Antibiotics: Classification and Uses

Antibiotics: Classification and Uses

An antibiotic is a chemical produced by or derived from microorganisms (i.e. germs such as bacteria and fungi or bugs). Antibiotics are among the most frequently prescribed medications in modern medicine. Antibiotics cure disease by killing the bacteria or by bacterial reproduction and growth inhibition. Some antibiotics can be used to treat a wide range of infectious diseases and are known as “broad-spectrum” antibiotics. Others are only effective against a few types of bacteria and are called “narrow-spectrum” antibiotics.

Bacterial resistance

The treatment of infectious disease is compromised by the development of antibiotic-resistant strains of microbial pathogens. Antibiotic-resistant bacteria are germs that cannot be killed by commonly used antibiotics. When bacteria are exposed to the same antibiotics over and over, a variety of biochemical processes occur within these bacteria. These processes may keep antibiotics out of the cell, alter the target of the drug, or disable the antibiotic allowing bacteria to change and no longer be affected by this drug.

The rapid spread or development of antibiotic resistance may compromise standard empiric treatment of many infectious diseases. Resistant bacteria do not respond to the antibiotics and continue to cause infection. Indeed, the greatest threat to successful antibiotic coverage, and hence the driving force behind the search for new therapies, is the evolution and spread of antibiotic resistance.

Beta-lactamases are enzymes produced by some bacteria and are responsible for their resistance to beta-lactam antibiotics like penicillins, cephalosporins, cephamycins and carbapenems. Beta-lactam antibiotics are mainly used to treat a broad spectrum of gram positive and gram-negative bacteria.

Often beta-lactam antibiotics are combined with bacterial beta-lactamase inhibitors in order to be able treat infections caused by bacteria resistant to beta-lactam antibiotics (for example ampicillin/sulbactam, ticarcillin/clavulanate, amoxicillin/clavulanic acid, piperacillin/tazobactam, etc). Nowadays these combined drugs are called next generation antibiotics.

Types of antibiotics

There are many different types of antibiotics. The type of prescribed antibiotics depends on the type of infection and what kinds of antibiotics are known to be effective.

The main classes of antibiotics include:

  • Macrolides
  • Aminoglycosides
  • Cephalosporins
  • Fluoroquinolones
  • Penicillins
  • Tetracyclines
  • Carbapenems


Macrolides are erythromycin-like antibiotics. They are used to treat respiratory tract infections, genital, gastrointestinal tract, soft tissue infections caused by susceptible strains of specific bacteria. Some of the more recent additions to the macrolide group are azithromycin and clarithromycin. They work the same way as the other macrolides, but typically are more effective with fewer side effects.

The most commonly-prescribed macrolides include: erythromycin, clarithromycin, azithromycin (one of the world’s best-selling antibiotics), roxithromycin


The aminoglycosides are related bactericidal antibiotics, which are used to treat infectious diseases caused by gram-negative bacteria.  The utility of aminoglycosides as antibacterial agents arises from their specificity of action. Sometimes these antibiotics may be used along with penicillins or cephalosporins to give more efficient attack on the bacteria. Aminoglycosides work fine, however bacteria can gain the resistance to them. These antibiotics must be injected and cannot be given by the mouth, because they are broken down easily in the stomach and lose their action. Generally, aminoglycosides are given for short periods of time.

The most commonly-prescribed aminoglycosides include: amikacin (has high resistance against bacterial inactivation), gentamicin, kanamycin, neomycin, streptomycin, tobramycin.

Cephalosporin antibiotics cover a broad range of organisms, are easy to administer and generally well-tolerated. That is why they are the most frequently used class of antibiotics. Cephalosporins are related to the penicillin’s and grouped together based upon a shared structural feature, the beta-lactam ring.

Based on their spectrum of antimicrobial activity cefalosporines are grouped into generations. The later generation cephalosporins have greater gram negative antimicrobial properties and greater effect against resistant bacteria.

Cephalosporins are used to treat pneumonia, strep throat, staph infections, tonsillitis, bronchitis, otitis media, various types of skin infections, gonorrhea.

The most commonly-prescribed cephalosporins are:

  • First generation: cephazolin; cefadroxil; cephalexin; cephradine
  • Second generation: cefaclor; cefamandole; cefonicid; ceforanide; cefuroxime
  • Third generation: cefotaxime; cefixime; cefpodoxime; ceftazidime; cefdinir, cefoperazone
  • Fourth generation: cefepime;  cefpirome


Fluoroquinolones are broad-spectrum (effective against many bacteria) antibiotics, that exhibit concentration-dependent bactericidal activity.  Fluoroquinolones are used to treat most common urinary tract infections, bacterial prostatitis, skin infections, and respiratory infections (like pneumonia, sinusitis, bronchitis, etc)

The most commonly-prescribed fluoroquinolones are: ciprofloxacin; gatifloxacin; levofloxacin; gemifloxacin; ofloxacin; trovafloxacin

For ages, doctors and scientists were looking for something to kill germs without harming the patients. Discovered by Alexander Fleming in 1929, penicillin was the first antibiotic to be able to provide this action.

Penicillins are used to treat skin infections, dental infections, ear infections, urinary tract infections, respiratory tract infections, gonorrhea.  To improve the efficiency of the drug, penicillins are sometimes combined with other ingredients called beta-lactamase inhibitors, which protect the penicillin from destructive bacterial enzymes.

The most commonly-prescribed penicillins: amoxicillin, ampicillin – has been used extensively to treat bacterial infections; ampicillin/sulbactam- combination of ampicillin and sulbactam, an inhibitor of bacterial beta-lactamase; pipperacillin; piperacillin/tazobactam; oxacillin; penicillin.

Tetracyclines are a family of broad spectrum antibiotics that are active against many species of bacteria.

Tetracyclines are commonly used in the treatment of infections of the respiratory tract, streptococcus pneumonia, sinuses, middle ear, urinary tract, intestines, and also anthrax infection, gonorrhea, plaque, malaria elephantiasis, cholera and others.

The most commonly-prescribed tetracyclines are: tetracycline, doxycycline, minocycline (has a broader spectrum than the other members of the group).


Carbapenems are members of the beta-lactam family of antibiotics. They are active against many important Gram-positive and Gram-negative pathogens (broad spectrum antibiotics). One of the important features of carbapenem antibiotics is their high resistance to several beta-lactamases. All these features make carbapenems are most significan class of antibiotics currently in clinical use.

The most commonly-prescribed carbapenems are: meropenem- a broad-spectrum antibacterial effective against a wide variety of microrganisms;  imipenem -has a broad spectrum of activity against aerobic and anaerobic Gram positive as well as Gram negative bacteria;  imipenem/cilastatin- is a broad spectrum beta-lactam antibiotic containing equal quantities of imipenem and cilastatin, addition of cilastatin prevents renal inactivation of imipenem, resulting in more effective antibiotic (imipenem resists actions of many enzymes that degrade most other penicillins and penicillin-like drugs).

How to use antibiotics

Antibiotics are usually taken orally; however, they can also be administered by injection, or applied directly to the affected part of the body.

Most antibiotics start to have an effect on an infection within a few hours after consumption. It is important to remember to complete the whole course of the medication to prevent the infection from coming back.