vids antibacterials Flashcards
drug target:
cell wall synthesis
penicillins
cephalosporins
glycopeptides
carbapenems
monobactams
drug target:
folic acid metabolism
sulphonamides
trimetoprim
drug target:
DNA gyrase
Quinolones
drug target:
DNA-directed RNA polymerase
rifampicin
drug target:
protein synthesis
30S
aminoglycosides
tetracyclines
drug target:
protein synthesis
50S
macrolides
chloramphenicol
clindamycin
peptidoglycan traps crystal violet
gram positive bacteria
crystal violet is easily rinsed away revealing the dye
gram negative bacteria
gram staining procedure
crystal violet- staining
gram iodine- mordant
95% etOH- decolorizing agent
saffranin- counter stain
(+) violet/blue
(-) red/pink
Cell Wall Synthesis Inhibitors
- Penicillins
- Cephalosporins
- β-lactams
- Glycopeptides
- Carbapenems
- Cycloserine
- Bacitracin
Resistance to Penicillins
- Physical Barrier
- Presence of β-lactamase enzymes
- High levels of transpeptidase enzyme produced
- Affinity of transpeptidase enzyme to penicillin
- Efflux process
- Mutations and genetic transfers
Resistance to Penicillins
* Presence of β-lactamase enzymes
-The β-lactamase enzyme recognizes and binds to the β-lactam ring present in antibiotics
-The enzyme breaks the amide bond in the β-lactam ring through hydrolysis.
-This destroys the antibiotic’s ability to bind to penicillin-binding proteins (PBPs), which are essential for bacterial cell wall synthesis.
Most bacteria have a cell membrane surrounded by a cell wall,
with some having an additional outer layer.
The cytoplasm contains ribosomes, a nuclear region, and
sometimes granules or vesicles.
have a periplasmic space between the membrane and cell wall
Gram-negative bacteria
only have periplasm where digestion and cell wall synthesis occur.
Gram-positive bacteria
The cell wall’s primary component is peptidoglycan, made of:
N-acetylmuramic acid (NAM)
N-acetylglucosamine (NAG)
Cross-linked by four amino acid chains
Functions of the cell wall:
Maintains bacterial shape
Prevents osmotic lysis
Peptidoglycan synthesis involves:
Addition of five amino acids to NAM.
NAG is added to NAM, forming a peptidoglycan precursor.
The precursor is transported across the membrane.
Cross-linking occurs via transpeptidase and D-alanyl carboxypeptidase (also called penicillin-binding proteins, PBPs).
β-lactam antibiotics (penicillins, cephalosporins) have a β-lactam ring that binds to PBPs, preventing
cross-linking of peptidoglycan.
β-lactam antibiotics (penicillins, cephalosporins) have a β-lactam ring that binds to PBPs, that leads to:
Weakened cell wall
Osmotic lysis, especially in Gram-positive bacteria (due to high internal osmotic pressure).
Autolysins activation, which digests the existing cell wall.
β-lactams are bactericidal, meaning
hey actively kill bacteria rather than just inhibiting growth.
Bacteria develop resistance to β-lactam antibiotics through two primary mechanisms:
A. Transformation (Gene Acquisition)
B. β-Lactamase Enzyme Production
A. Transformation (Gene Acquisition)
Bacteria can take up naked DNA containing resistance genes from dead bacteria.
The resistance genes integrate into the host DNA via homologous recombination.
If these genes alter PBPs, the new PBPs bind poorly to β-lactams, making the bacteria resistant.
Example: Penicillin-resistant Streptococcus pneumoniae acquired resistance from other streptococcal species.
B. β-Lactamase Enzyme Production
β-lactamases hydrolyze the β-lactam ring, inactivating the antibiotic.
The genes for these enzymes can be located on:
-Chromosomal DNA (inherent resistance)
-Plasmids (acquired resistance via conjugation)
Conjugation: A bacterium transfers a resistance plasmid to another through a pili-mediated bridge.
β-Lactamase Production Gram-Positive
Inducible (produced in response to the drug)
β-Lactamase Production Gram-Negative
Constitutive (always produced)
Location of Enzyme β-Lactamase Gram Positive
secreted into the extracellular space
Location of Enzyme β-Lactamase Gram Negative
Retained in the periplasmic space
Efficiency β-Lactamase Gram-Positive
less efficient
Efficiency β-Lactamase Gram-Negative
more efficient due to enzyme localization
release β-lactamases outside the cell, reducing drug effectiveness before entry.
Gram-positive bacteria
keep β-lactamases in the periplasmic space, breaking down antibiotics before they reach PBPs.
Gram-negative bacteria
β-lactamase inhibitors
(e.g., clavulanic acid, sulbactam, tazobactam, avibactam) can be combined with β-lactam antibiotics to counteract resistance.
resistant to nearly all β-lactams.
Some bacteria, like carbapenem-resistant Enterobacteriaceae (CRE), produce metallo-β-lactamases (MBLs)
Bacteria reproduce through __________, where one cell splits into two identical daughter cells.
binary fission
Before division, the bacterium must copy its entire circular DNA to ensure
each new cell gets a complete genome.
