The Action of Colicins on Eukaryotic Cells

Production of bioactive substances by intestinal bacteria as a basis for explaining probiotic mechanisms: bacteriocins and conjugated linoleic acid

The mechanisms by which intestinal bacteria achieve their associated health benefits can be complex and multifaceted. In this respect, the diverse microbial composition of the human gastrointestinal tract (GIT) provides an almost unlimited potential source of bioactive substances (pharmabiotics) which can directly or indirectly affect human health. Bacteriocins and fatty acids are just two examples of pharmabiotic substances which may contribute to probiotic functionality within the mammalian GIT. Bacteriocin production is believed to confer producing strains with a competitive advantage within complex microbial environments as a consequence of their associated antimicrobial activity. This has the potential to enable the establishment and prevalence of producing strains as well as directly inhibiting pathogens within the GIT. Consequently, these antimicrobial peptides and the associated intestinal producing strains may be exploited to beneficially influence microbial populations. Intestinal bacteria are also known to produce a diverse array of health-promoting fatty acids. Indeed, certain strains of intestinal bifidobacteria have been shown to produce conjugated linoleic acid (CLA), a fatty acid which has been associated with a variety of systemic health-promoting effects. Recently, the ability to modulate the fatty acid composition of the liver and adipose tissue of the host upon oral administration of CLA-producing bifidobacteria and lactobacilli was demonstrated in a murine model. Importantly, this implies a potential therapeutic role for probiotics in the treatment of certain metabolic and immunoinflammatory disorders. Such examples serve to highlight the potential contribution of pharmabiotic production to probiotic functionality in relation to human health maintenance.

Human tumor cells are selectively inhibited by colicins

The activity in vitro of four types of colicins (A, E1, E3, U) against one human standard fibroblast line and against 11 human tumor-cell lines carrying defined mutations of the p53 gene was quantified by MTT (tetrazolium bromide) assay. Flow cytometry showed that the pore-forming colicins A, E1 and U affected the cell cycle of 5 of these cell lines. Colicins E3 and U did not show any distinct inhibitory effects on the cell lines, while colicins E1 and especially A inhibited the growth of all of them (with one exception concerning colicin E1). Colicin E1 inhibited the growth of the tumor lines by 17-40% and standard fibroblasts MRC5 by 11%. Colicin A exhibited a differentiated 16-56% inhibition, the growth of standard fibroblasts being inhibited by 36%. In three of the lines, colicins A and E1 increased the number of cells in the G1 phase (by 12-58%) and in apoptosis (by 7-58%). These results correlated with the data from sensitivity assays. Hence, the inhibitory effect of colicins on eukaryotic cells in cell-selective, colicin-specific and can be considered to be cytotoxic.

In vitro assessment of the cytotoxicity of nisin, pediocin, and selected colicins on simian virus 40-transfected human colon and Vero monkey kidney cells with trypan blue staining viability assays

Gram-positive bacterial bacteriocins (nisin and pediocin) and gram-negative bacterial bacteriocins (colicins [Col] E1, E3, E6, E7, and K) were evaluated for cytotoxicity against cultured simian virus 40-transfected human colon (SV40-HC) and Vero monkey kidney (Vero) cells. Bacteriocin-treated cells were assessed for viability by trypan blue staining. Monolayers of SV40-HC and Vero cells were cultured in tissue culture plates (35 degrees C, 10% CO2 in humidified air) with the use of Dulbecco's modified Eagle's medium supplemented with 10% (vol/vol) calf serum. Actively growing cells in the log phase (ca. 10(4) cells per ml) were treated with individual partially purified bacteriocin preparations at 170, 350, and 700 activity units per ml. Duplicate culture plates for each bacteriocin treatment and untreated controls were withdrawn after 16, 32, and 48 h of incubation. Cells were dissociated with trypsin and treated with trypan blue and were then counted in a hemocytometer with the use of a phase-contrast microscope. Viability assays indicated dose-dependent toxicity for some bacteriocins. Nisin, pediocin, and Col E6 were the most cytotoxic bacteriocins; SV40-HC cells demonstrated greater sensitivity than Vero cells did. Some bacteriocins can be toxic to mammalian cells; therefore, bacteriocins intended for use as biopreservatives must be evaluated for toxicity to mammalian cells and for other toxicities. Col E1, Col E3, Col E7, and Col K demonstrated little toxicity at the activities tested, indicating that they are safe and thus have potential for use as food biopreservatives.

