Microbiology Study Guide: Genetics, Microbial Growth, Media, and Pathogen Identification

Microbial Genetics and Transformation

Bacterial Reproduction

Bacteria primarily reproduce through a process called binary fission, which is an asexual method of cell division. This process results in two genetically identical daughter cells.

• Binary Fission: A single bacterial cell duplicates its DNA and divides into two cells.

• No Sexual Reproduction: Bacteria do not reproduce sexually; genetic variation arises through other mechanisms.

Genetic Exchange: Transformation

Transformation is a process where bacteria take up free DNA from their environment, leading to genetic changes.

• Definition: Uptake of naked DNA fragments by a bacterial cell from its surroundings.

• Significance: Can introduce new traits, such as antibiotic resistance.

• Healthcare Importance: Transformation is a mechanism by which antibiotic resistance genes can spread among bacterial populations.

• Example: Escherichia coli growing on ampicillin-containing media indicates acquisition of an ampicillin resistance gene.

Plasmids and Trait Acquisition

Plasmids are small, circular DNA molecules that can carry genes conferring advantageous traits.

• Traits: Antibiotic resistance, fluorescence (e.g., GFP gene), and other phenotypes can be encoded on plasmids.

• Laboratory Application: Introduction of plasmids into bacteria (transformation) is used to study gene function and confer new traits.

Lab 3 Transformation Table (Predicted Outcomes)

This table summarizes expected bacterial growth and colony color based on the presence of plasmids and ampicillin in the growth medium.

Plasmid Present	Ampicillin Present	Growth	Colony Color
Yes	Yes	Yes	Depends on plasmid (e.g., green if GFP present)
No	Yes	No	None
Yes	No	Yes	Depends on plasmid
No	No	Yes	Normal (white)

Additional info: Table entries inferred based on standard transformation experiments.

Microbial Growth and Media

Selective and Differential Media

Specialized media are used to isolate and identify microbes based on their physiological properties.

• Selective Media: Favors the growth of certain microbes while inhibiting others.

• Differential Media: Distinguishes between microbes based on observable traits (e.g., color change).

Examples of Media

• Mannitol Salt Agar (MSA): Selective for salt-tolerant bacteria (e.g., Staphylococcus species); differential for mannitol fermentation (yellow color indicates fermentation).

• MacConkey Agar: Selective for Gram-negative bacteria; differential for lactose fermentation (pink colonies indicate lactose fermenters; colorless colonies are non-fermenters).

• Rogosa Agar: Highly acidic, used for isolating lactic acid bacteria.

Colony Morphology and Identification

• Serratia marcescens: Produces shiny red colonies due to the pigment prodigiosin.

Microbial Metabolism and Enzyme Activity

Catalase Test

The catalase enzyme breaks down hydrogen peroxide into water and oxygen gas, which is observed as bubbling during the catalase test.

• Reaction:

$2H_2O_2 \rightarrow 2H_2O + O_2$

• Significance: Used to differentiate between catalase-positive and catalase-negative bacteria.

Pathogen Identification and Classification

Common Pathogens and Their Types

Disease/Agent	Type
Strep throat	Bacteria
Giardia	Protozoa
Influenza	Virus
MRSA	Bacteria
Measles	Virus
Tuberculosis	Bacteria

Prevalence of Viral Diseases

• Influenza is more widespread globally than polio.

Laboratory Techniques and Safety

Petri Dish Handling

• Keep the lid closed to prevent contamination from airborne microbes.

Microbial Control Methods (Extra Credit)

Disinfection vs. Sterilization

• Disinfection: Reduces or eliminates most pathogens on surfaces (e.g., bleach, ethanol).

• Sterilization: Destroys all forms of microbial life, including spores (e.g., autoclave).

Gram Stain Interpretation

• Gram-negative bacteria: Appear pink after Gram staining due to a thin peptidoglycan layer and the presence of an outer membrane.

Fungal Example

• Rhizopus: A genus of common fungi, often studied in laboratory settings.