Microbiology Lab Practical 2 Study Guide: Antibiotic Resistance, Biofilms, DNA Damage & Repair, and Genetic Exchange

Antibiotic Resistance and Susceptibility Testing

Antibiotic Resistance

Antibiotic resistance occurs when bacteria evolve mechanisms to withstand the effects of antibiotics, often due to misuse or overuse. This can lead to ineffective treatments and persistent infections.

• Bacteriostatic: Inhibits bacterial growth without killing the organisms.

• Bactericidal: Kills bacteria directly.

Minimum Inhibitory and Bactericidal Concentrations

• MIC (Minimum Inhibitory Concentration): The lowest concentration of an antibiotic that inhibits visible growth of a microorganism.

• MBC (Minimum Bactericidal Concentration): The lowest concentration of an antibiotic that kills 99.9% of the bacteria.

Relationship:

• If MBC is close to MIC, the agent is bactericidal.

• If MBC is much greater than MIC, the agent is bacteriostatic.

Example: Antibiotic Dilution Test

Serial dilutions of antibiotics are used to determine MIC and MBC by observing bacterial growth in tubes with increasing concentrations.

Kirby-Bauer Disk Diffusion Method

Principle

This method tests bacterial sensitivity to antibiotics by measuring the zone of inhibition around antibiotic disks on an agar plate.

• Requires standardization (Mueller-Hinton agar and McFarland standard).

• Cannot distinguish between bacteriostatic and bactericidal effects.

Standardization Factors

• Sensitivity of the organism

• Antimicrobial molecule size and concentration

• Density of culture

• Diffusion rate of agent

• Incubation time and temperature

• Size of inoculum

• Interactions

Biofilm Formation

Definition and Importance

Biofilms are structured communities of microorganisms encapsulated within a self-produced matrix of polysaccharides, proteins, lipids, and DNA. They are common in environments with excess nutrients and can contribute to persistent infections.

• Planktonic cells: Free-floating bacteria.

• Sessile cells: Bacteria attached to a surface.

Stages of Biofilm Development

1. Surface attachment: Planktonic cells attach to a surface and may detach or stay.

2. EPS matrix production: Cells produce extracellular polymeric substances (EPS) to anchor themselves.

3. Maturation: Biofilm grows and develops complex structures.

4. Dispersion: Cells leave the biofilm to colonize new surfaces.

Disinfectants and Antiseptics

Definitions

• Disinfectants: Used on inanimate objects to destroy bacteria by damaging cell membranes and interfering with metabolism.

• Antiseptics: Used on living tissue; must be less toxic and are often broad-spectrum.

DNA Damage and Repair in Bacteria

Chemical-Induced DNA Damage

• Deaminating agents: Convert cytosine to uracil, leading to mutations.

• Cytosine deamination: Removal of an amine group from cytosine, resulting in uracil incorporation into DNA.

Bacterial DNA Repair Mechanisms

• Base Excision Repair (BER): Repairs mutations before DNA replication to prevent further damage.

UV Radiation and DNA Damage

UV Radiation

UV radiation is a physical method for killing microbes by inducing DNA damage. Wavelengths below 400 nm can cause mutations, with optimal germicidal effects at 260–265 nm.

Mechanisms of DNA Damage

• Thymine dimer formation: Covalent bonds form between adjacent thymine bases, blocking DNA polymerase and halting replication.

• Thymine 6-4 photoproduct: A dimer forms between carbons 6 and 4 of adjacent thymines, distorting the DNA backbone.

Bacterial Protection Mechanisms

• Formation of endospores

• DNA repair mechanisms

Endospores

Definition and Formation

Endospores are highly resistant structures formed by gram-positive bacteria such as Bacillus and Clostridium species. They allow survival in extreme conditions.

• Formed under nutrient deprivation or environmental stress.

• Resistant to heat, chemicals, and UV radiation.

• Can revert to vegetative cells when conditions improve.

