Microbiology Study Guide: Molecular Techniques, Immunoassays, Pathogens, and Clinical Applications

Molecular Identification and Analysis of Microbes

Ribotyping

Ribotyping is a molecular technique used to identify microbial species based on differences in ribosomal RNA (rRNA) gene sequences.

• Definition: Ribotyping involves amplifying rRNA genes using PCR, digesting the DNA with restriction enzymes, and analyzing fragment patterns via gel electrophoresis.

• Purpose: To distinguish bacterial species by their unique DNA fragment patterns.

• Example: Identifying Escherichia coli versus Staphylococcus aureus by comparing ribotype patterns.

PCR (Polymerase Chain Reaction)

PCR is a method to amplify specific DNA regions, making millions of copies for analysis.

• Setup: Use primers (short DNA sequences) designed to flank the target region.

• Process:

  1. Denaturation: Heat to 95°C to separate DNA strands (1 min).

  2. Annealing: Cool to 55°C so primers bind to target DNA (1.5 min).

  3. Extension: Heat to 72°C for DNA polymerase to synthesize new DNA (1 min).

• Thermocycler: Machine that cycles through these temperatures.

• Equation: $N = 2^n$ (where N = number of DNA copies, n = number of cycles)

Restriction Enzyme Digestion

Restriction enzymes cut DNA at specific sequences, generating fragments that differ between species.

• Definition: Proteins from bacteria that recognize and cut specific nucleotide sequences.

• Application: Used after PCR to create unique fragment patterns for each species.

Gel Electrophoresis

Gel electrophoresis separates DNA fragments by size using an electric current.

• Process:

  1. Mix DNA with stain and load into gel wells.

  2. Apply electric current; DNA moves toward positive end due to negative charge.

  3. Smaller fragments travel faster and farther.

• DNA Ladder: Standard fragments used to estimate sample sizes.

Interpreting Ribotyping Data

Compare fragment patterns to identify species.

• Example: Unique banding patterns indicate different bacterial species.

CRISPR and Genetic Engineering

CRISPR is a bacterial defense mechanism adapted for gene editing.

• Natural Function: Bacteria use CRISPR to defend against bacteriophage infection.

• Scientific Use: CRISPR-Cas9 system edits DNA by guiding Cas9 to specific sequences using guide RNA (gRNA).

• Molecular Players:

  • Cas9: DNA-cutting enzyme

  • gRNA: Guide RNA directs Cas9

  • Donor DNA: Used for repair or insertion

• Example: Disrupting lacZ gene in bacteria; white colonies indicate disruption, blue colonies indicate intact gene.

CRISPR Lab Results Interpretation

Condition	Colony Color	Result
gRNA + Donor DNA	White	Disrupted lacZ, DNA repair
No gRNA, Donor DNA	Blue	Intact lacZ, no cut
gRNA, No Donor DNA	Dead	Cut, no repair
No gRNA, No Donor DNA	Blue	No cut, intact gene

Additional info: Arabinose is required to express enzymes for DNA repair.

RT-PCR in COVID-19 Testing

RT-PCR is used to detect viral RNA, such as SARS-CoV-2, by converting RNA to DNA and amplifying it.

• Process:

  1. Isolate RNA from sample.

  2. Use reverse transcriptase to convert RNA to complementary DNA (cDNA).

  3. Amplify cDNA using PCR with primers specific to viral genes (e.g., N gene).

  4. Label DNA with fluorescent molecules; fluorescence indicates presence of virus.

• Enzymes: Reverse transcriptase, DNA polymerase.

• Interpretation: Fluorescence = positive for COVID-19; no fluorescence = negative.

Immunoassays and Rapid Tests

Immunoassays

Immunoassays use antibodies to detect microbial antigens or patient antibodies.

• Staph Agglutination Assay: Antibodies attached to blue latex beads detect S. aureus antigen; clumping indicates positive result.

• Strep Lateral Flow: Antibodies attached to colored beads detect S. pyogenes antigen; colored line indicates positive.

• COVID-19 Lateral Flow: Detects SARS-CoV-2 nucleocapsid protein; colored line indicates positive.

ELISA (Enzyme-Linked Immunosorbent Assay)

ELISA detects antigens or antibodies using enzyme-linked antibodies and color change.

• Direct ELISA: Detects antigen; color change indicates positive.

• Indirect ELISA: Detects patient antibodies; color change indicates positive.

