Antibiotics that are effective against both Gram-positive and Gram-negative bacteria are called broad-spectrum antibiotics.
Narrow spectrum antibiotic:
Antibiotics that are effective against Gram-positive or Gram-negative bacteria are called narrow-spectrum antibiotic
How Anti-Microbials act?
5 principal sites of action:
1. The cell wall. Example: Penicillin e Cephalosporins
2. The cytoplasmic membrane. Example: Antifungals like Amphotericin & Nystatin
3. Inhibition of protein synthesis. Example: chloramphenicol, Tetracyclines, Erythromycin, Aminoglycosides
4. Nucleic acid metabolism. Example: Nalidixic acid, Rifampicin, and Quinolones
5. Intermediary metabolism. Example: Trimethoprim and Sulfonamides
A mechanism by which micro-organism develops resistance to a drug to which it was previously sensitive. Bacteria develop resistance against antibiotics (drugs).
Drug (antibiotic) is resistant against that bacteria.
How microorganism acquires resistance?
1. By spontaneous mutation (an alteration in a gene)
2. By transmission of genes from other organisms Plasmid-mediated resistance.
Plasmid Mediated Resistance:
Occurs predominantly among Enterobacteriaceae, Staphylococci, Pneumococci, and Neisseria
Plasmid (extrachromosomal DNA) can easily pass from one bacterium to another; lack of plasmid-mediated resistance results in a low incidence of developing resistance.
Microorganism showing resistance to one drug may also show resistance to a second drug that has a similar mechanism of action or with a similar structure even without being exposed to the second drug. Such a phenomenon is called cross-resistance.
Penicillin, ampicillin, Amoxycillin are effective against them. They are called Penicillin sensitive Staphylococcus Aureus.
Staphylococcus aureus that does not produce penicillinase enzyme is called non-penicillinase producing Staphylococcus aureus.
The strains of Staphylococcus aureus that can produce penicillinase enzyme are called Penicillinase producing Staphylococcus aureus. Over 90% of strains of the Staphylococcus aureus are beta-lactamase producing.
They can break down the beta-lactam ring of penicillin, amoxicillin but not that of cloxacillin. So they are called penicillin-resistant Staphylococcus.
Methicillin-resistant strains of Staphylococcus Aureus (MRSA):
Staphylococcus aureus resistant to all beta-lactam antibiotics such as penicillin, ampicillin, cloxacillin, are called Methicillin-resistant strains of Staphylococcus aureus (MRSA). Generally resistant to all beta-lactam agents including cephalosporins. Often resistant to Erythromycin, Gentamicin, etc. Ciprofloxacin is effective against MRSA.
It is derived from Penicillium notatum.
Example: Semi-synthetic penicillin is Benzylpenicillin Penicillin V, Ampicillin, Amoxycillin, etc.
The basic structure of these agents contains a 4 membered beta-lactam ring and a 5 membered thiazolidine ring.
Penicillinase Resistant Penicillin:
Penicillins that are stable or resistant to beta-lactamase enzyme because of the addition of a side chain to the penicillin nucleus, are called penicillinase-resistant penicillin.
Chemical name: Isoxazolyl penicillins. These are semisynthetic penicillins.
Chemically related to penicillins. Both have a beta-lactam ring in their structure. Both penicillin and cephalosporin act on the cell wall of bacteria.
Dissimilarity with penicillin:
Penicillins contain a 5 membered ring fused within beta-lactam ring. Cephalosporins contain 6 membered rings.
The difference in structure gives cephalosporins more resistance to the action of the beta-lactamase enzyme. As penicillins and cephalosporins have similarities in structure, they may very rarely show cross allergenicity.
Divided into four major groups or generations depending mainly on the microbial spectrum of activity.
1st generation: Cephalexin, Cephradine
2nd generation: Cefuroxime, Cefaclor
3rd generation: Cefixime, Ceftriaxone, Cefdinir
4th generation: Cefepime
Mechanism of action:
Mechanism of action:
Sulfonamide: This is the first chemical substance obtained by chemical synthesis. Example: Cotrimoxazole
Cotrimoxazole is a combination of Trimethoprim and Sulfamethoxazole. Broad-spectrum and Bactericidal
Cotrimoxazole inhibits folic acid synthesis and ultimately nucleic acid synthesis is inhibited.
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