Key Concepts in Antimicrobial Agents and Photosynthesis

Chemotherapeutic Agents

Chemical agents used to treat disease by destroying pathogenic microbes or inhibiting their growth within a host. Most are antibiotics.

Selective Toxicity

The drug's ability to kill or inhibit a pathogen while minimizing damage to the host.

Therapeutic Index

The ratio of the toxic dose to the therapeutic dose. A higher index indicates greater selective toxicity and a safer drug.

Minimal Inhibitory Concentration (MIC)

The lowest drug concentration that inhibits pathogen growth.

Minimal Lethal Concentration (MLC)

The lowest drug concentration that kills the pathogen.

Kirby-Bauer Method (Disk Diffusion Test)

A standardized method where antibiotic-impregnated disks are placed on an inoculated agar plate. The diameter of the zone of inhibition indicates sensitivity or resistance.

E Test

It uses a strip with a gradient of antibiotic. The intersection of the elliptical zone of inhibition with the strip directly indicates the MIC.

Penicillins

They block transpeptidation (cross-link formation in peptidoglycan), leading to cell lysis. They act only on growing bacteria.

$eta$-lactam ring

The $eta$-lactam ring is crucial for penicillin's activity.

$eta$-lactamase

An enzyme produced by resistant organisms that hydrolyzes the $eta$-lactam ring, inactivating the antibiotic.

Glycopeptide antibiotics

They inhibit bacterial cell wall synthesis, and are often used for resistant staphylococcal and enterococcal infections.

Protein Synthesis Inhibitors

Bacterial ribosomes (specifically the 30S or 50S subunits).

Tetracyclines

They are broad-spectrum, bacteriostatic drugs that combine with the 30S ribosomal subunit, inhibiting aminoacyl-tRNA binding to the A site.

Macrolides

They bind to the 23S rRNA of the 50S ribosomal subunit, inhibiting peptide chain elongation.

Metabolic Antagonists

They act as antimetabolites, competitively inhibiting key enzymes in metabolic pathways, often by being structural analogs of natural metabolites (e.g., PABA for folic acid synthesis).

Quinolones

They are synthetic drugs that inhibit bacterial DNA-gyrase and topoisomerase II, which are essential for DNA replication.

Antifungal Drugs

Fungal cells are eukaryotic, similar to human cells. This similarity makes it difficult to find selectively toxic targets without harming the host.

Antifungal Drugs mechanisms

They can disrupt membrane permeability (e.g., Amphotericin B) or inhibit sterol synthesis (e.g., Fluconazole).

Antiviral Drugs

It is difficult to specifically target viral replication processes without affecting host cell functions, as viruses utilize host cellular machinery.

Anti-HIV Drugs

Reverse Transcriptase (RT) inhibitors block the enzyme that converts viral RNA to DNA. Protease inhibitors prevent viral maturation by blocking the protease enzyme.

Drug Resistance

Drug Resistance is increasing. It can be overcome by appropriate drug concentrations, combination therapy, and judicious drug use.

Drug Resistance Mechanisms

Bacteria can inactivate the drug (e.g., β-lactamase breaking down penicillin) or alter the drug's target (e.g., modifying the enzyme or organelle the drug normally binds to). Other mechanisms include drug exclusion or pumping the drug out.

Primary Metabolites

Primary metabolites are produced during the exponential growth phase (e.g., alcohol).

Secondary Metabolites

Secondary metabolites are produced during the stationary phase, are not essential for growth, and are often overproduced (e.g., penicillin).

Photosynthesis Parts

Light Reactions (light energy converted to ATP and NADPH) and Dark Reactions (Calvin Cycle) (ATP and NADPH used to reduce CO2 and synthesize sugars).

Photolithotrophy

In photolithotrophy, reducing power comes from inorganic substances (e.g., H2S).

Photoorganotrophy

In photoorganotrophy, reducing power comes from organic compounds (e.g., malate).

Oxygenic Photosynthesis

Performed by photosynthetic eukaryotes and cyanobacteria. Uses water (H2O) as the electron donor and produces oxygen (O2). Involves two photosystems (PS I and PS II) and chlorophylls.

Anoxygenic Photosynthesis

Performed by phototrophic green and purple bacteria. Does not use H2O, so O2 is not produced; uses H2S or organic compounds. Involves only one photosystem and bacteriochlorophylls.

Antennas in Photosynthesis

Highly organized arrays of chlorophylls and accessory pigments that capture light energy and transfer it to the reaction-center chlorophyll.

Bacteriorhodopsin-Based Phototrophy

Found in some archaea, it involves bacteriorhodopsin, a membrane protein that functions as a light-driven proton pump. A proton motive force is generated without an electron transport chain.

Microbial Products of Industrial Interest

Microbial Cells, Enzymes, Antibiotics, Steroids, Alkaloids, Food additives, Commodity chemicals (e.g., ethanol, citric acid). (Any three are acceptable)

Primary Metabolites Production Timing

During the exponential growth phase (log phase) of microbes.

Example of a Primary Metabolite

Alcohol (e.g., ethanol).

Characteristics of Secondary Metabolites

Not essential for growth; formation depends on specific growth conditions; produced as a group of related compounds; often significantly overproduced; often produced by spore-forming microbes during sporulation; large organic molecules requiring many specific enzymatic steps.

Example of a Secondary Metabolite

Penicillin.

Glucose levels and penicillin production

High glucose levels typically repress penicillin production, which is why its production begins after near-exhaustion of the carbon source.

Photosystems in oxygenic photosynthesis

Photosystem I (PS I) and Photosystem II (PS II).

Important pigments in oxygenic photosynthesis

Chlorophylls (e.g., chlorophyll a).

Initial event in light reactions of oxygenic photosynthesis

Light energy splits water molecules.

Energy-carrying molecules produced during light reactions

ATP (via photophosphorylation) and NADPH.

Electron donors in anoxygenic photosynthesis

Inorganic substances like H2S or organic compounds are used as electron donors. Oxygen (O2) is not produced.

Pigments used in anoxygenic photosynthetic bacteria

Bacteriochlorophylls.

Chlorosomes

Membrane vesicles where bacteriochlorophylls are located in some anoxygenic photosynthetic bacteria.

Function of Reaction-Center Chlorophyll

It's a special chlorophyll where captured light energy is transferred; it is directly involved in photosynthetic electron transport.

Outcome of electron flow in photosynthesis

Proton Motive Force (PMF).

Chloramphenicol usage

Used only in life-threatening situations because it is highly toxic, despite being a potent inhibitor of bacterial protein synthesis.

Prodrug

A pharmacologically inactive compound that is metabolized in the body to produce an active drug. An example is Valacyclovir, which is a prodrug of Acyclovir.

Fusion inhibitors

They prevent HIV from entering host cells by blocking the fusion of the viral envelope with the host cell membrane.

Role of autolysins in bacteria

Bacterial enzymes that degrade peptidoglycan. Some penicillins can activate these autolysins, contributing to cell lysis.

Modes of antimicrobial action

Inhibitors of Cell Wall Synthesis; Protein Synthesis Inhibitors; Metabolic Antagonists; Nucleic Acid Synthesis Inhibition.