Microbial Genetics

Bacterial DNA is circular. Any component of DNA that replicates as a singular unit is a replicon.

Bacterial protein synthesis is a target of antibiotics. These often bind at the 30S or 50S ribosome.

Common Antibiotics

• clindamycin
• aminoglycosides
• macrolides
• tetracycline

Bacterial Plasmids

Circular pieces of DNA. They have resistance to antibiotics between and among bacterial species. This is referred to as an R factor. They also encode for production of toxins, like the heat-labile/heat-stable E. coli enterotoxins, the exfoliative Staphylococcus aureus toxin, and the Clostridium tetani tetanus toxin.

R plasmids emerged from selection pressure from antibiotics. They can contain multiple resistant genes which can be transferred between bacteria (transposons).

Genetic Exchange

Transformation

Genetic altering of a cell from the uptake of genetic material from surrounding environment.

1. Exogenous DNA (dsDNA) is spliced into fragments and taken up by cells
2. One DNA is digested by exonuclease, while the other is incorporated into the bacterial chromosome via recombination (RegA gene)

Transduction

DNA is transferred between bacteria via a bacterial virus (bacteriophage). These viruses parasitize and kill bacteria. This is a method through which antibiotic resistance can be transferred between bacteria.

The lytic lifecycle is when the virus infects the bacteria, uses existing machinery to replicate, and then self-destructs the organism. This destruction releases new viruses. Lytic phages have a role in targeting antibiotic resistant bacteria.

The lysogenic lifecycle is when the virus DNA is incorporated into the bacterial genome (becoming a prophage) and turns it into a virus factory. This is associated with production of toxins/virulence factors.

Lysogenic associated bacterial toxins

• C. diphtheria
• C. botulinum
• V. cholerae
• E. coli Shiga
• [[Gram Positive Cocci#Group A Streptococcus pyogenes|S. pyogenes]]

In C. diphtheria, the lysogenic bacteriophage carries the gene encoding the exotoxin which inactivates elongation factor 2. This results in pharyngitis with pseudomembrane, myocarditis, and respiratory failure.

In C. botulinum, botulinum toxin is produced from the lysogenic bacteriophage and results in diarrhea, diplopia (double vision), dysphasia (speech impairment), dysphagia (swallowing impairment), and paralysis.

Conjugation

When bacteria form cytoplasmic connections, in which DNA is transferred directly. This is achieved using pilus.

Clinical pearl: Treatment of antimicrobial resistance

Bacteriophages can infect resistant bacteria like MDR Acinetobacter and Klebsiella.

Transposons

Can be transferred from plasmids to bacterial chromosomes and can carry antibiotic resistant genes.