Aminoglycosides are classified as broad-spectrum antibiotics, with greater activity against Gram-negative than Gram-positive bacteria

Pharmacology of essential antibacterials

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Aminoglycosides

Aminoglycosides are classified as broad-spectrum antibiotics, with greater activity against Gram-negative than Gram-positive bacteria. Resistance develops very rapidly. They are poorly absorbed from the gut, hence given by injection for systemic infections. All have the potential to produce ototoxicity and nephrotoxicity. Excretion is principally via the kidneys

Gentamicin

Gentamicin is currently the most important aminoglycoside for the treatment of infections caused by Gram-negative bacteria. Intrathecal gentamicin is indicated in the treatment of CNS infections. The medicine is not effective against most species of streptococci, diplococcic and anaerobic organisms

Precaution and side effects

Patients hypersensitive to one aminoglycoside may be hypersensitive to another aminoglycoside also. Patients under prolonged treatment with gentamicin should be observed for impairment of hearing or vertigo, as the agent is nephrotoxic. Gentamicin should be used with caution in patients with impaired renal function and dosage adjustment may be necessary

Drug interactions

Concurrent ad/or sequential administration of gentamicin with aspirin, frusemide and quinine should be avoided since the potential for ototoxicity and nephrotocity may be increased

Some aminoglycosides related to gentamicin are amikacin, kanamycin and tobramycin

Neomycin

This antibiotic is indicated in a wide variety of local infections including infected dermatoses, burns, wounds, ulcers, impetigo and conjunctivitis. Also for irrigation of the bladder and urethra during catheterization as prophylaxis. It is also incorporated into topical steroid preparations to control secondary infections in inflammatory disorders. Orally the medicine is used pre-operatively during surgery of the large bowel and in the treatment of intestinal infections

Precautions and side effects

Because of its potential toxicity the parenteral administration of neomycin is not recommended. Application to raw areas, wound and cavities and/or prolonged administration should be avoided as it may lead to ototoxicity

Drug interactions

Neomycin potentiates the action of phenindione, warfarin and other coumarin derivatives and some muscle relaxants like tuborcurarine, ethacrynic acid, frusemide and piretanide which increase the ototoxic effects of neomycin

The aminoglycoside which is related to neomycin is paromomycin

Streptomycin

Streptomycin is an aminoglycoside antibacterial agent. The medicine is effective in the treatment of both Gram-negative and Gram-positive bacteria. A major disadvantage and cause of failure of streptomycin therapy is the development of bacterial resistance to the medicine.

Indications

Its use has become limited after the development of more effective and less toxic antibacterial medicines. However, streptomycin still has important applications in the therapy of tuberculosis. It must always be used in combination with other effective antituberculous medicines to reduce the development of medicine-resistant bacteria.

Precautions and side effects

The unwanted effects include; skin rash, fever and blood dyscrasia, vertigo, dysfunction of the optic nerve and nausea.

Drug interactions

Streptomycin combined with penicillin G, ampicillin, cephalosporins or sulfonamide produces additive or supra-additive antimicrobial effects

Chloramphenicol

Chloramphenicol is broad-spectrum antibiotic. The medicine should be reserved for serious infections in which less toxic antibacterials are ineffective or contraindicated.

Chloramphenicol is indicated in meningitis, acute infections of typhoid fever, brain abscess and eye infections

Precautions and side-effects

Chloramphenicol is not indicated in routine treatment of typhoid carrier states and in the treatment of trivial infections (cold, influenza, or throat infections) due to risk of Grey syndrome. Chloramphenicol is not recommended for neonates, in pregnancy and in nursing mothers unless it is life-saving and there is no alternative due to serious side effects. Adverse effects include born marrow depression, blood dyscrasia, optic and peripheral neuritis.

Drug interactions

Concurrent use of chloramphenicol with anticoagulants, oral hypoglycemic drugs or phenytoin may increase the effects or toxicity of these medications because of inhibition of microsomal enzyme activity. In this case dosage adjustment may be necessary during and after chloramphenicol therapy.

Penicillins

Penicillins arebactericidal compounds with low toxicity. They act by inhibiting the bacterial cell wall synthesis. Penicillins difuse to most body fluids and tissues; penetration into the eye and across the normal meninges is poor. Inflammation of the meninges increases the amount of penicillin which cross the blood-brain barrier

Penicillins are excreted in the urine in therapeutic concentrations. Probenecid blocks renal tubular excretion of penicillins, resulting in high and prolonged plasma concentrations.