Enzymes called __________ break hydrogen bonds between DNA strands, unwinding and stabilizing them.
helicases
The points where the strands separate are called
replication forks.
DNA polymerase moves along each separated strand, synthesizing a
new complementary strand in red.
-As replication forks move, positive superhelical twists build up in the DNA
-____________ removes these twists to allow replication to proceed.
DNA gyrase (topoisomerase II)
DNA gyrase is essential and consists of
GYRA and GYRB gene products
The two replication forks move in opposite directions until they meet, forming two complete chromosomes.
Each chromosome consists of one old and one new DNA strand—a process called
semi-conservative replication.
The two new chromosomes become interlinked and need to be separated.
_______ encoded by PARC and PARE genes, helps separate them for proper cell division.
Topoisomerase IV
are synthetic, bactericidal antibiotics.
Their effectiveness is enhanced by the addition of a fluorine atom at position 6, hence the name.
They rapidly inhibit bacterial DNA synthesis, leading to cell death.
Fluoroquinolones
Fluoroquinolones target two key bacterial enzymes:
DNA gyrase (primary target in gram-negative bacteria)
Topoisomerase IV (primary target in gram-positive bacteria like streptococci and staphylococci)
Fluoroquinolones bind to the ____________, stabilizing it.
This causes DNA breaks, which are fatal to bacteria.
DNA gyrase-DNA complex
In gram-positive bacteria, fluoroquinolones primarily target _________
This disrupts the separation of newly replicated chromosomes, halting bacterial division.
topoisomerase IV
Resistance occurs due to __________________ in bacterial genes.
These mutations alter DNA gyrase or topoisomerase IV, reducing fluoroquinolone binding
spontaneous mutations
Mutation in GYRA or GYRB genes →
Alters DNA gyrase → Fluoroquinolone no longer binds effectively.
Mutation in PARC or PARE genes →
Alters topoisomerase IV → Reduced Fluoroquinolone binding.
Some fluoroquinolones target both enzymes equally—in these cases,
mutations in both enzymes are needed for full resistance.
Resistant bacteria can continue __________________________ despite fluoroquinolone exposure.
This allows them to survive and proliferate, making treatment more difficult.
semi-conservative DNA replication
Inside the bacterial cell, DNA exists as a circular double-stranded molecule. Like the DNA of all living organisms, it contains the unique genetic code necessary for:
Protein production
Growth and repair
Regulation of metabolism
It also codes for the three types of RNA essential for protein synthesis:
Ribosomal RNA (rRNA) – Forms part of the ribosome
Messenger RNA (mRNA) – Carries genetic instructions from DNA
Transfer RNA (tRNA) – Delivers amino acids for protein assembly
Process of Protein Synthesis
1. Transcription
The double-stranded DNA unwinds and separates in the region that codes for a specific protein.
Only one strand of DNA serves as a template.
mRNA is synthesized as a mirror copy of the DNA sequence.
Once transcription is complete, mRNA detaches and attaches to ribosomes.
Process of Protein Synthesis
2. Translation
Bacterial ribosomes consist of a small (30S) and a large (50S) subunit.
The two subunits join around the mRNA strand.
tRNA molecules align along the mRNA sequence, bringing specific amino acids.
Amino acids are linked together to form a polypeptide chain.
The ribosome continues adding amino acids until it reaches a stop signal on the mRNA.
At this point, it releases the completed protein molecule.
antibiotics such as azithromycin, inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit. This prevents the ribosome from synthesizing the polypeptide chain.
Macrolides
Macrolides are bacteriostatic (stop bacterial growth).
At higher concentrations or during rapid bacterial growth, they may become bactericidal (kill bacteria)
Bacterial Resistance to Macrolide Antibiotics
Bacteria can develop resistance through two key mechanisms:
- Target Site Modification (MLS Phenotype)
- Efflux Pumps (M Phenotype)
- Target Site Modification (MLS Phenotype)
This mechanism is mediated by the erm (erythromycin ribosome methylation) gene, found on plasmids or transposons (small genetic elements that can move between bacteria).
Copies of the erm gene are transferred to other bacteria via pore channels.
Once inside the new bacterium, the gene is transcribed and translated, producing an enzyme that methylates the 50S ribosomal subunit at a specific position.
This modification reduces macrolide binding affinity, making the antibiotic ineffective.
This resistance can also extend to lincosamides and streptogramin B, creating the MLS phenotype.
As a result, the bacterium continues protein synthesis unaffected.
- Efflux Pumps (M Phenotype)
Resistance is mediated by the mef(A) gene, a transposable element.
This gene codes for efflux pumps, which are energy-dependent transporters that remove macrolide antibiotics from the bacterial cell.
In Streptococcus pneumoniae, these pumps cause moderate resistance to macrolides but do not affect lincosamides or streptogramins.
However, in Staphylococcus aureus, a different plasmid-mediated efflux system encoded by the msr(A) gene provides resistance to macrolides, lincosamides, and streptogramins.
Even if macrolide antibiotics enter the bacterial cell, efflux pumps
actively expel them before they reach the 50S ribosomal subunit, allowing bacteria to continue protein synthesis and survive.
