Aminoglycosides: Mode of Action and Mechanism of Resistance

A patient with gram-negative sepsis is started on gentamicin, and two things about the order puzzle a new student. The whole day's dose is given once, as a single large infusion, and the patient is booked for blood-level checks and a baseline hearing test.

Both make sense once you understand how aminoglycosides work: they kill harder the higher the peak concentration climbs, so one big dose beats several small ones, and the same power that kills bacteria can damage the kidney and the inner ear if levels run too high for too long.

Aminoglycosides are among the oldest antibiotics still in front-line use, and almost everything about them, their dosing, their toxicity, their spectrum, follows from their mechanism.

What are aminoglycosides?

Aminoglycosides are a class of bactericidal antibiotics that kill bacteria by binding the 30S subunit of the ribosome and corrupting protein synthesis. The group includes gentamicin, tobramycin, amikacin, streptomycin, neomycin, kanamycin, netilmicin, and the newer plazomicin.

They are highly polar, positively charged molecules, which is why they are not absorbed from the gut and must be given by injection for systemic infection.

They are active mainly against aerobic gram-negative bacteria, including Pseudomonas aeruginosa, and are used together with cell-wall agents against certain gram-positive infections.

Why aminoglycosides matter

Aminoglycosides remain essential for serious gram-negative infections, and they do something most antibiotics cannot: they act in synergy with cell-wall agents such as penicillins and vancomycin, which is why a low "synergy" dose of gentamicin is added to penicillin for enterococcal endocarditis.

Streptomycin and amikacin are also important antituberculosis and antimycobacterial drugs. Understanding the mechanism explains their unusual once-daily dosing, their signature kidney and ear toxicity, and why they simply do not work against anaerobes.

Structure

Aminoglycosides are built from an aminocyclitol ring (most often 2-deoxystreptamine) linked by glycosidic bonds to two or more amino sugars. The many amino and hydroxyl groups make the molecule strongly cationic and water-soluble. That charge is central to the whole class: it drives the drug's binding to the ribosome, prevents oral absorption, and helps explain how the drug is taken up (and how bacteria resist it).

Classification and members

Mode of action of aminoglycosides

Aminoglycosides are taken up into the bacterium in a step that depends on oxygen-driven transport across the membrane, then bind the 16S ribosomal RNA of the 30S subunit at the aminoacyl (A) site. This binding does two damaging things: it causes misreading of the messenger RNA, so the ribosome inserts the wrong amino acids and builds faulty proteins, and it interferes with initiation and translocation. The mistranslated proteins include membrane proteins, and once these defective proteins are inserted into the cell membrane they increase its permeability, which lets in still more drug in a self-amplifying loop. That is why aminoglycosides are bactericidal, an unusual property for a protein-synthesis inhibitor, and why their binding is effectively irreversible.

Two practical consequences follow directly from the mechanism. Because uptake needs oxygen-dependent transport, anaerobes cannot take the drug up and are intrinsically resistant to it (see Bacteria Associated with Intrinsic Antibiotic Resistance). And because cell-wall agents damage the envelope and let more aminoglycoside in, beta-lactams and vancomycin act in synergy with aminoglycosides.

Aminoglycosides are one of the two antibiotic classes that inhibit protein synthesis at the 30S subunit. To see where this sits among the five mechanisms of action of antibiotics, and how the 30S drugs compare with the 50S drugs, see the overview hub.

Bactericidal, concentration-dependent killing, and once-daily dosing

Aminoglycosides show concentration-dependent killing: the higher the peak concentration relative to the pathogen's MIC, the more bacteria are killed. They also have a long post-antibiotic effect, meaning bacterial growth stays suppressed even after drug levels fall. Together these two properties are the reason for once-daily (extended-interval) dosing: a single large dose maximizes the killing peak, and the low trough between doses gives the kidney and inner ear time to recover, which reduces toxicity. This is the opposite of the time-dependent beta-lactams, which are dosed frequently to keep levels above the MIC.

Clinical uses

Aminoglycosides are used for serious aerobic gram-negative infections, often in combination, including Pseudomonas infections; in synergy with a cell-wall agent for enterococcal and some streptococcal or staphylococcal endocarditis; for tuberculosis and other mycobacterial disease (streptomycin, amikacin); and for tularemia and plague (streptomycin, gentamicin). Neomycin is used topically and orally (it is not absorbed) for skin preparations and bowel preparation before surgery.

