05.& 05.01 Bacterial Genetics

Streptococcus pneumoniae

Gram/Morphology: Gram-positive cocci.

​Key Virulence Factor: Capsule; loss of the capsule (via mutation) results in an avirulent strain.

​Diagnostic/Genetic Marker: Used to demonstrate Transformation (uptake of naked DNA).

​Treatment: Penicillin

Corynebacterium diphtheriae

Gram/Morphology: Gram-positive rod.

​Key Virulence Factor: Diphtheria Toxin.

​Mechanism of Acquisition: Lysogenic Conversion; the toxin gene is carried by a prophage (temperate bacteriophage) integrated into the bacterial chromosome.

​Diagnostic: Toxin production (Elek test) is the gold standard for pathogenicity.

Escherichia coli

Gram/Morphology: Gram-negative rod.

​Key Genetic Feature: Hfr (High Frequency of Recombination) strains.

​Gold Standard Genetic Use: Used for mapping the bacterial chromosome via conjugation (the entire chromosome takes 100 minutes to transfer).

​Example Regulon: Maltose utilization is a key example of a regulon in E. coli.

Shigella, Listeria, Yersinia

Key Virulence Factor: Invasion Plasmids.

​Mechanism: These bacteria carry large plasmids that encode the genes necessary for invading host cells.

​Gram Stain: Shigella and Yersinia (Gram-negative rods); Listeria (Gram-positive rod).

Borrelia (Genomic Exception)

Gram/Morphology: Spirochete.

​Unique Feature: Unlike most bacteria with circular DNA, Borrelia has a linear chromosome/plasmid.

Pathogenicity Islands (PAIs)

Definition: Large genomic regions carrying virulence genes (toxins, adhesins).

​Key Identifiers: Often have a different G+C content than the rest of the genome and are flanked by direct repeats.

​Movement: Mobilized by integrase enzymes or associated with tRNA genes.

Integrons

Key Components: Integrase gene (intI), an attachment site (attI), and a promoter (Pc).

​Function: They capture and express gene cassettes (usually antibiotic resistance genes).

​Note: Gene cassettes lack their own promoters and must use the integron's promoter to be expressed.

The SOS Response

Definition: A Global Regulatory Network triggered by heavy DNA damage (e.g., from UV radiation).

​Mechanism: Induces approximately 29 genes to attempt DNA repair.

​Consequence: Can lead to the induction of the lytic cycle in temperate phages.

Staphylococcus aureus (Gram stain/Morphology)

Gram-positive cocci in clusters ("bunches of grapes").

Staphylococcus aureus (Key Virulence Factors)

Protein A (binds Fc region of IgG to prevent phagocytosis), Coagulase, and TSST-1 (superantigen).

Staphylococcus aureus (Gold Standard Diagnostic)

Catalase (+) and Coagulase (+).

Gold standard: Culture on blood agar or Mannitol Salt Agar (ferments mannitol).

Staphylococcus aureus (First-line Treatment)

Nafcillin or Oxacillin (MSSA); Vancomycin (MRSA).

Streptococcus pyogenes / Group A Strep (Gram stain/Morphology)

Gram-positive cocci in chains.

Streptococcus pyogenes (Key Virulence Factor)

M protein (antiphagocytic; mimics heart tissue leading to Rheumatic Fever) and Streptolysin O.

Streptococcus pyogenes (Gold Standard Diagnostic)

Catalase (-), Bacitracin Sensitive, and Beta-hemolytic.

Streptococcus pyogenes (First-line Treatment)

Penicillin G or Amoxicillin.

Streptococcus pneumoniae (Gram stain/Morphology)

Gram-positive lancet-shaped diplococci.

Streptococcus pneumoniae (Key Virulence Factor)

Polysaccharide Capsule (major factor for evasion).

Streptococcus pneumoniae (Gold Standard Diagnostic)

Alpha-hemolytic, Optochin Sensitive, and Quellung reaction positive.

Streptococcus pneumoniae (First-line Treatment)

Ceftriaxone or Penicillin (if sensitive); Vancomycin for resistant strains.

Neisseria meningitidis (Gram stain/Morphology)

Gram-negative kidney-bean shaped diplococci.

Neisseria meningitidis (Key Virulence Factors)

Polysaccharide Capsule and IgA protease (cleaves IgA at mucosal surfaces).

Neisseria meningitidis (Gold Standard Diagnostic)

Culture on Thayer-Martin agar (VPN agar).

Neisseria meningitidis (First-line Treatment)

Ceftriaxone.

Escherichia coli (Gram stain/Morphology)

Gram-negative rod.

Escherichia coli (Key Virulence Factors)

P-pili (UTIs), K1 Capsule (neonatal meningitis), and LPS/Endotoxin (septic shock).

Escherichia coli (Gold Standard Diagnostic)

Pink colonies on MacConkey agar (Lactose fermenter); Indole positive.

Escherichia coli (First-line Treatment)

Fluoroquinolones (UTI) or Ceftriaxone (systemic).

Mycobacterium tuberculosis (Gram stain/Morphology)

Acid-fast bacilli (AFB); high mycolic acid content in cell wall.

Mycobacterium tuberculosis (Key Virulence Factor)

Cord Factor (inhibits macrophage migration and induces granuloma formation).

Mycobacterium tuberculosis (Gold Standard Diagnostic)

Lowenstein-Jensen (LJ) medium or BACTEC culture. (Note: Ziehl-Neelsen stain for rapid ID).

Mycobacterium tuberculosis (First-line Treatment)

RIPE Regimen: Rifampin, Isoniazid, Pyrazinamide, Ethambutol.

Antibiotic targeting DNA Gyrase/Topoisomerase II

Fluoroquinolones (e.g., Ciprofloxacin)

Antibiotic targeting RNA Polymerase (Transcription)

Rifampin

Genetic element containing virulence genes

Pathogenicity Islands (PAIs).

Result of UV radiation damage to DNA

Thymine Dimers