Top 15 Antibacterials | Animals | Pharmacology

Here is a list of top fifteen antibacterials.

1. Ionophores:

These polyether antibiotics are fermentation products of Streptomyces species. Monensin, salinomycin, narasin and lasalocid are the commonly used substances in this group. Maduramycin and semduracin are the newer ionophore antibiotics.

Monensin:

This antibiotic alters the cell permeability. It is obtained from Streptomyces cinnamonensis. By complexing with the sodium of cell membrane, it causes passive transport of potassium ions outside the cell and their replacement by hydrogen ions. This results in lowering of intracellular pH and lysis of the cell.

The antibacterial spectrum of this antibiotic includes Gram positive bacteria, some Campylobacter species and Serpuline hyodysenteriae. It is also effective against coccidia and toxoplasma.

Monensin is highly toxic to horses and to ruminants and other animals when the dose is 5 to 10 times the recommended dose or when mixed with certain incompatible drugs. Muscle weakness and myoglobinuria are marked features of poisoning in dogs, pigs and sheep.

In cattle, sheep and goats, progressive cardiomyopathy develops and animals die of generalized cardiac failure. The predominant lesions of monensis toxicosis in chickens and turkeys are necrosis of skeletal and cardiac muscle, associated clinically with posterior paresis, inability to rise, incoordination, leg weakness and reluctance to move.

The clinical uses of this antibiotic are:

1. In ruminants it is used to improve the feed conversion efficiency. When fed in small quantities, it changes the rumen microflora towards more Gram negative flora which shifts the volatile fatty acid production, decrease rumen protein breakdown, primary hydrogen or formic acid production and decrease lactase production.

2. In poultry and other species it is used in the control of coccidiosis.

Other Ionophores:

lasalocid, Maduramycin, Narasin, Salinomycin and Semudracin also possess anti-coccidial activity. They are also used for their effect in improving the efficiency of ruminal fermentation. Salinomycin is used as a growth promoter in swine. Mode of action and clinical uses and toxicity are similar to Monessen.

2. Novobiocin:

Novobiocin is a semisynthetic bacteriostatic agent effective against Gram positive bacteria. In higher concentrations it is bactericidal. It is stable in acid solution. Activity is decreased by alkaline conditions and in the presence of magnesium. It inactivates the beta subunit of DNA gyrase inhibiting supercoiling. Chromosomal resistance develops readily.

It is well absorbed from the gastrointestinal tract in humans. Skin eruptions are common. Eosinophilia, thrombocytopenia and leucopaenia also occur occasionally. It is synergistic with tetracyclines. Novobiocin is useful in anthrax, mastitis and cellulitis. In poultry it is used to beat staphylococcal infections and fowl cholera. For the treatment of mastitis it is combined with Penicillin G, especially in dry cow therapy.

3. Polymyxins:

Polymyxins are the products of Bacillus polymyxia. Originally these antibiotics were used for their action against Pseudomonas aeruginosa. But now, they are used topically. Five members of Polymyxins A, B, C, D and E have been produced. But among these only B and E are useful because of less toxic effects. Polymyxin E is also known as Colistin.

They are basic cyclic decapeptides which inactivate the endotoxin of gram negative bacteria. Polymyxines are used as the sulphates and colistin is used as methane sulphonate. Polymyxins are stable, highly water soluble drugs.

They are stable at pH 5 to 7. They are surface active agents which disrupt the structure of cell membrane phospholipids and increase cell permeability by a detergent like action. Bivalent metallic ions block this action.

They dis-organise the outer membrane of Gram negative bacteria by binding to phospholipids through interaction of the cationic drugs with anionic lipoid region. They are bactericidal effective against Gram negative organisms like E. coli, Salmonella and Pseudomonas.

Although intrinsic bacterial resistance to polymyxins is recognised, resistance is uncommon and chromosome dependent only. Acquired resistance is rare. Cross resistance occurs among the polymxins. Very slowly absorbed from the gastrointestinal tract. Diffuse poorly through the membranes. Excreted unchanged in the urine.

Synergistic with sulphonamides and trimethoprim against many gram negative organisms. It also acts synergistically with tertracyclines. Synergism is also noticed with chelating agents like EDTA, cationic chlorhexidine and is used in topical treatment of local infections caused by Pseudomonas.

Reduces the activity of endotoxins in body fluids and hence is useful in toxaemia. Systemic use causes nephrotoxicity, neurotoxicity and neuromuscular blocking effects. Polymyxins should be avoided for use in animals because of the toxicity. In local treatment of otitis externa, superficial eye infections it is used in dogs and cats.

4. Bacitracin:

Bacitracin is isolated from Bacillus subtilis. It inhibits the formation of bacterial cell wall peptidoglycan. It is bactericidal to gram positive bacteria. Bactericidal activity requires presence of divalent cations such as zinc.

Most gram negative organisms are not susceptible probably due to lack of penetration of the drug through the cell membrane. The spectrum of activity is similar to that of penicillin. Resistance develops slowly. Penicillin resistant strains are also susceptible to bacitracin.

The drug is not absorbed orally and is highly toxic exhibiting symptoms of nephrotoxicity. It is generally used in the treatment of superficial infections of the skin and mucosal surfaces. The drug is not affected by the presence of blood and tissue fluids. Zinc bacitracin is used as a growth promoter in poultry, cattle and swine and also in the prevention and treatment of enteritis.

