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* They are active mainly against gram-negative species, including some of the hardier species.
* They are active mainly against gram-negative species, including some of the hardier species.
* There is variable gram-positive activity.
* They are inactive against anaerobes, this is because their action is dependent on oxygen-dependent active transport systems.
* They are inactive against anaerobes, this is because their action is dependent on oxygen-dependent active transport systems.
* They tend not to work well in purulent material.
* They tend not to work well in purulent material.
* If the local tissue environment is acidic the efficacy of the drug will be reduced.
* If the local tissue environment is acidic the efficacy of the drug will be reduced.
===Pharmacokinetic Considerations===
===Pharmacokinetic Considerations===
Aminoglycosides are very hydrophilic molecules and weak bases and so aren't very well absorbed in the gastrointestinal tract. They distribute poorly and are confined to mainly extracellular fluid. Elimination is performed by the kidneys and the drugs are excreted unchanged in the urine and so have very short half-lives. They also bind to renal tissue and so shouldn't be used in patients with renal failure.
Aminoglycosides are very hydrophilic molecules and weak bases and so aren't very well absorbed in the gastrointestinal tract. They distribute poorly and are confined to mainly extracellular fluid. Elimination is performed by the kidneys and the drugs are excreted unchanged in the urine and so have very short half-lives. They also bind to renal tissue and so shouldn't be used in patients with renal failure.
===Side Effects and Contraindications===
===Side Effects and Contraindications===
* Ototoxicity is a problem with the Vestibulocochlear Nerve (Cranial Nerve VIII) affected.
* Ototoxicity is a problem with the Vestibulocochlear Nerve (Cranial Nerve VIII) affected.
* They can create non-depolarising neuromuscular junction blocks leading to acute paralysis and cardiovascular collapse if given intravenously.
* They can create non-depolarising neuromuscular junction blocks leading to acute paralysis and cardiovascular collapse if given intravenously.
==Streptomycin==
* The weakest spectrum of activity of the aminoglycosides. It is active against some ''Staphylococcal'' species and is active against mycobacteria.
* Reistance to this drug is widespread.
* It's use is limited by its toxicity, especially in cats. It is main toxicity problem is with ototoxicity. If used post-anaesthesia there is a higher risk of the patient experiencing a neuromuscular block.
==Neomycin==
* More active than Streptomycin but still not very active. Good activity against ''S.aureus'' but is poor against other gram-positives. Though it is active against most gram-negatives including ''Pseudomona''.
* This is the most toxic of all aminoglycosides and is mainly nephrotoxic and toxic to the cochlear.
==Kanamycin==
* Similar potency to Neomycin but is active against mycoplasma and mycobacteria. It isn't effective against ''Pseudomonas''.
* It has a wider therapeutic index than neomycin, but the normal toxicity risks still stand.
* Cross-resistance exists with neomycin.
==Gentamicin==
* The most broad spectrum of the group. It works well against ''Pseudomonas'' and gram-negative rods.
* It is very nephrotoxic and shouldn't be used for more than seven days and it is recommeneded to only give once daily doses.
* Resistance is currently uncommon in veterinary pathogens.
==Amikacin==
* More active than Kanamycin but not as broad spectrum as Gentamicin.
* Standard concerns about toxicity remain.
==Tobramycin==
* Similar level activity against pathogens as Gentamicin but with the least toxicity of all aminoglycosides.
* It is currentlt too expensive to use in the veterinary field.
==Aminocyclitols==
This is a closely related group of drugs that are essentially the same as aminoglycosides.
===Apramycin==
* It's full spectrum is not yet known but it is active against ''S.aureus'', many ''Streptococci'', ''Treponema hyodysenterae'' and some mycoplasmas.
* There are few reports of toxicity except nephrotoxity in cats.
==Spectinomycin==
* Poor gram-positive activity but works well against gram-negatives and mycoplasmas.
* Not very toxic but has been known to cause neuromuscular junction blockades.
* It has the highest oral bioavailability out of any of the drugs in this group.