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Microbiology

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Microbiology - Complete Interactive Lesson

Part 1: Bacteria Structure & Function

Microbiology for the MCAT

Part 1 of 7 — Bacteria: Structure & Classification

Bacterial Cell Structure

Structure	Function	Notes
Cell wall	Protection, shape	Peptidoglycan
Plasma membrane	Selective barrier	No cholesterol
Nucleoid	Circular DNA	No membrane-bound nucleus
Ribosomes	Protein synthesis	70S (target for antibiotics!)
Plasmid	Accessory genes	Often carry antibiotic resistance
Flagella	Motility	Chemotaxis
Pili	Attachment, conjugation	Sex pili for DNA transfer
Capsule	Immune evasion	Prevents phagocytosis

Gram Stain Classification

Feature	Gram Positive	Gram Negative
Stain color	Purple/Blue	Pink/Red
Cell wall	Thick peptidoglycan	Thin peptidoglycan
Outer membrane	No	Yes (contains LPS)
LPS (endotoxin)	No	Yes

LPS (Lipopolysaccharide) — HIGH YIELD

Found ONLY in Gram-negative outer membrane
Released when bacteria lyse → triggers massive immune response
Can cause septic shock, fever, disseminated intravascular coagulation (DIC)

Bacteria Structure 🎯

Key Takeaways — Part 1

Gram-positive: thick peptidoglycan, no outer membrane (stains purple)
Gram-negative: thin peptidoglycan + outer membrane with LPS (stains pink)
LPS = endotoxin → fever, shock
Bacterial ribosomes = 70S (antibiotics target these)

Part 2: Viruses & Prions

Microbiology for the MCAT

Part 2 of 7 — Bacterial Growth & Metabolism

Bacterial Growth Curve

Phase	Description
Lag	Adapting to environment, synthesizing enzymes
Log (Exponential)	Rapid binary fission, most sensitive to antibiotics
Stationary	Growth rate = death rate (resources depleted)
Death	Death rate > growth rate

Binary Fission

$N = N_0 \times 2^n$

Part 3: Fungi & Parasites

Microbiology for the MCAT

Part 3 of 7 — Bacterial Genetics & Antibiotic Resistance

Horizontal Gene Transfer

Mechanism	How it works
Transformation	Bacteria picks up free DNA from environment
Transduction	Bacteriophage transfers DNA between bacteria
Conjugation	Direct DNA transfer via sex pilus (F plasmid)

Antibiotic Resistance Mechanisms

Mechanism	Example
Enzyme degradation	$\beta$ -lactamase destroys penicillin
Target modification	Altered ribosome binding site → macrolide resistance
Efflux pumps	Pump drug out of cell → tetracycline resistance
Decreased permeability	Porin mutations → reduced drug entry

Part 4: Microbial Genetics

Microbiology for the MCAT

Part 4 of 7 — Viruses

Virus Structure

NOT cells — obligate intracellular parasites
Nucleic acid (DNA or RNA, never both) + protein coat (capsid)
Some have a lipid envelope (from host membrane)

Viral Classification

Feature	Types
Genome	dsDNA, ssDNA, dsRNA, ssRNA (+) or (-)
Envelope	Enveloped or naked
Shape	Icosahedral, helical, complex

Replication Cycles

Lytic cycle: Attach → Inject DNA → Replicate → Assemble → Lyse → Release Lysogenic cycle: Viral DNA integrates into host genome (prophage) → replicates with host → can switch to lytic under stress

Baltimore Classification (Important for MCAT)

Class	Genome	Key Feature
I	dsDNA	Direct transcription (herpes, adenovirus)
IV	(+)ssRNA	mRNA-ready → immediate translation (COVID-19, Zika)

Part 5: Antimicrobial Agents

Microbiology for the MCAT

Part 5 of 7 — Fungi, Parasites & Prions

Fungi

Feature	Details
Cell wall	Chitin (not peptidoglycan!)
Cell membrane	Contains ergosterol (target for antifungals)
Nutrition	Heterotrophs, absorptive feeding
Forms	Yeasts (unicellular), molds (multicellular), dimorphic (both)

Fungal Reproduction

Asexual: Budding (yeasts), spore formation
Sexual: Occurs under stress conditions

Parasitology (Key MCAT Parasites)

Organism	Type	Disease	Transmission
Plasmodium	Protozoan	Malaria	Mosquito (Anopheles)
Trypanosoma	Protozoan	Sleeping sickness

Part 6: Host-Pathogen Interactions

Microbiology for the MCAT

Part 6 of 7 — Immune Response to Infection

First Line of Defense (Barriers)

Skin (physical), mucous membranes, stomach acid, lysozyme (tears/saliva), normal flora

Second Line (Innate Immune Response)

Component	Function
Neutrophils	First responders, phagocytosis (most abundant WBC)
Macrophages	Phagocytosis + antigen presentation (APC)
NK cells	Kill virus-infected and tumor cells (no antigen specificity)
Complement	Opsonization, membrane attack complex (MAC), inflammation
Inflammation	Vasodilation, increased permeability, cell recruitment

Third Line (Adaptive Immune Response)

