Inhibition of colicin e2 activity by bacterial lipopolysaccharide

Mode of inhibitory action of a bacteriocin produced by Streptococcus mutans C3603

The basis for the lethal activity of a bacteriocin produced by Streptococcus mutans C3603 (serotype c) was studied. Bacteriocin C3603 was found to adsorb to cells of representative strains of the seven serotypes of S. mutans. S. mutans BHT (serotype b) was used to study the adsorption and the lethal properties of bacteriocin C3603. The adsorption of bacteriocin to cells of S. mutans BHT was inhibited by treatment of cells with protease and beta-glucosidase and by such ligands as poly-L-lysine, poly-L-arginine, L-aspartic acid, L-glutamic acid, glutathione, oxidized glutathione, poly-L-aspartic acid, and poly-L-glutamic acid. The adsorption to cells was also inhibited by oligosaccharides and glucosamine. Mixtures of anionic and cationic amino acids or polyamino acids did not greatly enhance or antagonize the inhibition of adsorption of bacteriocin C3603 to cells. Sodium hydroxide extracts of cell walls and cell wall-membranes contained carbohydrates and proteins; however, only proteins were found to bind to bacteriocin or to a bacteriocin affinity column. The sodium hydroxide extracts contained about 35 protein bands as determined by sodium dodecyl sulfate-polyacrylamide disc gel electrophoresis. Bacteriocin C3603 was found to immediately inhibit the synthesis of proteins, DNA, and RNA of cells and to slowly release DNA from cells of S. mutans BHT.

Novel approaches to the mode of action of colicins

According to the theory of Fredericq (1949) and Nomura (1964), colicins are attached by specific receptor sites in the cell walls of sensitive bacteria, which mediate their inhibitive effects. During last years, a great variety of experimental data have been accumulated, some of which cannot be easily interpreted in terms of this theory. There exist considerable discrepancies concerning the chemical nature and molecular weight of isolated receptors. The attachment of a colicin onto its receptor need not be irreversible. The inhibition of numerous membrane-associated functions in colicin-tolerant mutants suggests their pleiotropic deletion nature. The difference between colicin resistance and colicin tolerance does not seem to be clear-cut. Cells of stable L-forms of protoplast type, completely devoid of their walls, retain in most cases the same patterns of sensitivity to colicins as rods of the same strains. Experimental changes in the relationship between the cell wall and the cytoplasmic membrane decrease colicin sensitivity of the cells. Colicin E3 has been found to be a specific endoribonuclease, able to cleave a terminal fragment from the 16 S rRNA also in isolated ribosomes in vitro: not only in ribosomes from sensitive bacteria, but also in those from resistant ones and from eukaryotic cells. A destabilization of the DNA helix was induced by colicin E2 in vitro as in vivo. It seems that there exist two distinct types of colicin receptors with different functions: those in the cell wall, and those in the cytoplasmic membrane. Only the contact of colicins with the latter ones is biologically effective and starts both stages of their inhibitive effect: the reversible and the irreversible ones.

[Nature of bacteriocins]

The bacteriocins comprise a large, heterogeneous group of bactericidal substances produced by bacteria from widely differing species. The best known bacteriocins are the colicins. Some aspects of research on colicins are discussed especially their chemical nature and mode of action, and their genetic determinants.

Colicin B: mode of action and inhibition by enterochelin

Adsorption of colicin B to a sensitive strain of Escherichia coli results in rapid cessation of deoxyribonucleic acid, ribonucleic acid, and protein synthesis. Some classes of mutants insensitive to colicin B hyperexcrete a colicin inhibitor into their growth medium. This inhibitor functions by preventing adsorption of colicin B and does not rescue cells to which colicin has already adsorbed. The inhibitor is insensitive to nucleases, proteolytic enzymes, and lysozyme and is not extracted into organic solvents. The inhibitory material has a low molecular weight, which rules out identification as lipopolysaccharide, although purified lipopolysaccharide has some inhibitory activity. Evidence is presented that the inhibitor is enterochelin, an iron chelator which is the cyclic trimer of 2,3-dihydroxybenzoylserine. Enterochelin does not inhibit colicin M, a colicin that is produced by many strains colicinogenic for colicin B.

Specificity, induction, and absorption of pesticin

Irradiation of pesticinogenic (Pst(+)) cells of Yersinia pestis with ultraviolet light resulted in a five- to eightfold increase in titer of intracellular pesticin. Extracellular activity in irradiated or control cultures never approached that found within the bacteria. Upon chromatography of crude extracts of Pst(+) cells on columns of diethylaminoethyl-cellulose, Sephadex G200, and calcium hydroxyapatite, fractions were recovered which inhibited the growth of indicator cells of Escherichia coli, Y. pseudotuberculosis, Y. enterocolitica, and Pst(-)Y. pestis. By means of these methods, in conjuntion with fractional precipitation with ammonium sulfate, the antibacterial activity was purified to homogeneity (as judged by disc gel electrophoresis and analysis by double diffusion in agar). The relative sensitivity of each cell type was constant and not affected by the state of purification. These findings indicate that the pesticin molecule alone accounts for lethality. Cells of E. coli and Y. pseudotuberculosis were about 100 and 10 times more sensitive to pesticin, respectively, than were those of Pst(-)Y. pestis. Activity directed against sensitive cells of these three species was enhanced by 0.1% ethylenediaminetetraacetate (EDTA) and completely inhibited by 0.01 m hemin or 0.001 m Fe(3+). The response of Y. enterocolitica to pesticin was variable in the presence of EDTA and was not influenced by Fe(3+). Attempts to detect common surface structures responsible for absorption of pesticin by the four cell types were not successful.

Localization and solubilization of colicin receptors

Envelope fractions isolated from Escherichia coli K-12 C600 and from colicin-resistant and colicin-tolerant (Tol II) mutants derived from this strain were separated on sucrose gradients into cell wall-enriched and cytoplasmic membrane-enriched fractions. These fractions were tested for their ability to neutralize colicins of the E and K groups. Neutralization activity was found in the cell wall-enriched fraction from the parent and the Tol II mutant but was absent from all fractions from the resistant mutant. This was also tested with several other E. coli strains. In all cases, sensitive strains contained the neutralization activity, whereas resistant strains did not. The neutralization activity was solubilized from cell walls or cell envelopes of sensitive or Tol II strains by extraction at room temperature with Triton X-100 plus ethylenediaminetetraacetic acid. The solubilized activity was precipitated by 20% ammonium sulfate, 70% ethanol, or 10% trichloroacetic acid. The activity was destroyed by treatment of the solubilized preparation with trypsin or periodate. These results suggest that this colicin-neutralization activity is due to the presence of specific receptors localized in the cell wall and that intact protein and a carbohydrate are required for this receptor to bind colicin.