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The drug bacitracin consists of a group of structurally similar peptide antibiotics originally isolated from Bacillus subtilis . Bacitracin blocks the activity of a specific cell-membrane molecule that is responsible for the movement of peptidoglycan precursors from the cytoplasm to the exterior of the cell, ultimately preventing their incorporation into the cell wall. Bacitracin is effective against a wide range of bacteria, including gram-positive organisms found on the skin, such as Staphylococcus and Streptococcus . Although it may be administered orally or intramuscularly in some circumstances, bacitracin has been shown to be nephrotoxic (damaging to the kidneys). Therefore, it is more commonly combined with neomycin and polymyxin in topical ointments such as Neosporin .

The top of the image shows diagrams of various antibiotics. All have a beta-lactam ring wich is a square made of 3 carbons and a nitrogen; one of the carbons has a double bonded O. The antibiotics shown are penicillin, cephalosporin, monobactam and carbapenem Below is a table with the rows: R group, Drug name, specrum of activity and route of administration. Penicillin G has an R group of a carbon linked to a  6 carbon ring; it is active on G+ and a few G- cells, and has a parenteral route of administration. Penicillin V has an R group of a carbon linked t an oxygen linked to a carbon ring. IT affects G+ and a few G- and is administered orally. Ampicillin has an R group of a Carbon attached to both an amine group and a carbon ring. It is affective agains G+ and more G- than penicillin. It is administered orally and parenterally. Amoxicillin has an R group similar to ampicillin but the carbon rign has an additional OH. It has similar activity to ampicillin and is administerd orally (better than ampicillin). Methiciliin has an R group of a carbon right with 2 CH3O attached to the ring. IT is affective against G+ only, including B-lactam producers. It is administered parenterally.
Penicillins, cephalosporins, monobactams, and carbapenems all contain a β-lactam ring, the site of attack by inactivating β-lactamase enzymes. Although they all share the same nucleus, various penicillins differ from each other in the structure of their R groups. Chemical changes to the R groups provided increased spectrum of activity, acid stability, and resistance to β-lactamase degradation.
Drugs that Inhibit Bacterial Cell Wall Synthesis
Mechanism of Action Drug Class Specific Drugs Natural or Semisynthetic Spectrum of Activity
Interact directly with PBPs and inhibit transpeptidase activity Penicillins Penicillin G, penicillin V Natural Narrow-spectrum against gram-positive and a few gram-negative bacteria
Ampicillin, amoxicillin Semisynthetic Narrow-spectrum against gram-positive bacteria but with increased gram-negative spectrum
Methicillin Semisynthetic Narrow-spectrum against gram-positive bacteria only, including strains producing penicillinase
Cephalosporins Cephalosporin C Natural Narrow-spectrum similar to penicillin but with increased gram-negative spectrum
First-generation cephalosporins Semisynthetic Narrow-spectrum similar to cephalosporin C
Second-generation cephalosporins Semisynthetic Narrow-spectrum but with increased gram-negative spectrum compared with first generation
Third- and fourth-generation cephalosporins Semisynthetic Broad-spectrum against gram-positive and gram-negative bacteria, including some β-lactamase producers
Fifth-generation cephalosporins Semisynthetic Broad-spectrum against gram-positive and gram-negative bacteria, including MRSA
Monobactams Aztreonam Semisynthetic Narrow-spectrum against gram-negative bacteria, including some β-lactamase producers
Carbapenems Imipenem, meropenem, doripenem Semisynthetic Broadest spectrum of the β-lactams against gram-positive and gram-negative bacteria, including many β-lactamase producers
Large molecules that bind to the peptide chain of peptidoglycan subunits, blocking transglycosylation and transpeptidation Glycopeptides Vancomycin Natural Narrow spectrum against gram-positive bacteria only, including multidrug-resistant strains
Block transport of peptidoglycan subunits across cytoplasmic membrane Bacitracin Bacitracin Natural Broad-spectrum against gram-positive and gram-negative bacteria

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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