How are antibiotics classified and what factors influence their selection?

From the Guidelines

Antibiotics are classified into three main groups: Access, Watch, and Reserve, based on their effectiveness, resistance potential, and side effects, as recommended by the World Health Organization's AWaRe framework 1. These groups are used to guide the selection of antibiotics for empiric treatment of clinical infections. The Access group includes antibiotics that are recommended as first- or second-choice treatment options for common clinical infections, such as beta-lactams (penicillins, cephalosporins, carbapenems), macrolides (azithromycin, erythromycin), and fluoroquinolones (ciprofloxacin, levofloxacin) 1. The Watch group includes antibiotic classes that have a higher risk of selection of antibiotic-resistant bacteria and should be targets of antimicrobial monitoring and stewardship programs, such as fluoroquinolones and carbapenems 1. The Reserve group includes antibiotics that should be used only for highly specific patient populations and settings when other alternatives would be inadequate or have already failed, such as last-resort options for multidrug-resistant bacteria 1.

When selecting an antibiotic, several factors must be considered, including:

• The suspected or confirmed pathogen
• The site of infection
• Patient allergies
• Local resistance patterns
• Pharmacokinetics
• Potential side effects
• Drug interactions
• Patient comorbidities
• Pregnancy status
• Cost

For example, amoxicillin (500mg three times daily for 7-10 days) might be appropriate for uncomplicated community-acquired pneumonia in a patient without penicillin allergy, while levofloxacin (750mg daily for 5 days) might be chosen for a patient with penicillin allergy 1. Narrow-spectrum antibiotics should be preferred when possible to reduce resistance development and minimize disruption to normal flora. Empiric therapy should be based on the most likely pathogens for a specific infection site, then narrowed once culture results are available. Appropriate dosing, timing, and duration are essential for effective treatment while minimizing adverse effects and resistance development.

It is also important to consider the principles of choice of antibiotics used, including the spectrum of activity of the antibiotic and the main bacteria involved in the surgical site infection 1. Additionally, factors such as clinical severity of the infection, presence of bone infection, and history of antibiotic therapy within the previous 3 months should be taken into account when selecting an antibiotic 1.

Overall, the selection of antibiotics should be guided by the most recent and highest-quality evidence, and should prioritize the use of narrow-spectrum antibiotics and the minimization of adverse effects and resistance development.

From the FDA Drug Label

To reduce the development of drug-resistant bacteria and maintain the effectiveness of clarithromycin extended-release tablets and other antibacterial drugs, clarithromycin extended-release tablets should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. In the absence of such data, local epidemiology and susceptibility patterns may contribute to the empiric selection of therapy.

Antibiotic Groups: Antibiotics can be classified into several groups based on their mechanism of action, spectrum of activity, and chemical structure. The main groups include:

• Macrolides: such as clarithromycin 2 and azithromycin 3, which inhibit protein synthesis by binding to the bacterial ribosome.
• Aminoglycosides: such as tobramycin 4, which inhibit protein synthesis by binding to the bacterial ribosome and interfering with the initiation complex.
• Penicillins: which inhibit cell wall synthesis by binding to penicillin-binding proteins.
• Cephalosporins: which inhibit cell wall synthesis by binding to penicillin-binding proteins.
• Fluoroquinolones: which inhibit DNA replication by binding to DNA gyrase and topoisomerase IV.
• Tetracyclines: which inhibit protein synthesis by binding to the bacterial ribosome.

Factors Influencing Selection: The selection of an antibiotic depends on several factors, including:

• Susceptibility of the causative organism: The antibiotic should be effective against the specific bacteria causing the infection.
• Severity of the infection: More severe infections may require broader-spectrum antibiotics or combination therapy.
• Patient factors: Such as age, renal function, and allergies, which may affect the choice of antibiotic.
• Local epidemiology and susceptibility patterns: The prevalence of antibiotic-resistant bacteria in the local area should be considered when selecting an antibiotic.

From the Research

Antibiotic Classification

Antibiotics can be classified based on their mechanism of action, which includes:

• Cell-wall inhibitors
• Nucleic acid inhibitors
• Protein-synthesis inhibitors 5
• Inhibitors of the biosynthesis of the bacterial cell wall
• Inhibitors of the bacterial protein synthesis
• Inhibitors of the nucleic acid metabolism
• Membrane-active antibiotics 6

Factors Influencing Antibiotic Selection

The selection of antibiotics is influenced by several factors, including:

• Antimicrobial spectrum
• Mechanism of action
• Generation of the antibiotic
• Pharmacokinetic characteristics
• Indications for use 5
• Patient population, such as hospitalized adults with community-acquired pneumonia 7, 8
• Combination therapy, such as beta-lactam plus aminoglycoside or beta-lactam plus fluoroquinolone 9

Antibiotic Groups

Some common antibiotic groups include:

• Beta-lactams (e.g. cefepime, piperacillin/tazobactam) 9
• Fluoroquinolones (e.g. ciprofloxacin, levofloxacin) 7, 9, 8
• Macrolides (e.g. azithromycin) 7, 8
• Aminoglycosides (e.g. gentamicin) 9