Side effects and monitoring

The two signature toxicities are nephrotoxicity and ototoxicity. Nephrotoxicity affects the proximal tubule and is usually reversible if caught early. Ototoxicity can be cochlear (hearing loss) or vestibular (balance problems) and is often permanent, which is why it is the more feared of the two. Aminoglycosides can also rarely cause neuromuscular blockade. Because the safe and toxic ranges are close, serum levels are monitored (or extended-interval dosing with nomograms is used), and hearing and kidney function are watched during prolonged therapy.

Mechanism of resistance to aminoglycosides

The ways bacteria resist aminoglycosides are specific examples of the general mechanisms of antibiotic resistance: enzymatic inactivation, target modification, and reduced uptake or efflux.

Enzymatic inactivation (the main mechanism). Bacteria produce aminoglycoside-modifying enzymes (AMEs) that chemically alter the drug so it can no longer bind the ribosome. There are three families, named for the chemical group they add: acetyltransferases (AAC), nucleotidyltransferases or adenylyltransferases (ANT), and phosphotransferases (APH). These enzymes are usually carried on plasmids and transposons and spread readily. Amikacin and plazomicin are deliberately designed to resist many of these enzymes, which is why they often remain active when gentamicin and tobramycin fail.

Target modification. The bacterium changes the ribosomal target. High-level, broad resistance comes from 16S rRNA methyltransferases (for example armA and rmt genes) that methylate the binding site so no aminoglycoside can bind. A single ribosomal protein mutation (rpsL) is a classic cause of streptomycin resistance.

Reduced uptake and efflux. Decreased membrane permeability lowers drug entry, and efflux pumps remove the drug; both are important in Pseudomonas aeruginosa.

Intrinsic resistance is a separate point: anaerobes are naturally resistant because aminoglycoside uptake needs oxygen, not because they acquired anything.

How to remember

The members, "GNATS": Gentamicin, Neomycin, Amikacin, Tobramycin, Streptomycin. A useful extension is "mean GNATS canNOT kill anaerobes," which captures both the toxicity (mean) and the intrinsic anaerobe gap (no oxygen-driven uptake, so no killing).

Where they act: 30S, paired with tetracyclines in the hub mnemonic "buy AT 30" (Aminoglycosides, Tetracyclines). The twist is that aminoglycosides kill while tetracyclines only stall.

The dosing logic in one line: high peak kills (concentration-dependent), long gap heals (post-antibiotic effect plus recovery time), so give it once a day.

Where students actually get confused

Why aminoglycosides are bactericidal when other protein-synthesis inhibitors are not. Most 30S and 50S drugs simply pause protein production, which is reversible. Aminoglycosides cause misread membrane proteins that damage the membrane and let in more drug, and their binding is effectively irreversible, so the cell dies.

Aminoglycosides and tetracyclines both hit the 30S, but with opposite results. Aminoglycosides cause misreading and kill; tetracyclines block tRNA entry and are only bacteriostatic.

Once-daily dosing feels wrong but is correct. Because killing is concentration-dependent and there is a long post-antibiotic effect, one large daily dose is both more effective and less toxic than several small ones.

Nephrotoxicity versus ototoxicity. Kidney toxicity is usually reversible; ear toxicity (hearing or balance) is often permanent. That is why hearing is the toxicity clinicians worry about most.

Why they fail against anaerobes. It is intrinsic resistance: uptake needs oxygen-dependent transport, which anaerobes lack, so the drug never gets in.

Key exam facts

References

1. Krause KM, Serio AW, Kane TR, Connolly LE (2016). Aminoglycosides: an overview. Cold Spring Harbor Perspectives in Medicine. 6(6): a027029.
2. Ramirez MS, Tolmasky ME (2010). Aminoglycoside modifying enzymes. Drug Resistance Updates. 13(6): 151-171.
3. Mingeot-Leclercq MP, Glupczynski Y, Tulkens PM (1999). Aminoglycosides: activity and resistance. Antimicrobial Agents and Chemotherapy. 43(4): 727-737.
4. Katzung BG (ed.) (2021). Basic and Clinical Pharmacology. 15th edn. McGraw Hill.