal vents, and hot springs.
• Temperature Profile: Thriving at optimum growth temperatures between 70°C and 110°C.
• Metabolism: Mostly Gram-negative, strict anaerobes that utilize sulphur compounds as electron acceptors (sulphate/sulfur metabolizers). They can be autotrophic or heterotrophic.

4. Bacterial Toxins: Exotoxins vs. Endotoxins

Bacterial toxins are specific chemical substances, often metabolic products, that damage host cells and tissues. Toxins can induce disease even in the absolute absence of the producing viable bacterial cells (a clinical state known as intoxication). The presence of toxins in the host bloodstream is referred to as toxemia. Bacterial toxins are divided into two primary categories: exotoxins and endotoxins.

4.1 Comparative Features of Exotoxins and Endotoxins

4.2 Major Structural and Functional Classes of Exotoxins

Exotoxins can be divided into different categories based on their structure and physiological activities. They are categorized based on the site affected: neurotoxins (acting on nervous tissue), enterotoxins (acting on the intestinal mucosa), and cytotoxins (acting on general tissues). Based on their molecular mode of action on target cells, exotoxins are divided into three functional categories:

1. Toxins That Act from the Cell Surface (Superantigens)

These toxins bind directly to receptors on the host cell surface and stimulate intracellular signaling pathways without entering the cytoplasm.

• Superantigens: These proteins bridge the class II MHC protein on antigen-presenting cells directly with the T-cell receptor (TCR) on T helper cells. They bypass normal antigen-processing pathways.
• Mechanism: Consequently, up to 30% of all T cells are non-specifically activated, leading to a massive, uncontrolled systemic secretion of cytokines (including IL-1, IL-2, and TNF-$\alpha$). This cytokine storm produces the clinical symptoms of toxic shock.
• Examples: Toxic shock syndrome toxin-1 (TSST-1) produced by Staphylococcus aureus and pyrogenic exotoxins from Streptococcus pyogenes.

2. Membrane-Damaging Toxins

These toxins disrupt the structural integrity of the host plasma membrane, causing cell lysis and death. They are subcategorized into:

• Channel-Forming Toxins: These insert themselves into the host cell membrane and polymerize to form an open channel (pore) through which cellular ions leak. An example is the alpha-toxin of Staphylococcus aureus.
• Enzymatically Membrane-Damaging Toxins: These digest membrane lipids. An example is the alpha-toxin of Clostridium perfringens, which possesses strong lecithinase (phospholipase) activity that enzymatically cleaves the charged polar head groups from membrane phospholipids, destabilizing the bilayer and causing gas gangrene.

3. AB Toxins

AB toxins are intracellularly acting toxins consisting of two distinct functional parts:

• B-Subunit (Binding): Attaches to specific receptor regions on host cell membranes and mediates the entry of the active subunit.
• A-Subunit (Active): Enters the host cytoplasm and possesses the specific enzymatic activity that alters host cell biochemistry.
• Subunit Properties: Isolated A-subunits are enzymatically active in cell-free systems but cannot bind or enter intact cells (rendering them non-toxic in vivo). Isolated B-subunits are completely non-toxic but can bind to cells and block the entry of intact wild-type toxins.
• Examples: Cholera toxin, pertussis toxin, Shiga toxin, tetanus toxin, botulinum toxin, anthrax toxin, and heat-labile enterotoxin from E. coli.

4.3 Key Bacterial Exotoxins and Molecular Modes of Action

5. Control of Microbial Growth: Physical and Chemical Methods

Control of microbial growth is defined as the inhibition or prevention of the growth of microorganisms. This control is affected in two basic ways:

• By killing microorganisms (mediated by cidal agents, which cause irreversible cell death).
• By inhibiting the growth of microorganisms (mediated by static agents, which arrest metabolic replication without killing; removing the agent allows growth to resume). The term bactericidal refers to killing bacteria, while bacteriostatic refers to inhibiting the growth of bacterial cells.

5.1 Definitions of Control Terms and Processes

• Disinfectants: Chemical agents that kill or eliminate microorganisms, used exclusively on inanimate objects. Disinfectants are usually too harsh and toxic for living tissues. Examples include hypochlorites, chlorine compounds, copper sulfate, formaldehyde (which reacts with $NH_2$, $SH$, and $COOH$ groups of proteins), phenolic compounds (which denature proteins and disrupt cell membranes), and mercuric chloride (which reacts with sulfide groups of enzymes).
• Antiseptics: Chemical agents that kill or inhibit the growth of microorganisms, formulated to be non-toxic enough to be used on living tissues. Examples include alcohols (ethanol denatures proteins and solubilises lipids), silver nitrate (precipitates bacterial proteins), iodine solutions (inactivates essential proteins), and mild detergents (disrupts plasma membranes).
• Sterilization: The process of total destruction or elimination of all viable organisms, including highly resistant bacterial endospores, from a substance or environment. Sterilization procedures involve the use of heat, radiation, chemicals, or physical removal of cells. Common processes include direct flaming, autoclaving (steam under pressure), tyndallization (fractional steam sterilization), and microfiltration.

