Antibiotic Modes of Action and Clinical Uses

Penicillin

Mode of Action: Penicillins include Penicillium notatum and Penicillium chrysogenum, as well as synthetic and semi-synthetic variants like penicillin V. Penicillin is a mold derivative that inhibits cell wall synthesis in bacteria and was the earliest known antibiotic to be studied scientifically. Members of the penicillin family have a ring-like structure formed primarily by the amino acids valine and cysteine. The penicillin family works by preventing the cross-linking of peptides on mucosaccharide chains in target cells. When this occurs, the cell wall is compromised, and bacterial cells fill with water, causing them to burst and die (Ophardt, 2003). Their direct effect on bacterial cell walls distinguishes penicillins from other antibiotic classes.

Clinical Uses: Penicillins are among the oldest known and classically prescribed antibiotics. However, because bacterial strains have become resistant to penicillin, their effectiveness is limited. Additionally, penicillins are mild and can only treat mild to moderate infections. They are effective only against gram-positive bacteria, not gram-negative organisms such as Mycobacterium tuberculosis (The Microbial World, n.d.). Semi-synthetic penicillins like ampicillin, carbenicillin, and oxacillin may be effective against some gram-negative bacteria.

Adverse Effects: Allergic reactions are common with penicillin use. Other side effects include indigestibility-related problems such as mild diarrhea, headaches, and sores on the tongue and in the mouth. Additional effects include rashes and joint pain ("Penicillin Side Effects," n.d.).

Gentamicin

Mode of Action: Gentamicin prevents bacterial growth and can also kill existing bacteria. Its primary mechanism is interrupting peptide synthesis, which is necessary for bacterial reproduction. It disrupts the ability of bacilli to accurately translate their genes. Gentamicin binds to the aminoacyl site of 16S ribosomal RNA within the 30S ribosomal subunit, "leading to misreading of the genetic code and inhibition of translocation" (Drew, 2014). Gentamicin also inhibits oxygen transport in target organisms and causes anaerobiasis (Itokazu, n.d.). Additionally, gentamicin prevents the biosynthesis of DNA present in the proteins of Staphylococcus aureus (Abou-Zeid, Eissa & Salem, 1978).

Clinical Uses: Gentamicin is one of the most frequently prescribed antibiotics in the aminoglycoside family, a group that also includes tobramycin and amikacin (Drew, 2014). Aminoglycosides like gentamicin work by interrupting peptide synthesis and disrupting gene translation, which is why they are indicated for serious infections that might not respond to other drugs, such as those caused by gram-negative bacilli. They are often effective for hospital-acquired infections like staph infections, which can be robust, resilient, and resistant to other antibiotics. Furthermore, aminoglycosides like gentamicin are broad-spectrum antibiotics, effective for both gram-positive and gram-negative infections.

Adverse Effects: Gentamicin causes known toxicity problems including nephrotoxicity, ototoxicity, and liver toxicity. Because toxicity is a significant concern, gentamicin is not a first-choice antibiotic but is prescribed only when necessary. Not all potentially developing toxicities are benign; some are irreversible. For example, ototoxicity related to vestibular issues and associated impacts on hearing are major complications of gentamicin use (Itokazu, n.d.). Likewise, tinnitus is a common adverse reaction. Other reactions include vomiting, nausea, loss of appetite, skin problems, and headache. The drug's effectiveness increases with dose, but so do the side effects.

Mode of Action: Azithromycin is processed in the liver and is part of the azalide class of macrolide antibiotics, related to erythromycin. It works by inhibiting bacterial protein synthesis by binding to ribosomes on bacterial cells. When binding occurs, peptidyl transferase activity cannot take place. This impedes amino acid translocation during RNA and DNA translation and prevents the growth and reproduction of the bacteria ("Azithromycin," n.d.). Like other antibiotics, azithromycin is effective only for bacteria and not for viral infections.

Azithromycin

Clinical Uses: Treating infections caused by Hemophilus influenzae, Streptococcus pneumoniae, Mycoplasma pneumoniae, Staphylococcus aureus, and Mycobacterium avium are among the most common prescribed uses for azithromycin ("Zaiqi Granules," n.d.). Pneumonia due to Streptococcus pneumoniae, Haemophilus influenzae, or Mycoplasma pneumoniae represents particularly important clinical applications. The antibiotic can also be used for certain types of urethritis, nasosinusitis, tympanitis, acute bronchitis, cervicitis, and acute tonsillitis that has caused acute pharyngitis or acute amygdalitis ("Zaiqi Granules," n.d.).

Adverse Effects: Liver problems are a major adverse effect due to the antibiotic's mode of action. Digestive issues such as diarrhea and loose stool also occur. Irritability and nervousness, as well as skin problems that may be enhanced by combinations with other drugs, have been reported ("Zaiqi Granules," n.d.).

Mode of Action: Like many antibiotics, doxycycline inhibits protein synthesis in target microorganisms by binding to the 30S ribosomal subunit. This prevents bacterial reproduction. It is primarily bacteriostatic in its action but may also be bacteriocidal.

Doxycycline

Clinical Uses: A tetracycline antibiotic, doxycycline is derived from oxytetracycline and has similar broad-spectrum antibacterial actions. It is indicated for both gram-negative and gram-positive organisms, making it a broad-spectrum antibiotic ("Doxycycline," n.d.). Common indications include respiratory infections such as sinusitis, chronic bronchitis, and various types of pneumonia, including Mycoplasma pneumoniae pneumonia, Streptococcus pneumoniae, Haemophilus influenzae, and Klebsiella pneumoniae. Doxycycline is also effective against bacteria causing urinary tract infections, including Klebsiella species, Enterobacter species, Escherichia coli, Streptococcus faecalis, and other organisms (Actavis, n.d.). It can also treat certain sexually-transmitted diseases, including infections caused by Chlamydia trachomatis and Calymmatobacterium granulomatis. Doxycycline is used to treat Rocky Mountain spotted fever, plague, cholera, anthrax, typhus fever, malaria, and other typhus group infections ("Doxycycline," n.d.).

Adverse Effects: Headache, dizziness, blurred vision, chills, diarrhea, jaundice, loss of appetite, vaginal discharge, and upset stomach are among the most commonly reported side effects. However, the prevalence of specific side effects is not well documented in the clinical literature, and patients should report all side effects to their physician immediately. Doxycycline is generally not recommended for children or pregnant women. Despite this, its broad application provides doxycycline with significant clinical utility and potential in numerous disease scenarios.

Mode of Action: Cotrimoxazole is a pyrimidine inhibitor of dihydrofolate reductase that interferes with folic acid metabolism in target organisms ("Trimethoprim," n.d.). Impeding folic acid metabolism kills bacteria and prevents reproduction. Cotrimoxazole is formed by the synergistic interaction of trimethoprim with sulfamethoxazole, typically combined in a 1:5 ratio ("Trimethoprim," n.d.). As a sulfonamide, cotrimoxazole inhibits folate synthesis and kills bacteria by blocking the production of tetrahydrofolic acid through "inhibition of the enzyme dihydrofolate reductase" ("Trimethoprim/Sulfamethoxazole," n.d.).

Clinical Uses: Urinary tract infections, shigellosis enteritis, Pneumocystis carinii pneumonitis, acute otitis, acute exacerbations of chronic bronchitis, gastrointestinal infections, and cholera represent only some of the many clinical uses for cotrimoxazole ("Trimethoprim/Sulfamethoxazole," n.d.). It can be prescribed as a prophylactic for travelers visiting regions where they may be exposed to pathogens such as Escherichia coli or infectious diseases.