The polypeptides antibiotics group includes bacitracin A, colistin, novobiocin, and polymyxin B. Bacitracin inhibits bacterial cell wall synthesis while polymyxins disrupt cell membranes. Bacitracin is effective against gram-positive bacteria but not gram-negatives. Polymyxins are effective against many gram-negative species like E. coli and Pseudomonas. Resistance can develop to bacitracin and novobiocin but is rare for polymyxins. Bacitracin is used to improve growth and feed efficiency in livestock while novobiocin treats mastitis in dairy cattle. However, polymyxins are nephrotoxic,

1. Antibiotics Groups
Polypeptides

2. Polypeptides Group
 The polypeptides include:
1. Bacitracin A
2. Colistin (polymyxin E)
3. Novobiocin
4. Polymyxin B.

3. Description
Bacitracin
– It is a complex mixture of branched, cyclic decapeptides produced by
Bacillus subtilise, which was first isolated in 1945.
Novobiocin
– first reported in 1955 as streptonivicin, is produced by the
actinomycete Streptomyces niveus.
The polymyxins
– Discovered in 1947, are synthesized by various strains of Bacillus
polymyxa.
– Colistin (polymyxin E) comprises a family of polymyxins and was
known as colimycin when first isolated from a broth of Bacillus
polymyxa var. colistinus in 1951.
– The polymyxins are cationic detergents.

4. Bacitracin
Mechanism of Action - Spectrum
 Bacitracin inhibits the synthesis of the bacterial cell wall by
preventing the transport of peptidoglycan precursors through
the cytoplasmic membrane.
 Bactericidal
 It is bactericidal to Gram-positive bacteria but exhibits little
activity against Gram-negative organisms.

5. Polymyxins
Mechanism of Action - Spectrum
 The antibacterial activity of the polymyxins is attributed to
their strong binding to phospholipids in cell membranes,
which disrupts their structure and alters membrane
permeability.
 Bactericidal
 Many species of Gram-negative bacteria, including:
1. E. coli
2. Salmonella
3. Pseudomonas aeruginosa

6. Resistance
Bacitracin
 Bacterial resistance to the polymyxins is rare.
 Resistance is common in pig and chicken isolates of
Enterococcus spp.
 Interestingly, bacitracin administered to pigs and chickens has
been shown to reduce the transfer of resistance plasmids
among enteric E. coli .
Novobiocin
 Resistance has developed in many species of bacteria.

7. Indications
Bacitracin
1. For prevention of necrotic enteritis in broilers.
2. To improve broilers and layers ability to withstand heat
stress.
3. To control of proliferative enteropathy in grower and finisher
pigs.
4. To decrease the incidence and severity of clostridial enteritis
in piglets born to sows treated during pregnancy.
5. As a feed additive for pigs, poultry, and ruminants. It
improves:
– Growth rate
– Feed conversion efficiency

8. Cont. …
Novobiocin sodium
 it is included with other agents in intramammary infusions for
treating mastitis in dairy cattle.

9. Side Effects
Polymyxins
 They are nephrotoxic and neurotoxic.
 They cause intense pain upon injection.
 Polymyxin B is a potent histamine releaser; however,
hypersensitivity reactions to all polymyxins are seen
occasionally.