5.2 Fractional Steam Sterilization: Tyndallization

Tyndallization is a specialized physical sterilization process used to sterilise culture media or heat-sensitive fluids that might be spoiled or denatured by exposure to the extremely high temperatures of an autoclave:

• Methodology: The material is steamed at atmospheric pressure (approximately 100°C) for 30 to 45 minutes each time for 3 consecutive days.
• Day 1: The initial steam exposure kills all active vegetative cells. However, highly resistant bacterial endospores survive. The medium is then incubated overnight at an optimal temperature, which triggers the surviving endospores to germinate into vulnerable vegetative cells.
• Day 2: The second steaming kills these newly germinated vegetative cells before they have time to sporulate again.
• Day 3: The final steaming session ensures complete sterilization by killing any remaining late-germinating cells, producing a completely sterile medium.

5.3 Pasteurisation: Standard vs. Flash Methods

Pasteurisation is a brief, controlled heat treatment used to reduce the number of spoilage organisms and to kill specific pathogens in milk, wine, beer, and other heat-sensitive liquids. Unlike sterilization, pasteurisation does not kill all microorganisms and is therefore not synonymous with sterilization:

• Batch Method (Low-Temperature Holding, LTH): The liquid is heated to 63°C for 30 minutes.
• Flash Method (High-Temperature Short-Time, HTST): The liquid is heated to 71°C for 15 seconds, followed by rapid cooling. This method is highly preferred in modern dairy processing as it preserves taste, color, and nutritional value while extending milk’s usable shelf life.

5.4 Physical and Chemical Control Agents

The physical and chemical methods used to control microbial growth are categorized by their biophysical action below:

Physical Agents

• Heat: The most rapid and efficient physical agent normally used to control microbial growth.
  • Moist Heat: Generally coagulates and denatures essential cellular proteins and enzymes. It is highly effective (e.g. boiling, autoclaving).
  • Dry Heat: Kills primarily by the oxidation of large intracellular molecules. It requires much higher temperatures and longer exposure times than moist heat.
• Radiation:
  • Ionizing Radiation (X-rays, Gamma rays): Highly energetic; collides with water molecules to form highly reactive hydroxyl free radicals ($OH^\bullet$) in the cytoplasm. These free radicals chemically damage proteins, enzymes, and the sugar-phosphate backbone of nucleic acids.
  • Non-Ionizing Radiation (Ultraviolet light): Possesses lower energy; specifically absorbed by purines and pyrimidines, leading to the formation of lethal pyrimidine dimers (thymine dimers) in DNA, which halt replication and transcription.
• Filtration: Involves the physical removal of all cells in a liquid or gas. It is especially important for the sterilization of solutions which would be denatured by heat (e.g., antibiotic solutions, amino acids, vitamins). Liquid is passed through a membrane filter with a sufficiently small pore diameter—generally 0.22 micron to remove the smallest known bacterial cells. Cellulose-ester membrane filters (made of cellulose acetate, cellulose nitrate, collodion, etc.) can range in pore size up to 0.75 micron.

Chemical Agents

• Alcohols: Denature proteins and solubilise lipids.
• Aldehydes (Formaldehyde, Glutaraldehyde): Highly reactive alkylating agents that cross-link proteins.
• Halogens (Chlorine, Iodine): Act as strong oxidizing agents that react with and oxidize amino acids in bacterial proteins, changing their structure and function.
• Heavy Metals (Mercury, Silver, Copper): Act via oligodynamic action, binding to sulfhydryl groups and precipitating essential enzymes.
• Gases (Ethylene oxide, Beta-propiolacton): Strong alkylating gases used to sterilize pre-packaged medical plastics and instruments.

6. Antibiotics and Chemotherapeutic Agents

6.1 Defining Antibiotics

Antibiotics are low-molecular-weight antimicrobial agents produced as secondary metabolites by specific microorganisms that kill or inhibit the growth of other microorganisms. The term was originally coined by Selman Waksman and historically described only those formulations derived from living organisms. Today, the term is broader:

• Natural Antibiotics: Extracted directly from microbial cultures (fungal or bacterial).
• Semi-synthetic Antibiotics: Natural antibiotics modified by organic chemists to enhance their antimicrobial properties, broaden their spectrum, or render them unique for a pharmaceutical patent.
• Spectrum of Action:
  • Broad-spectrum: Effective against a wide range of both Gram-positive and Gram-negative bacteria (e.g., tetracycline).
  • Narrow-spectrum: Effective against only a restricted group of bacteria (e.g., penicillin G, which is primarily effective against Gram-positive organisms).
  • Limited-spectrum: Highly restricted to a single specific pathogen or group (e.g., streptomycin).

