Inhibitors of Cell Wall Synthesis: Polypeptide Antibiotics & Isoniazid: Videos & Practice Problems

Polypeptide antibiotics like bacitracin and vancomycin disrupt bacterial cell wall synthesis by targeting peptidoglycan formation, differing from beta-lactam antibiotics that inhibit cross-linking. Bacitracin is a topical, narrow-spectrum antibiotic effective mainly against gram-positive bacteria. Vancomycin, a glycopeptide, treats resistant infections such as MRSA but faces emerging resistance. Isoniazid, an antimycobacterial drug, inhibits mycolic acid synthesis, crucial for mycobacteria like tuberculosis and leprosy pathogens. These antibiotics exemplify targeted antimicrobial strategies, emphasizing the importance of understanding bacterial cell wall components and antibiotic resistance in clinical treatment and microbiology.

Polypeptide Antibiotics & Isoniazid

Some antibiotics disrupt bacterial cell wall synthesis without containing the characteristic beta-lactam ring found in penicillins and cephalosporins. These drugs include polypeptide antibiotics and antimycobacterial agents, which target different stages or components of cell wall formation.

Polypeptide antibiotics, composed of amino acid chains, are bactericidal and interfere with earlier steps in peptidoglycan synthesis rather than the cross-linking targeted by beta-lactam antibiotics. Bacitracin is a well-known example, commonly found in over-the-counter topical ointments like Neosporin. It is effective primarily against gram-positive bacteria due to its narrow spectrum of activity. Another important polypeptide antibiotic is vancomycin, classified as a glycopeptide antibiotic. Vancomycin is also narrow spectrum and targets gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), a pathogen notorious for its antibiotic resistance. However, resistance to vancomycin has begun to emerge, highlighting the need for careful use.

Antimycobacterial antibiotics specifically inhibit the synthesis of mycolic acid, a unique lipid component of the cell walls of mycobacteria. Isoniazid (abbreviated INH) is a key drug in this class, used primarily to treat tuberculosis (TB) and leprosy, diseases caused by mycobacteria. Because mycolic acid is exclusive to this bacterial group, isoniazid exhibits a very narrow spectrum of activity, targeting only mycobacteria. This specificity makes it crucial in managing TB, a major global health concern.

Understanding these antibiotics emphasizes the diversity of mechanisms by which bacterial cell wall synthesis can be disrupted. While beta-lactam antibiotics inhibit the cross-linking of peptidoglycan strands, polypeptide antibiotics like bacitracin and vancomycin interfere with earlier synthesis steps or peptide components, and antimycobacterial drugs like isoniazid block unique lipid synthesis essential for mycobacterial cell walls. This knowledge is vital for selecting appropriate treatments based on bacterial type and resistance patterns.

Which of the following drugs is available without a prescription in the US and targets cell wall synthesis?

• A. Vancomycin.
• B. Bacitracin.
• C. Neomycin.
• D. Isonizid.

Answer: (B)

A patient presents with a cough that has lasted 5 weeks, accompanied by blood in the sputum and fever. The pathogenic microbe determined to be bacterial is only visible with an acid-fast stain. Based on your knowledge of staining techniques and bacterial cell structure, which antibacterial drug would be appropriate?

• A. Vancomycin.
• B. Bacitracin.
• C. Monobactam.
• D. Isoniazid.

Answer: (D)

Here’s what students ask on this topic:

What are polypeptide antibiotics and how do they disrupt bacterial cell wall synthesis?

Polypeptide antibiotics are bactericidal drugs composed of amino acids that target bacterial cell wall synthesis but differ from beta-lactam antibiotics. Instead of inhibiting the cross-linking of peptidoglycan, polypeptide antibiotics interfere with an earlier step in peptidoglycan synthesis. Two key examples are bacitracin and vancomycin. Bacitracin is commonly used topically and is effective mainly against gram-positive bacteria. Vancomycin, a glycopeptide antibiotic, is also effective against gram-positive bacteria and is especially important for treating resistant infections like MRSA. Both antibiotics are narrow-spectrum, focusing on specific bacterial structures essential for cell wall integrity, which ultimately leads to bacterial death.

How does bacitracin work and what is its typical clinical use?

Bacitracin works by inhibiting the synthesis of peptidoglycan, an essential component of the bacterial cell wall, at an early stage before cross-linking occurs. It is primarily effective against gram-positive bacteria and is considered a narrow-spectrum antibiotic. Clinically, bacitracin is most commonly used as a topical antibiotic in over-the-counter first aid ointments, such as Neosporin, to prevent infection in minor cuts and wounds. Its topical use limits systemic exposure, reducing the risk of toxicity and resistance development.

What makes vancomycin an important antibiotic in treating resistant bacterial infections?

Vancomycin is a glycopeptide antibiotic that targets the synthesis of peptidoglycan in the bacterial cell wall, but unlike beta-lactams, it binds directly to the peptide chains, preventing cell wall formation. It is especially important for treating infections caused by methicillin-resistant Staphylococcus aureus (MRSA), a strain resistant to many other antibiotics. Vancomycin is narrow-spectrum, effective mainly against gram-positive bacteria. However, the emergence of vancomycin-resistant strains is a growing concern, highlighting the need for careful use and ongoing research into new treatments.

How does isoniazid target mycobacterial cell walls and why is it important in tuberculosis treatment?

Isoniazid specifically targets the synthesis of mycolic acid, a unique and essential lipid component of the mycobacterial cell wall. This action disrupts the integrity of the cell wall in mycobacteria, which include the bacteria that cause tuberculosis (TB) and leprosy. Because mycolic acid is unique to mycobacteria, isoniazid is a very narrow-spectrum antibiotic, effective only against this group. It is a cornerstone drug in TB treatment due to its ability to inhibit bacterial growth and kill the pathogen, making it critical in controlling and curing tuberculosis infections.

Why are polypeptide antibiotics and isoniazid considered narrow-spectrum antibiotics?

Polypeptide antibiotics like bacitracin and vancomycin are considered narrow-spectrum because they primarily target gram-positive bacteria by interfering with peptidoglycan synthesis, which is more accessible in these bacteria. Isoniazid is also narrow-spectrum but for a different reason: it targets mycolic acid synthesis, a component unique to mycobacteria. This specificity means these antibiotics are effective against a limited range of bacteria, focusing on particular structural features such as peptidoglycan or mycolic acid. This narrow spectrum helps reduce the impact on beneficial bacteria and limits the development of broad resistance.