• Sandwich ELISA: Most sensitive for antigen detection.

Test Type	Detects	Result
Direct ELISA	Antigen	Color change = infection
Indirect ELISA	Antibody	Color change = exposure
Sandwich ELISA	Antigen	Most sensitive

White Blood Cells and Clinical Interpretation

Types of White Blood Cells (WBCs)

WBCs are immune cells with distinct morphology and functions.

• Neutrophils: Largest proportion; multi-lobed nucleus, salmon-pink granules.

• Lymphocytes: Slightly larger than RBCs; spherical nucleus, no granules.

• Monocytes: Largest WBC; kidney-shaped nucleus, no granules.

• Eosinophils: Two-lobed nucleus, bright red granules.

• Basophils: Two-lobed nucleus, deep blue granules.

WBC Type	Frequency (Most to Least)
Neutrophils	Most
Lymphocytes	Second
Monocytes	Third
Eosinophils	Fourth
Basophils	Least

Note: Health conditions can alter WBC frequencies.

Vaccines and Public Health

Vaccine Concerns and Disease Prevention

Vaccines are essential for preventing infectious diseases, but concerns persist.

• Common Concerns: Autism, religious/moral objections, ingredients, side effects.

• Measles Increase: Due to decreased vaccination rates.

• Vaccine-Preventable Cancer: HPV-related cancers.

• Scientific Evidence: No link between vaccines and autism.

Epidemiology and Disease Metrics

Incidence, Prevalence, and Index Case

Epidemiology uses metrics to track disease spread.

• Incidence: Number of new cases in a time period.

• Prevalence: Total cases in a time period.

• Index Case: First known case in an outbreak.

• Equations:

  • Incidence: $\text{Incidence} = \frac{\text{New cases}}{\text{Total population}} \times K$

  • Prevalence: $\text{Prevalence} = \frac{\text{Total cases}}{\text{Total population}} \times K$

Pathogens and Clinical Microbiology

Pathogen Definition and Classification

A pathogen is a microbe that causes disease. Parasites include eukaryotic pathogens such as fungi, protozoans, and worms.

• Identification: Use microscopy and scientific names to classify pathogens.

• Example: Streptococcus pyogenes causes strep throat.

Throat Culture and Hemolysis Patterns

Blood agar is used to differentiate bacteria based on hemolysis.

• Alpha (α) Hemolysis: Greenish, partial clearing; reduction of hemoglobin.

• Beta (β) Hemolysis: Complete clearing; lysis of RBCs, associated with pathogens.

• Gamma (γ) Hemolysis: No clearing; no hemolysis.

Urinary Tract Infections (UTIs)

Types and Diagnosis

UTIs are most commonly caused by E. coli and can affect the bladder or kidneys.

• Cystitis: Infection of the bladder.

• Pyelonephritis: Infection of one or more kidneys.

• Glomerulonephritis: Infection from inflammation of kidney glomeruli.

Clean Catch Method

• Minimizes contamination by washing area, discarding initial urine, and collecting midstream.

Semi-Quantitative Streak Method

• Allows estimation of colony-forming units (CFU) per mL; >$10^5$ CFU/mL indicates UTI.

• Equation: $\text{CFU/mL} = \frac{\text{Number of colonies} \times \text{Dilution factor}}{\text{Volume plated}}$

MacConkey Agar

• Selective: Inhibits Gram-positive bacteria; allows Gram-negative (e.g., E. coli) to grow.

• Differential: Lactose fermentation turns colonies pink.

Dental Microbiology

Snyder Test for Tooth Decay

The Snyder Test assesses risk for dental caries by measuring acid production from oral microbes.

• Process: Microbes metabolize sucrose, producing acid that lowers pH.

• pH Indicator: Bromocresol green turns yellow as acid is produced.

• Interpretation: Faster color change = higher risk for tooth decay.

Result	Interpretation
Negative	No color change; low risk
Weak Positive	Slow color change; moderate risk
Strong Positive	Rapid color change; high risk

Antibiotics and Microbial Competition

Origin and Purpose of Antibiotics

Most antibiotics are natural products made by bacteria and fungi to inhibit competitors.

• Reason: Microbes produce antibiotics to gain advantage in resource competition.

• Example: Streptomyces species produce streptomycin.

Testing Antibiotic Activity

• Design experiments to assess inhibition of microbial growth by antibiotics.

• Measure zones of inhibition on agar plates.