5. Vancomycin:

It is a complex polypeptide which binds to precursors of the peptidoglycan layer in bacterial cell wall. This effect prevents cell wall synthesis and produces a rapid bactericidal effect. This drug is active against gram positive and not against gram negative bacteria because of non permeability.

Resistance does not occur readily. It is widely distributed in the body and excreted in active form through the kidneys. In cases of renal insufficiency the drug gets accumulated. It is useful parenterally in methicillin resistant Staphylococcus aureus.

6. Virginamycin:

It is an antibiotic mixture produced as a fermentation product of Streptomyces virginea. The drug is active against gram positive aerobic and anaerobic bacteria. Mycoplasma are also often susceptible. It is not absorbed after oral administration in animals. Has no adverse effects. Used to promote growth.

7. Methanamine:

Methanamine (Hexamine) is a basic drug which decomposes under acidic conditions to release formaldehyde. It is available as a salt of mandelic acid or hippuric acid. After administration as enteric coated tablets, it is well absorbed and excreted unchanged in urine. If urine is strongly acidic, it releases formaldehyde which acts as a non specific urinary antiseptic. Urine is rendered acidic by concurrent administration of ascorbic acid or ammonium chloride.

8. Isoniazid:

It is most potent anti-tuberculosis drug. The drug is bactericidal to Mycobacterium tuberculosis. Mycbacterium bovis is also susceptible. The mechanism of action is not known. It is well absorbed from the intestine and distributed well into the tissue cavities including the cerebrospinal fluid.

9. Rifamycins:

This group of drugs is produced from Streptomyces mediterranei. Rifamycins (rifamycin SV, rifampin and rifamide) interfere with the synthesis of RNA in microorganisms by binding to subunits of sensitive DNA dependent RNA polymerase.

They are effective against gram positive bacteria, mycobacteria and some gram negative bacteria, aerobes and chlamydiae. Higher concentration is effective against viruses. Resistance to rifamycin may develop as a one step process. Hence, rifamycins are often administered with penicillin, erythromycin, miconazole and amphotericin B.

10. Mupirocin:

This drug is obtained from P. fluorescens. It is effective against gram positive bacteria and mycoplasma. The mode of action is active inhibition of isoleucyl tRNA synthetase. This prevents incorporation of isolecucine during protein synthesis.

It is useful in topical and soft tissue infections. Its activity increases with decreasing pH. So it is useful for topical application to skin that is slightly acidic. It is inactivated by body fluids including pus and mucosal secretions.

11. Nitrofurans:

These are synthetic antibacterial compounds which are based on the furan ring to which a 5 nitro group has been added. They are bactericidal and cause inhibition of carbohydrate metabolism in the bacterial cell. They are effective against gram positive and gram negative bacteria.

This group is also effective against coccidial infection. Have low tissue toxicity. Toxic manifestations include peripheral neuritis, gastrointestinal irritation, depressed spermatogenesis, weight gain and sensitisation. They are not used parenterally.

Furaltadone:

It is well absorbed after oral administration. Used in poultry to treat salmonellosis, colibacillosis, caecal coccidiosis, infective synovitis and black head. Treatment should be restricted to a maximum of 10 days.

Nitrofurazone:

It is active against gram positive and negative bacteria. Also effective against protozoa especially coccidia.Used in enteric infections especially salmonella in poultry and pigs, pasteurellosis in rabbits and grey diarrhoea in mink.

Furazolidone:

This drug is effective against Clostridia, Salmonella, Shigella, Staphylococci, Streptococci and E.coli. It is also useful against Eimeria and Histomonas. It is poorly absorbed after oral administration. But, it is useful in treating gastrointestinal infections.

12. Sodiumfusidate:

Fusidic acid is a lipophilic steroid antibiotic obtained from Fusidium coccineum. It is highly active against gram positive bacteria especially Staphylococcus aureus. It acts by inhibition of bacterial protein synthesis and may be bacteriostatic or bactericidal.

Well absorbed from the gastrointestinal tract and widely distributed throughout the body. It does not diffuse into the cerebrospinal fluid. About 95% of the drug in circulation is bound to plasma proteins. Excreted by the liver mainly.

13. Tiamulin Furmarate:

It is a derivative of pleuromutilis obtained from Pleurotus mutilis, It is active against gram positive bacteria, mycoplasma and anaerobes. The drug is a bacteriostatic inhibiting the protein synthesis of bacterial cell. It has a great affinity to the 50s subunit of the bacterial ribosome which causes a break in the peptide chain and hence bacteriostatic.

Useful in treating swine dysentry and mycoplasmal arthritis. The drug is well absorbed after oral administration. In poultry tiamulin interferes with monensin and salinomycin metabolism and hence when they are fed together they become toxic. In pigs it is also useful for the treatment of enzootic pneumonia.

14. Carbadox:

Carbadox is a quinoxaline NN dioxide derivative used to promote growth and prevent dysentery in swine. It inhibits bacterial DNA synthesis and denatures persisting DNA. More effective under anaerobic conditions than aerobic conditions.

15. Arsenicals:

Arsenic containing compound (arsenilic acid, sodium arsenilate, 3-nitro- 4hydroxyphenyl arsenic acid) are used to promote growth in pigs and poultry and to treat swine dysentery.