6.2 Microbial Sources of Common Antibiotics

6.3 Molecular Targets and Mechanisms of Action

Antibiotics inhibit microbial populations by five major methods:

1. Disrupting cell wall synthesis.
2. Interfering with cell membrane function and permeability.
3. Inhibiting protein synthesis (translation) or interfering with its completion.
4. Disrupting the replication or transcription of nucleic acids.
5. Interrupting selected metabolic pathways (metabolic antagonism).

The table below catalogs the exact mechanisms of action and molecular targets of key antibiotics:

6.4 Molecular Profiles of Key Antibiotic Classes

1. Sulfonamides (Sulfa Drugs)

• Structural Mimicry: Sulfonamides are structurally related to sulfanilamide, which is a chemical analogue of p-aminobenzoic acid (PABA).
• Mechanism: In bacteria, PABA is a critical starting material used in the enzymatic synthesis of the coenzyme folic acid. When sulfonamides enter a bacterial cell, they competitively bind to the active site of the enzyme dihydropteroate synthase (DHPS).
• Impact: This competitive inhibition leads to a severe decrease in intracellular folate concentrations. Because folic acid is an absolute requirement for the synthesis of purine and pyrimidine nucleotides, the bacterium is unable to replicate its DNA, arresting cell growth (bacteriostatic effect). Humans are unaffected because we lack this pathway and must import pre-formed dietary folic acid.

2. Penicillins

• Discovery: Penicillin was first discovered by Alexander Fleming in 1928 as a natural metabolic by-product of the filamentous fungus Penicillium notatum.
• Chemical Structure: All penicillins share a common bicyclic core nucleus composed of a $\beta$-lactam ring fused to a five-membered sulfur-containing thiazolidine ring. The generic structural nucleus of penicillin is illustrated below:

                                Penicillin Nucleus

                                  O       H   S   CH3
                                  ║       │ ┌─┴─┐
                              R ─ C ─ NH ─ CH─CH │   CH3
                                          │ │   │
                                          O=C─N─┼─COOH
                                                H
                                           └─┬┘
                                      Beta-Lactam Ring
  

• Mechanism of Action: The $\beta$-lactam ring acts as a structural mimic of the terminal D-alanyl-D-alanine peptide stem of peptidoglycan precursors. Penicillin binds irreversibly to the active site of DD-transpeptidase enzymes (also known as Penicillin-Binding Proteins, PBPs). This completely blocks the transpeptidation cross-linking reaction of adjacent glycan backbones during cell wall synthesis.
• Lysis: As the bacterium continues to grow and express autolysins to remodel its wall, the newly synthesized peptidoglycan lacks peptide cross-links. Under normal turgor pressure, the weakened cell wall ruptures, inducing osmotic lysis of the bacterium.

3. Cephalosporins

• Source: Cephalosporins were originally isolated from the marine fungus Cephalosporium.
• Structure: Like penicillins, they possess a core $\beta$-lactam ring, but it is fused to a six-membered dihydrothiazine ring instead of a five-membered thiazolidine ring.
• Efficacy: Their molecular mechanism is identical to penicillin (blocking transpeptidation), but they are highly effective against a broader range of Gram-positive bacteria and exhibit greater resistance to specific bacterial $\beta$-lactamases.

4. Tetracyclines

• Structure: Composed of a common hydronaphthacene skeleton containing a linear, fused four-ring structure to which various functional side chains are attached.
• Efficacy: Tetracyclines are highly effective broad-spectrum bacteriostatic antibiotics. They are active against Gram-positive bacteria, Gram-negative bacteria, and specialized wall-deficient or intracellular pathogens such as rickettsias, chlamydiae, and mycoplasmas. They act by binding to the 30S ribosomal subunit, preventing the attachment of incoming aminoacyl-tRNAs.

5. Aminoglycosides

• Structure: A family of bactericidal antibiotics characterized by amino sugar molecules connected via glycosidic bonds to a central aminocyclitol ring.
• Members: Includes gentamicin, kanamycin, tobramycin, and streptomycin.
• Spectrum: They are most active against Gram-negative pathogens and work by binding irreversibly to 16S rRNA on the 30S subunit, forcing the ribosome to misread codons, resulting in defective, toxic proteins.

6. Macrolides (Erythromycin)

• Structure & Source: Erythromycin is a macrolide antibiotic synthesized by the soil actinomycete Streptomyces erythraeus. It contains a large, multi-membered (12- to 22-carbon) lactone ring linked to one or more sugars.
• Mechanism: It is highly useful against Gram-positive bacteria, acting as a bacteriostatic agent that binds to the 23S rRNA of the 50S subunit to block ribosomal translocation during protein synthesis.

7. Chloramphenicol

• Source: Originally produced from cultures of Streptomyces venezuelae, but because of its simple structure, it is now manufactured entirely through chemical synthesis.
• Spectrum: It is a broad-spectrum bacteriostatic antibiotic that binds to the 50S subunit, inhibiting peptidyltransferase activity, thereby halting translation.