Endospore Formation Cycle

Includes stages such as asymmetric division, engulfment, cortex and coat formation, and maturation.

Types of Endospores

Endospore Shape	Cell Deformation	Endospore Position
Spherical	None	Central, Subterminal, Terminal
Oval	Swollen	Central, Subterminal, Terminal
Additional info: Some species have unique positions and shapes for identification.

Endospore Structure

Component	Function
Spore Coat	High protein content; enzymatic and chemical resistance
Cortex	Peptidoglycan layer; high-temperature resistance
Outer and Inner Membranes	Permeability barriers
Core	Contains DNA, ribosomes, and dipicolinic acid for resistance
Small acid-soluble proteins (SASP)	Bind DNA and protect against UV damage

DNA Damage Repair Mechanisms

Photoactivation

• Uses visible light to activate photolyase enzyme, breaking thymine dimers.

• Allows DNA replication to continue after repair.

Nucleotide Excision Repair (NER)

• Uses ATP to remove damaged DNA segments.

• DNA polymerase fills in the gap using the undamaged strand as a template.

Repair Mechanism	Description
Photoactivation	Light-dependent repair of thymine dimers
Nucleotide Excision Repair	ATP-dependent excision and replacement of damaged DNA

Genetic Exchange in Bacteria

Conjugation

Conjugation is the direct transfer of DNA between bacteria via cell-to-cell contact, typically mediated by a pilus.

• Vertical gene transfer: Parent to offspring via cell division.

• Horizontal gene transfer: Transfer between unrelated cells.

• F plasmid: Fertility plasmid that enables conjugation.

• Plasmids: Small, circular DNA molecules that replicate independently.

Acquiring a Plasmid

• Plasmids may carry antibiotic resistance, virulence factors, or metabolic genes.

• Replication of extra DNA can slow growth, so plasmids must confer a benefit.

Example: F'128 Plasmid

• Contains genes for lactose fermentation and chloramphenicol resistance (Cat).

• Kanamycin resistance (Kan) may be present in the chromosome.

Antibiotic	F-	F+	TC
Cat	No growth	Growth, pink	*Growth, pink
Kan	No growth	Growth, yellow	*Growth, yellow
Cat + Kan	No growth	No growth	*Growth, pink/yellow
Additional info: TC = transconjugant cells (acquire both resistances)

Transformation

Definition and Mechanism

Transformation is the process by which bacteria acquire DNA from their environment. Cells must be competent to take up DNA, which can be induced chemically or naturally.

• Competence can be induced by CaCl2 treatment and heat shock.

• Plasmids must confer a selective advantage, such as antibiotic resistance.

Example: pGlo Plasmid

• Contains genes for ampicillin resistance and green fluorescent protein (gfp) under the control of the arabinose promoter.

• Expression of gfp requires arabinose in the medium.

Results Interpretation

• Cells without plasmid do not grow on ampicillin plates.

• Cells with plasmid grow on ampicillin; only those with arabinose express gfp and glow.

Summary Table: Key Laboratory Concepts

Concept	Definition	Application
MIC	Minimum Inhibitory Concentration	Determines lowest antibiotic concentration to inhibit growth
MBC	Minimum Bactericidal Concentration	Determines lowest antibiotic concentration to kill bacteria
Kirby-Bauer	Disk diffusion method	Tests antibiotic sensitivity
Biofilm	Microbial community in matrix	Persistence in environment and infection
Endospore	Resistant bacterial structure	Survival in harsh conditions
Conjugation	DNA transfer via pilus	Spread of resistance genes
Transformation	Uptake of environmental DNA	Genetic engineering, adaptation

Key Equations

• Serial dilution calculation for MIC determination: $C_n = C_0 \times D^n$ Where $C_n$ is the concentration after n dilutions, $C_0$ is the initial concentration, and $D$ is the dilution factor.

Additional info:

• Some details on endospore types and positions are inferred for completeness.

• Summary tables are expanded for clarity and exam preparation.