Arm	Cells	Function
Humoral	B cells → Plasma cells	Produce antibodies (target extracellular pathogens)
Cell-mediated

Part 7: Review & MCAT Practice

Microbiology for the MCAT

Part 7 of 7 — Immune Disorders & Clinical Microbiology

Immune System Disorders

Disorder	Type	Description
Allergies	Hypersensitivity Type I	IgE-mediated, mast cell degranulation (histamine)
Autoimmune diseases	Self-tolerance failure	Immune system attacks own tissues (lupus, MS, T1DM)
HIV/AIDS	Immunodeficiency	Destroys CD4+ T cells → opportunistic infections
SCID	Immunodeficiency	No functional T or B cells (severe combined)

Antibody Classes (HIGH YIELD)

Class	Function	Location
IgG	Most abundant, crosses placenta	Blood
IgM	First to respond, pentamer	Blood

Where $N$ = final number, $N_0$ = initial number, $n$ = number of generations

Bacterial Metabolism

Type	Energy Source	Carbon Source
Photoautotroph	Light	CO $_2$
Photoheterotroph	Light	Organic compounds
Chemoautotroph	Inorganic chemicals	CO $_2$
Chemoheterotroph	Organic compounds	Organic compounds

Type	O $_2$ Needed?	Example
Obligate aerobe	Yes	M. tuberculosis
Obligate anaerobe	No (O $_2$ is toxic!)	Clostridium spp.
Facultative anaerobe	Either way (prefers O $_2$ )	E. coli

Bacterial Growth 🎯

Key Takeaways — Part 2

Growth curve: Lag → Log → Stationary → Death
Binary fission: $N = N_0 \times 2^n$ (exponential growth)
Obligate aerobes need O $_2$ ; obligate anaerobes are killed by O $_2$
Antibiotics most effective during log phase (active division)

MCAT Connection: Antibiotic Targets

Antibiotic Class	Target	Spectrum
$\beta$ -lactams (penicillin)	Cell wall synthesis	Bacteria only
Aminoglycosides	30S ribosomal subunit	Bacteria only
Macrolides (erythromycin)	50S ribosomal subunit	Bacteria only
Fluoroquinolones	DNA gyrase (topoisomerase)	Bacteria only
Sulfonamides	Folate synthesis	Bacteria only

Antibiotics DON'T work against viruses — viruses use host machinery!

Bacterial Genetics 🎯

Key Takeaways — Part 3

Horizontal gene transfer: Transformation (naked DNA), Transduction (phage), Conjugation (pilus)
Antibiotic resistance: enzyme degradation, target modification, efflux pumps
Know antibiotic targets: cell wall, 30S/50S ribosome, DNA gyrase, folate
Antibiotics target bacteria, NOT viruses

Key Takeaways — Part 4

Viruses are obligate intracellular parasites (not alive by themselves)
(+)ssRNA can be directly translated. (-)ssRNA needs RNA-dependent RNA Pol.
Retroviruses (HIV): RNA → DNA via reverse transcriptase
Enveloped viruses = fragile. Naked viruses = environmental survivors.

Misfolded proteins (PrP $^{Sc}$ ) — NO nucleic acid
Convert normal PrP $^{C}$ to misfolded form
Cannot be sterilized by standard methods (resist heat, UV, chemicals)
Cause spongiform encephalopathies (BSE, CJD, kuru)

Fungi & Parasites 🎯

Key Takeaways — Part 5

Fungi: chitin cell wall, ergosterol in membrane (drug target)
Malaria (Plasmodium) transmitted by Anopheles mosquito
Prions: misfolded proteins with NO nucleic acid — unique infectious agents
Antifungals target ergosterol (azoles) or chitin — not peptidoglycan

Active immunity: Exposure to antigen → immune response → memory (vaccines, natural infection)
Passive immunity: Receiving pre-formed antibodies (maternal IgG, antiserum)
- Passive = immediate but temporary. Active = delayed but long-lasting.

Immune Response 🎯

Key Takeaways — Part 6

Three lines of defense: barriers → innate (neutrophils, complement) → adaptive (B and T cells)
Active immunity: long-lasting, requires time. Passive: immediate but temporary.
Neutrophils = first responders. Macrophages = APCs + phagocytes.
Vaccines = active immunity (memory cells formed)

Mnemonic: "Greatest amount, M is fiMrst, A on All surfaces, E for allErgies"

Organism found in all cases of disease
Organism isolated and grown in pure culture
Cultured organism causes disease in healthy host
Organism re-isolated from new host = original organism

Immune Disorders 🎯

Microbiology — Complete! ✅

From bacteria to viruses to immune function, microbiology bridges molecular biology with clinical medicine. The MCAT loves testing infectious disease mechanisms, immune responses, and antibody functions. Know Koch's postulates, vaccine types, and immune disorders.

Microbiology

Bacterial Metabolism

Oxygen Requirements

Key Takeaways — Part 2

MCAT Connection: Antibiotic Targets

Key Takeaways — Part 3

Key Takeaways — Part 4

Prions

Key Takeaways — Part 5

Vaccination

Key Takeaways — Part 6

Koch's Postulates

Microbiology — Complete! ✅