Microbial Growth, Genetics, Control, and Virology: Study Guide

Microbial Growth

Physical and Chemical Requirements for Microbial Growth

Microbial growth is influenced by a variety of physical and chemical factors that determine the ability of microorganisms to survive and reproduce in different environments.

• pH and Temperature: Microorganisms have optimal pH and temperature ranges for growth. Extreme conditions can inhibit or kill microbes.

• Carbon and Energy Sources: Microbes are categorized based on their carbon and energy requirements, such as autotrophs (use CO2 as carbon source) and heterotrophs (use organic compounds).

• Chemical Requirements: Essential elements include nitrogen, sulfur, phosphorus, trace elements, and organic growth factors.

Example: Escherichia coli grows best at neutral pH and moderate temperatures.

Biofilms

Biofilms are structured communities of microorganisms attached to surfaces and embedded in a self-produced extracellular matrix.

• Definition: Biofilms provide protection to microbes and facilitate nutrient exchange.

• Importance: Biofilms are significant in medical and industrial contexts due to their resistance to antimicrobial agents.

Example: Dental plaque is a common biofilm formed by oral bacteria.

Binary Fission and Population Growth

Microbial population growth occurs primarily through binary fission, a process where a single cell divides into two identical daughter cells.

• Binary Fission: The main method of reproduction in bacteria.

• Generation Time: The time required for a population to double in number.

Equation:

$N = N_0 \times 2^n$

Where $N$ is the final number of cells, $N_0$ is the initial number, and $n$ is the number of generations.

Estimating Microbial Growth

Several methods are used to estimate microbial growth, including direct and indirect techniques.

• Direct Methods: Plate counts, microscopic counts.

• Indirect Methods: Turbidity measurements, metabolic activity assays.

Microbial Genetics

Prokaryotes vs. Eukaryotes: Genome Structure and Organization

Prokaryotes and eukaryotes differ significantly in their genetic organization and genome structure.

• Prokaryotes: Typically have a single, circular chromosome located in the nucleoid region.

• Eukaryotes: Possess multiple, linear chromosomes within a membrane-bound nucleus.

DNA Replication, Transcription, and Translation

These are the fundamental processes by which genetic information is maintained and expressed in cells.

• Replication: Copying of DNA prior to cell division.

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of proteins from mRNA.

Gene Expression and Regulation

Gene expression involves the transcription and translation of genes, regulated by various mechanisms.

• Operons: In prokaryotes, genes are often organized into operons for coordinated regulation.

• Regulatory Proteins: Control transcription by binding to DNA sequences.

Mutations and Genetic Transfer

Mutations are changes in the DNA sequence, while genetic transfer allows exchange of genetic material between organisms.

• Types of Mutations: Point mutations, insertions, deletions.

• Genetic Transfer: Includes transformation, transduction, and conjugation.

Controlling Microbial Growth in the Environment

Physical and Chemical Control Methods

Microbial control is essential in healthcare, industry, and research to prevent contamination and infection.

• Physical Methods: Heat (autoclaving, pasteurization), filtration, radiation.

• Chemical Methods: Disinfectants, antiseptics, sterilants.

Selection and Application of Control Methods

Choosing appropriate control methods depends on the target microorganism, environment, and desired outcome.

• Factors: Type of microbe, level of contamination, material compatibility.

Levels of Hostility

Microbial control agents are classified by their effectiveness in killing or inhibiting microbes.

• High-level: Kill all microbes, including spores.

• Intermediate-level: Kill most microbes, not spores.

• Low-level: Kill some microbes, not spores or mycobacteria.

Controlling Microbial Growth in the Body: Antimicrobial Drugs

Mechanisms of Action of Antimicrobial Agents

Antimicrobial drugs target specific cellular structures or processes to inhibit or kill microorganisms.

• Cell Wall Inhibitors: Prevent synthesis of peptidoglycan (e.g., penicillins).

• Protein Synthesis Inhibitors: Target ribosomes (e.g., tetracyclines).

• Nucleic Acid Synthesis Inhibitors: Block DNA/RNA synthesis (e.g., quinolones).

Development of Resistance

Microorganisms can develop resistance to antimicrobial drugs through genetic mutations or acquisition of resistance genes.

• Mechanisms: Enzyme production (e.g., beta-lactamases), altered drug targets, efflux pumps.

Outcomes of Antimicrobial Therapy

Successful therapy eliminates infection, but misuse can lead to resistance and treatment failure.

• Example: Overuse of antibiotics can select for resistant strains of Staphylococcus aureus.

Viruses and Prions

Viruses vs. Prions

Viruses and prions are infectious agents with distinct characteristics and modes of replication.

• Viruses: Composed of nucleic acid (DNA or RNA) and protein coat; require host cells for replication.

• Prions: Infectious proteins lacking nucleic acids; cause neurodegenerative diseases.

Structure of Viruses

Viruses can be enveloped or non-enveloped, affecting their stability and infectivity.

• Enveloped Viruses: Surrounded by a lipid membrane derived from the host cell.

• Naked Viruses: Lack an envelope; generally more resistant to environmental stress.

Viral Replication

Viral replication involves distinct steps, varying between bacteriophages and animal viruses.

• Bacteriophage Replication: Includes lytic (host cell lysis) and lysogenic (integration into host genome) cycles.

• Animal Virus Replication: Entry, uncoating, replication, assembly, and release.

Viral Infections and Cancer

Certain viruses are associated with cancer development in humans.

• Oncogenic Viruses: Viruses such as Human papillomavirus (HPV) can cause cancer by disrupting normal cell regulation.

Table: Comparison of Viruses and Prions

Feature	Viruses	Prions
Genetic Material	DNA or RNA	None
Structure	Protein coat (capsid), sometimes envelope	Protein only
Replication	Requires host cell machinery	Converts normal proteins to prion form
Diseases	Wide range (e.g., influenza, HIV)	Neurodegenerative (e.g., Creutzfeldt-Jakob disease)