BIO 220: General Microbiology Midterm Study Guide

Pioneers of Microbiology & Microscopy

Major Scientists and Their Contributions

This section covers the foundational experiments and discoveries that shaped microbiology, focusing on disproving spontaneous generation and the development of microscopy and staining techniques.

• Francesco Redi: Demonstrated that maggots arise from fly eggs, not spontaneous generation, by placing meat in open and gauze-covered jars.

• Lazzaro Spallanzani: Boiled broth in sealed flasks, showing no microbial growth, further challenging spontaneous generation.

• Louis Pasteur: Swan-neck flask experiment allowed air in but trapped dust/microbes, proving broth remained sterile unless exposed to contaminants.

• Edward Jenner: Used material from cowpox lesions to protect against smallpox, pioneering vaccination.

• John Tyndall: Showed some bacteria form heat-resistant endospores; developed Tyndallization (intermittent heating for sterilization).

• Joseph Lister: Introduced carbolic acid (phenol) as an antiseptic in surgery, reducing infection rates.

• Robert Koch: Developed Koch’s postulates to link specific microbes to specific diseases.

Microscopy & Staining

• Microscopy: Essential for observing microorganisms; includes light and electron microscopy.

• Staining: Enhances contrast; Differential stains distinguish cell types.

• Gram stain: Separates bacteria into Gram-positive (thick peptidoglycan, purple) and Gram-negative (thin peptidoglycan, pink).

• Acid-fast stain: Identifies bacteria with waxy mycolic acids (e.g., Mycobacterium).

Example:

Pasteur’s swan-neck flask experiment conclusively disproved spontaneous generation by showing that broth remained sterile unless exposed to airborne microbes.

Cell Structure

Prokaryotic and Eukaryotic Cell Structures

Understanding cell structure is fundamental to microbiology. This section compares prokaryotic and eukaryotic cells, focusing on cell walls, organelles, and specialized structures.

• Peptidoglycan: Found in bacterial cell walls, provides structural support.

• Gram-negative bacteria: Have an outer membrane containing lipopolysaccharide (LPS), absent in Gram-positive bacteria.

• Ribosomes: Prokaryotes have 70S ribosomes (free in cytoplasm); eukaryotes have 80S ribosomes (some attached to ER).

• Bacterial capsule: Protects against phagocytosis, aids in attachment.

• Eukaryotic cells: Contain membrane-bound organelles (e.g., mitochondria), absent in prokaryotes.

Specialized Structures

• Flagellum: Long, whip-like appendage for motility.

• Pili (fimbriae): Short, hair-like projections for attachment and DNA transfer.

• Plasmid: Small, circular DNA molecule carrying extra genes (often antibiotic resistance).

• Nucleoid: Region in prokaryotes where the chromosome is located.

Example:

Gram-negative bacteria are distinguished by their outer membrane with LPS, which is absent in Gram-positive bacteria.

Cell & Colony Morphology

Bacterial Shapes and Arrangements

Bacteria are classified by their shapes and arrangements, which are observed under the microscope and in culture.

• Bacillus: Rod-shaped.

• Coccobacillus: Short, plump rod intermediate between coccus and bacillus.

• Vibrio: Curved, comma-shaped rod.

• Spirillum: Rigid spiral.

• Spirochete: Flexible spiral, moves via internal axial filaments.

Arrangements

• Diplococci: Cocci in pairs.

• Streptococci: Cocci in chains.

• Staphylococci: Cocci in clusters.

• Tetrads: Cocci in groups of four.

Colony Morphology

• Describes the appearance of bacterial colonies on solid media (e.g., color, shape, margin).

Example:

Staphylococci are cocci arranged in grape-like clusters, while colony morphology refers to features like raised, cream-colored growth with irregular margins.

The Three Domains & Other Infectious Agents

Domains of Life

All cellular life is classified into three domains: Bacteria, Archaea, and Eukarya. Other infectious agents include viruses, viroids, and prions.

• Bacteria: Prokaryotes, reproduce by binary fission, cell walls with peptidoglycan.

• Archaea: Prokaryotes, lack peptidoglycan, often live in extreme environments.

• Eukarya: Eukaryotes, include algae, fungi, protozoa.

Other Infectious Agents

• Virus: Acellular, DNA or RNA inside protein coat (capsid), replicates only in host cell.

• Viroid: Short, single-stranded RNA, no protein coat, infects plants.

• Prion: Misfolded infectious protein, no nucleic acid, causes diseases like mad cow disease.

Example:

Prions are unique infectious agents composed only of protein, unlike viruses (DNA/RNA + protein coat) and viroids (RNA only).