The most common side effect of penicillins is hypersensitivity, which causes rash and occasionally anaphylaxis, which may be fatal. Patients hypersensitive to one penicillin may be hypersensitive to other penicillins also. Other side-effets include fungal overgrowth, pseudomonas suprainfections and diarrhea for oral dosage forms. Due to sensitizing effects penicillins should never be used topically.

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Penicillins include: Ampicillin, penicillin G, Penicillin V, cloxacillin, amoxicillin

Cephalosporins

Cephalosporins belong to the broad-spectrum antibiotics and they are similar to penicillins in their mechanism of action. Some cephalosporins are inactivated by penicillinase. No cephalosporin is a first drug of choice for any infection. These medications are used when resistance to penicillins occurs

Cephalosporins include: cephalexin, cephalogylcin, cephaloridine, cephalothin

Sulphonamides

Sulphonamides have a wide range of antimicrobial activities against both Gram-positive and Gram-negative bacteria. In general these agents exert only bacteriostatic effects in the body, by inhibiting folic acid synthesis in bacteria. Sulphonamides in which the amino group is free readily absorbed via small intestine. Those with an ionisable N-acyl group are poorly absorbed. Sulphonamides are the medicines of choice for few infections because of the development of resistance.

Precautions and side effects

Sulphonamides are not recommended during pregnancy and in new born or premature infants and should be used with caution in nursing mothers due to the possibility of forming crystals. Adequate fluid intake should be maintained during therapy due to the possibility of causing crystalluria. Adverse effects during therapy with sulphonamides represent the renal failure, blood dyscrasia and Stevens Johnson Syndrome

Drug interactions

Sulphonamide action may be antagonized by para-aminobenzoic acid and related derivatives, particularly local anaesthetics. Effects of sulphonamides may be increased by being displaced from plasma binding sites by more highly bound acid substances such as anticoagulants, oral hypoglycaemics and phenytoin. Dosage adjustments may be necessary

Sulphonamides include: co-trimoxazole, sulphadimidine, sulphadimethoxine, succinylsulphathiazole or phthalysulphathiazole

Nitrofurantoin

This medicine is effective in the treatment of uncomplicated urinary tract infections. It is also used in long-term suppressive therapy of chronic complicated urinary-tract infections after the infecting bacteria have been eliminated or markedly reduced with other, more effective agents e.g. co-trimoxazole or amoxicillin

Precautions and side effects

Nitrofrantoin is contraindicated in patients with impaired renal function or with glucose-6-phosphate-dehydrogenase (G6PD) deficiency and in infants of less than month. Nausea and vomiting occur frequently. Lowering the dose and administering the medicine with meals control this. Other side effects include headache, skin rashes, blood disorders and allergic reactions

Drug interactions

Concurrent use with probenecid reduces the excretion and increases the side effects of nitrofurantoin

Spectinomycin

Spectinomycin shows activity against a wide range of Gram-positive and Gram-negative organisms but its main effects is against Neisseria gonorrhea when treatment with penicillin is undesirable or the organism develops resistance.

Precautions and side effects

Nausea, vomiting, headache, dizziness, fever, slight pruritus and urticarial have been reported in few patients with a single dose of spectinomycin. Alterations in kidney and liver function and decrease in hemoglobin and blood count have been observed with repeated dose. Spectinomycin should not be used in treatment of syphilis

Tetracyclines

Tetracycline is used in the treatment of relapsing fever, rickettsia, trachoma, mycoplasma pneumonia, gonorrhea, severe acne and falciparum malaria in combination with antimalarias. When used in the treatment of cholera concurrent rehydration is a must. Topically it is applied to treat some localized infections.

Tetracyclines are very much alike with respect to their antimicrobial spectra and their side effects. They differ in their absorption, duration of action and suitability for parenteral administration

Tetracyclines stains developing teeth and reversibly impair bone growth through complexation with bone salts and fixation with matrix protein. They are not recommended for children under 12 years and during pregnancy. Cross sensitivity with other tetracyclines does occur. Other side effects caused by tetracyclines include gastrointestinal disturbances with flatulence, nausea, vomiting and diarrhea.

Macrolides

Mcrolides contain a large lactone ring with 12 or more carbon atoms. Erythromycin and rifampicin are in common use in this group.

The erythromycin-type macrolides interfere with bacterial ribosomal protein synthesis. Amphotericin B and nystatin are antifungal drugs. These medicines cause potassium leakage through the micro-organisms cell membrane. Rifampicin inhibits ribonucleic acid synthesis

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