Topical Antibiotics

• Topical antibiotics help prevent infections caused by bacteria that get into minor cuts, scrapes, and burns.

• Treating minor wounds with antibiotics allows quicker healing.

• If the wounds are left untreated, the bacteria will multiply, causing pain, redness, swelling, itching, and oozing.

• Untreated infections can eventually spread and become much more serious.

Which bacteria?

• Most topical antibiotics are directed against Staphylococcus aureus and Streptococcus pyogenes.

• The anaerobic Gram-positive bacterial species Propionibacterium acnes has been linked to acne.

Which topical antibiotics are common?

• Some widely used topical antibiotics are bacitracin, neomycin, mupirocin, and polymyxin B.

• Among the products that contain one or more of these ingredients are Bactroban (a prescription item), Neosporin, Polysporin, and Triple Antibiotic Ointment or Cream.

Classes of topical antibiotics

• Cell wall synthesis inhibitors

• Ribosome function inhibitors

• Sulfa drugs

• Burn treatment agents

• Miscellaneous

Mupirocin (90% Pseudomonic acid A)

• Isolated from Pseudomonas fluorescens – Antibacterial activity of substance from P. fluorescens noted in

1887– Purified in the 1960’s.

• Mupirocin works against Gram-positive bacteria only• Can be used to treat MRSA (although resistance is rising)

• Ester linkage is rapidly hydrolyzed hepatically, thus precluding utility as an oral or intravenous antibiotic

• Mupirocin inhibits bacterial isoleucyl-tRNA synthetase.

Products containing Mupirocin

Polymyxin B

Polymixin B

• Member of the lipopeptide class of antibiotics, similar to daptomycin

Daptomycin Polymixin B

Polymyxin: Antibacterial activity

• However, the polymyxins are only active against gram negative bacteria (P. aeruginosa, E. coli, K. pneumoniae), while daptomycin is used to treat gram positive bacteria

• The polymyxins are highly nephrotoxic and are thus only used topically

Polymyxins: Mechanism of action

• Bind the the lipopolysaccharide in the outer membrane, thus destroying OM integrity.

• Bind to the cytoplasmic membrane (to the phosphatidylethanolamine) and make the membrane more permeable.

Products containing polymyxin B

Bacitracin A

Bacitracin: History

• Isolated by John T. Goorley in 1943

• Found in the infected compound fracture of the patient Margaret Tracy

Bacitracin: Antibacterial Activity

• Primarily used against gram positive bacteria S. aureus and Streptococci spp.

• Most gram negative organisms are resistant

Bacitracin: Mechanism

• Bacitracin interferes with bacterial cell wall synthesis

• Acts by blocking a step in the process whereby the key subunits are transferred from the cytoplasm

• Specifically bacitracin tightly binds undecaprenyl pyrophosphate, preventing the hydrolysis into undecaprenyl phosphate

• This step is essential for recycling of the carrier

• Link

Products containing Bacitracin

Gramicidin

Gramicidin S

Gramicidins

• The Gramicidins are small peptides (15 amino acids)

• Some, such as gramicidin S, are cyclic

• Others, including Gramicidin A, B, C, and D, are linear

• Commercial gramicidin is a mixture of compounds, with gramicidin A being major

Gramicidins

• Gramicidin S is a powerful antibacterial agent, with broad range against a number of Gram positive and Gram negative microorganisms.

• Unfortunately, Gramicidin S is hemolytic, and thus is limited to topical use.

• Mechanism of action is believed to be at the cytoplasmic membrane.

Gramicidins: Mechanism of action

• The gramicidins behave as ionophoric substances

• The gramicidins self associate, thus forming small pores that cause leakage of essential cations from the cytoplasm

A gramicidin channel

Gramicidin is an unusual peptide, with alternating D & L amino acids.

In lipid bilayer membranes, gramicidin dimerizes & folds as a right-handed -helix.

The dimer just spans the bilayer.

Primary structure of gramicidin (A):Gramicidin dimer(PDB file 1MAG)

HCO-L-Val-Gly-L-Ala-D-Leu-L-Ala-D-Val-L-Val-D-Val-L-Trp-D-Leu-L-Trp-D-Leu-L-Trp-D-Leu-L-Trp-NHCH2CH2OH Note: The amino acids are all hydrophobic; both peptide ends are modified (blocked).

The outer surface of the gramicidin dimer, which interacts with the core of the lipid bilayer, is hydrophobic.

Ions pass through the more polar lumen of the helix.

Ion flow through individual gramicidin channels can be observed if a small number of gramicidin molecules is present in a lipid bilayer separating 2 compartments containing salt solutions.

Gramicidin dimer(PDB file 1MAG)

Neomycin

Historical: Aminoglycosides

• Waksman and Schatz demonstrated the antibacterial activity of Streptomyces griseus in 1943

• Streptomycin isolated in 1944

• Neomycin isolated from Streptomyces fradiae in 1949

Neomycin

• Neomycin is extremely nephrotoxic, thus limiting its use to a topical antibiotic

• Neomycin has excellent activity against gram negative bacteria and partial activity against gram positive strains

• Some people have allergies to neomycin

• Like other aminoglycosides, neomycin works by binding to the bacterial 30S ribosomal subunit, thus inhibiting protein synthesis.

Silver sulfadiazine

• Sulfa drug works by normal mechanism of interfering with the biosynthesis of folic acid

• Heavy metals, like silver, seem to be toxic to bacteria, probably due to their ability to denature proteins through reaction with disulfide bonds

Uses

• Used to treat burn patients

Treatment of Acne Vulgaris

What Causes Acne?

• Acne is a result of clogging of a hair follicle, and simultaneous activation of the sebaceous gland (thus producing more sebum).

• A commensal bacterium, Propionibacterium acnes, which lives on the skin, but is also present in the follicle, causes inflammation and thus contributes to the problem.

Propionibacterium acnes

• Killing the bacteria can help with treatment of acne

Benzoyl Peroxide

• Exact antibacterial mechanism is unknown, but presumably involves oxidation of essential bacterial structures.

Clindamycin

The antibiotic clindamycin is commonly used topically in the treatment of acne

• Recall that clindamycin is a member of the lincosamide class of antibacterial agents and acts at the bacterial ribosome.

• Clindamycin is commonly used to treat aerobic Gram-positive bacteria.

Assigned Reading

• Noah Scheinfeld A primer on topical antibiotics for the skin and eyes. Journal of drugs in dermatology : JDD (2008), 7(4), 409-15.

Homework Question

• List the primary target organism and the mechanism of action of the topical antibiotics discussed in this presentation.