Media & Laboratory Methods

Laboratory Techniques and Media Types

Microbiologists use various media and methods to isolate, identify, and study microorganisms.

• Streak plate method: Used to obtain isolated colonies for pure cultures.

• TSA (Tryptic Soy Agar): Solid medium; TSB (Tryptic Soy Broth): Liquid medium.

• Differential media: Distinguish organisms based on biochemical reactions (e.g., color change).

• Selective media: Inhibit growth of some organisms, allow others to grow.

• TSI (Triple Sugar Iron): Tests for sugar fermentation and hydrogen sulfide production.

Example:

MacConkey agar is both selective (inhibits Gram-positive bacteria) and differential (lactose fermenters turn pink).

Medium	Purpose	Key Feature
TSA	General purpose	Solid, contains agar
TSB	General purpose	Liquid broth
MacConkey agar	Selective & Differential	Inhibits Gram-positive, lactose fermenters turn pink
TSI	Differential	Detects sugar fermentation, H2S production

Microbial Genetics

Flow of Genetic Information

Genetic information flows from DNA to RNA to protein. Bacteria achieve genetic diversity through recombination.

• Replication: DNA is copied; semiconservative (each new molecule has one old and one new strand).

• Transcription: DNA is used as a template to make RNA (mRNA, tRNA, rRNA).

• Translation: mRNA is decoded by ribosomes; tRNA brings amino acids to build proteins.

Genetic Diversity in Bacteria

• Transformation: Uptake of free DNA from environment.

• Conjugation: DNA transfer via direct cell contact (bacterial "mating").

• Transduction: DNA transfer by bacteriophage (virus).

• Plasmid: Extra-chromosomal DNA, often carries antibiotic resistance.

• Genetic engineering: Uses restriction nucleases (enzymes that cut DNA at specific sites) and reverse transcription (making DNA from RNA).

Key Equations:

• DNA Replication: $\text{DNA}_{\text{parent}} \rightarrow \text{DNA}_{\text{daughter}}$

• Central Dogma: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Example:

Restriction nucleases are used in genetic engineering to cut DNA at specific sequences, enabling gene cloning.

Factors Affecting Microbial Growth

Environmental Factors

Microbial growth is influenced by oxygen, pH, and water availability. These factors are used to control microbial growth in food and laboratory settings.

• Oxygen: Obligate aerobes require oxygen; obligate anaerobes cannot tolerate it; facultative anaerobes grow with or without oxygen.

• pH: Most pathogens are neutrophiles (grow best at pH 6.5–7.5); acidophiles prefer acidic environments.

• Water activity: High salt or sugar reduces water activity, inhibiting microbial growth.

Type	Growth Requirement
Obligate aerobe	Requires oxygen
Obligate anaerobe	Cannot tolerate oxygen
Facultative anaerobe	Grows with or without oxygen
Acidophile	Grows best at low pH
Halophile	Grows best in high salt
Microaerophile	Requires low oxygen

Example:

Adding salt to food reduces water activity, preventing microbial growth and preserving the food.

Medically Important Bacteria

Pathogenic Bacteria and Associated Diseases

This section lists key bacteria and the diseases they cause, along with transmission and characteristics.

Organism	Disease	Transmission	Key Characteristic
Escherichia coli	Gastroenteritis, UTI	Fecal-oral	Gram-negative rod
Salmonella enterica (Typhi)	Typhoid fever	Fecal-oral	Gram-negative rod
Yersinia pestis	Plague	Flea bite	Gram-negative rod
Helicobacter pylori	Peptic ulcers	Oral-oral, fecal-oral	Gram-negative spiral
Vibrio cholerae	Cholera	Contaminated water	Gram-negative comma-shaped
Neisseria gonorrhoeae	Gonorrhea	Sexual contact	Gram-negative diplococcus
Bacillus anthracis	Anthrax	Contact, inhalation	Gram-positive rod, endospore-forming
Clostridium botulinum	Botulism	Foodborne	Gram-positive rod, endospore-forming, neurotoxin
Clostridium tetani	Tetanus	Wound contamination	Gram-positive rod, endospore-forming, neurotoxin
Staphylococcus aureus	Skin infections, food poisoning	Contact	Gram-positive cocci
Streptococcus pyogenes	Strep throat, skin infections	Contact, respiratory	Gram-positive cocci
Mycobacterium tuberculosis	Tuberculosis	Respiratory	Acid-fast rod
Treponema pallidum	Syphilis	Sexual contact	Spirochete
Borrelia burgdorferi	Lyme disease	Tick bite	Spirochete
Mycoplasma pneumoniae	Walking pneumonia	Respiratory	Lacks cell wall

Example:

Yersinia pestis causes plague, transmitted by fleas, and is a Gram-negative rod.