Antibiotics are chemotherapeutic agents used to inhibit or kill bacteria (prokaryotic organisms). How do they destroy bacteria without hurting our cells?

There are many different classes of antibiotics, each exerting a different type of inhibitory effect that specifically impacts bacteria. Antibiotic aminoglycosides work by inhibiting protein synthesis in bacteria and compromising the structure of the bacterial cell wall.

Antibiotic Class Article Series

Each article in this series covers a specific class of antibiotic and includes the following information:

• a list of the specific antibiotics that fall in the class (generic and brand names)
• mode of action
• type of infection the antibiotic class to against
• type of bacteria inhibited or killed
• pros and cons of using the class of antibiotics

Some of the main classes of antibiotics include:

• Aminoglycosides
• Cephalosporins (Beta-lactam)
• Macrolides
• Penicillin (Beta-lactam)
• Quinolones
• Sulfonamides
• Tetracyclines

Aminoglycosides

Aminoglycoside antibiotics are used to treat serious infections caused by gram-negative bacteria.

Some commonly used aminoglycosides include:

• amikacin (Amikin®)
• apramycin
• capreomycin
• gentamicin (Garamycin®)
• kanamycin (Kantrex®)
• neomycin (Mycifradin®)
• netilmicin (Netromycin®)
• paromomycin (Humatin®)
• streptomycin
• tobramycin (TOBI Solution®, TobraDex®, Nebcin®)

Mode of Action

Although aminoglycosides stop bacteria from making proteins, it is uncertain whether this is the action that results in bacterial cell death.

Disruption of Protein Synthesis

Aminoglycosides bind to the bacterial 30S ribosomal subunit. Ribosomes are the protein factory of the cells. They are composed of two subunits, in bacteria a 30S and a larger 50S. By binding to the ribosome, aminoglycosides inhibit the translocation of tRNA during translation and leaving the bacterium unable to synthesize proteins necessary for growth.

Although the eukaryotic cells of humans also have ribosomes, these cellular protein factories differ in size and structure from the ribosomes of prokaryotes. That is why aminoglycosides do not interfere with protein synthesis in human cells.

Bacterial Cell Wall Damage

But it may not be the inhibition of protein synthesis that produces the bactericidal (bacteria killing) effect. Aminoglycosides also appear to displace cations in the bacterial cell biofilm that are responsible for linking the lipopolysaccharide (LPS) molecules characteristic of Gram-negative bacterial cell walls.

This creates holes in the cell wall that may kill the bacteria before the aminoglycoside even reaches the ribosome. Because our cells do not have cell walls (some of the chemical structural components of the bacterial cell wall are found only bacteria) human cells are not susceptible to these destructive actions.

Type of Infection Aminoglycosides Are Used For

Aminoglycosides are most often used in empiric therapy (treatment initiated before a firm diagnosis is made) for serious infections including septicemia, complex infections within the abdominal cavity, severe urinary tract infections, and nosocomial (hospital acquired) respiratory infections.

Because of the toxicity of aminoglycosides, are only given for only a short period of time. Less toxic antibiotics are substituted once the causative agent of infection is positively identified.

Antimicrobial Spectrum of Aminoglycosides

Gram-negative Aerobic Bacteria

Aminoglycosides exert their antibiotic effect against aerobic (oxygen requiring), gram-negative bacteria, including Pseudomonas, Acinetobacter, and Enterobacter.

Some species of Mycobacteria, including the causative agent of tuberculosis, are also susceptible to aminoglycosides.

Gram-positive Bacteria

Gram-positive bacterial infections can also be treated with aminoglycosides, but are typically not used because of the toxicity. Aminoglycosides have a synergistic effect when combined with beta-lactam antibiotics (penicilins) and have been used to treat streptococcal infections. The ampicillin (a beta-lactam antibiotic) gentamicin combination is referred to as "amp and gent" or "pen and gent" for penicillin and gentamicin.

Pros and Cons of Using Aminoglycosides

Because aminoglycosides are quickly broken down in the stomach, these antibiotics can't be given orally, but instead must be injected.

The adverse effects of aminoglycosides include potential damage to the ears and kidneys. The risk can be reduced by monitoring blood levels of the drug and maintaining the dose at levels that will kill bacteria without causing harmful side effects.

This article is for informational purposes only and is not mean to be used in the treatment of bacterial infections. If you are ill, see a medical professional.

More Antibiotic Resources

The information in this article was obtained from a variety of sources, including the textbook Foundations in Microbiology by Kathleen Park Talero and Yuri Bayarski’s article Antibiotics and Their Types, Uses and Side Effects. To learn more about bacteria and prokaryotes in general, see the microbiology information on the science website Science Prof Online.