310
Apart from being causatives of some serious life-threatening infections, they rapidly develop resistance to antimicrobials used for therapy.
12.4.4 Antimicrobials from Pseudomonas
Pyocins Pyocins are high-molecular-weight bacteriocins produced by P.s aeruginosa as defense molecules against other strains of the same species. Besides being
antimicrobial against strains of P. aeruginosa, they are reported to have limited
antibacterial activity against other bacteria. Pyogenecity or the ability to produce
pyocins is widely distributed in the strains of P. aeruginosa, but the level of spontaneous production of pyocin is very low because only a fraction of cells in a culture take part in the production of pyocins. The production of pyocins is enhanced
by mutagenesis either using ultraviolet irradiation or addition of mitomycin C to
actively growing cultures. Pyocins insert themselves into target cell walls after
binding to specific receptors on the cell surface and disrupt the membrane potential
of the target cells causing cell death. A single pyocin molecule is capable of producing such bactericidal effects. Three types of pyocins are described. R-type pyocins resemble the rod-like, contractile tail structure of microviridiae bacteriophages
with no head structure and DNA content. They are protease and acid resistant.
Examples of R-type pyocins include pyocins R1-R5, pyocin C9, 21 and 430 c.
R-type pyocins are active against Neisseria gonorrhoeae, N. meningitidis,
Haemophilus ducreyi, and H. influenzae. F-type pyocins are particulate, flexuous
pyocins similar to the tail structure of noncontractile bacteriophages like λ phage.
F-type pyocins include pyocin 28, pyocin F1 and F2 and 430 f. The third group of
pyocins is called S-type pyocins. They are soluble pyocins that include pyocin
S1–S5. Unlike bacteriocins, which are plasmid encoded, the structural genes for
pyocins are chromosomally located. The synthesis of pyocins is a regulated genetic
event with inducible regulator genes located near the structural genes (MichelBriand and Baysse 2002).
Siderophores Siderophores are ferric (Fe
3+
)-ion-binding, low-molecular-weight
molecular chelators (>1500 Da) that facilitate the transport of iron into microbial
cells under conditions of low iron stress. Siderophores are used as biocontrol agents
due to their ability to sequester and render iron unavailable to microbial pathogens
along with other applications. Siderophores are used as agents for selective drug
delivery in multidrug-resistant bacteria. The transportation abilities of siderophores
are exploited for this by the production of drug-siderophore conjugates. Fluorescent
Pseudomonas spp. produce two important classes of siderophores, namely, pyoverdines and pyochelins. Pyoverdines contain a dihydroquinoline-like chromophore
attached to a peptide. At least 60 different pyoverdines are described with variable
peptide lengths and sequences. Under specific conditions, pyoverdines function as
a diffusible bacteriostatic or fungistatic antibiotic. Pyochelins are sparingly waterD. Francis
Apart from being causatives of some serious life-threatening infections, they rapidly develop resistance to antimicrobials used for therapy.
12.4.4 Antimicrobials from Pseudomonas
Pyocins Pyocins are high-molecular-weight bacteriocins produced by P.s aeruginosa as defense molecules against other strains of the same species. Besides being
antimicrobial against strains of P. aeruginosa, they are reported to have limited
antibacterial activity against other bacteria. Pyogenecity or the ability to produce
pyocins is widely distributed in the strains of P. aeruginosa, but the level of spontaneous production of pyocin is very low because only a fraction of cells in a culture take part in the production of pyocins. The production of pyocins is enhanced
by mutagenesis either using ultraviolet irradiation or addition of mitomycin C to
actively growing cultures. Pyocins insert themselves into target cell walls after
binding to specific receptors on the cell surface and disrupt the membrane potential
of the target cells causing cell death. A single pyocin molecule is capable of producing such bactericidal effects. Three types of pyocins are described. R-type pyocins resemble the rod-like, contractile tail structure of microviridiae bacteriophages
with no head structure and DNA content. They are protease and acid resistant.
Examples of R-type pyocins include pyocins R1-R5, pyocin C9, 21 and 430 c.
R-type pyocins are active against Neisseria gonorrhoeae, N. meningitidis,
Haemophilus ducreyi, and H. influenzae. F-type pyocins are particulate, flexuous
pyocins similar to the tail structure of noncontractile bacteriophages like λ phage.
F-type pyocins include pyocin 28, pyocin F1 and F2 and 430 f. The third group of
pyocins is called S-type pyocins. They are soluble pyocins that include pyocin
S1–S5. Unlike bacteriocins, which are plasmid encoded, the structural genes for
pyocins are chromosomally located. The synthesis of pyocins is a regulated genetic
event with inducible regulator genes located near the structural genes (MichelBriand and Baysse 2002).
Siderophores Siderophores are ferric (Fe
3+
)-ion-binding, low-molecular-weight
molecular chelators (>1500 Da) that facilitate the transport of iron into microbial
cells under conditions of low iron stress. Siderophores are used as biocontrol agents
due to their ability to sequester and render iron unavailable to microbial pathogens
along with other applications. Siderophores are used as agents for selective drug
delivery in multidrug-resistant bacteria. The transportation abilities of siderophores
are exploited for this by the production of drug-siderophore conjugates. Fluorescent
Pseudomonas spp. produce two important classes of siderophores, namely, pyoverdines and pyochelins. Pyoverdines contain a dihydroquinoline-like chromophore
attached to a peptide. At least 60 different pyoverdines are described with variable
peptide lengths and sequences. Under specific conditions, pyoverdines function as
a diffusible bacteriostatic or fungistatic antibiotic. Pyochelins are sparingly waterD. Francis