Incidence of colicinogenic strains among human Escherichia coli

During the years 1993-1999, altogether 1,043 Escherichia coli strains from colons of different persons were screened for colicinogeny using a most susceptible procedure and indicator system. In control persons (with healthy colons), 41.37% producers of colicins were found. In patients suffering from salmonelloses, the proportion of colicinogenic Escherichia coli was the same. In patients with non-specific inflammatory colon diseases, the proportion of colicinogenic Escherichia coli strains appeared slightly though weakly, significantly or unsignificantly increased: to 47.50% in morbus Crohn and to 56.10% in colitis ulcerosa. These results suggest some sort of engagement of colicinogeny in the pathogenesis thereof. In malignant tumours of the colon, the incidence of colicinogenic Escherichia coli was not altered (40.58%). This does not indicate any colicin participation in the pathology of malignant tumours. In colitis ulcerosa, the incidence of colicinogenic Escherichia coli strains inhibiting Shigella sonei 17 (the indicator for colicin Js which generally inhibits interoinvasive strains of both species) increased from 21.94% (control samples) to 41.46%. Uropathogenic Escherichia coli strains shared the same incidence of colicinogeny as controls (42.08%), if they were not haemolytic; haemolytic ones were colicinogenic with only 22.37%. This difference was highly significant. The patterns of some colicin activities in the set of five indicator strains used suggested that several wild strains produced new, so far unknown types of colicins and/or combinations thereof.

Colicins--exocellular lethal proteins of Escherichia coli

Colicins are toxic exoproteins produced by bacteria of colicinogenic strains of Escherichia coli and some related species of Enterobacteriaceae, during the growth of their cultures. They inhibit sensitive bacteria of the same family. About 35% E. coli strains appearing in human intestinal tract are colicinogenic. Synthesis of colicins is coded by genes located on Col plasmids. Until now more than 34 types of colicins have been described, 21 of them in greater detail, viz. colicins A, B, D, E1-E9, Ia, Ib, JS, K, M, N, U, 5, 10. In general, their interaction with sensitive bacteria includes three steps: (1) binding of the colicin molecule to a specific receptor in the bacterial outer membrane; (2) its translocation through the cell envelope; and (3) its lethal interaction with the specific molecular target in the cell. The classification of colicins is based on differences in the molecular events of these three steps.

Acute lymphoblastic leukemia of childhood monitored by bacteriocin and flowcytometry

Bacteriocin and flowcytometric analysis of 106 blood samples from children with acute lymphoblastic leukemia were correlated with clinical stages of disease. Bacteriocins interacted selectively with malignant, and not with normal, lymphocytes causing cell cycle perturbation which was rapidly and objectively recorded by the flowcytometer. The patients were grouped as: (A) newly-diagnosed (15); (B) early induction (11); (C) remission with viral infection (7); (D) remission (64); (E) bone marrow relapsed (5); (F) extramedullary relapsed (3); (G) non-malignant pediatric controls (8). Bacteriocin reacted usually with groups A, B, C, E and not with groups D, F and G. Repeated testing correlated well with the clinical status. Blood from 7 patients in remission and from 3 normal individuals, each with transient viral infection, reacted with bacteriocin. A quantitative correlation between peripheral blood blasts or surface markers for ALL and bacteriocin reactivity was not established. Unexpected results were obtained only in 13% (false-positive 11% and false-negative 3%). This test can be recommended for preliminary diagnosis and possibly prognosis of lymphoblastic leukemia and provides means of monitoring progress during chemotherapy.

Recognition of T-cell murine leukemia by bacteriocin (colicin); correlation with transplantation experiments

The presence of potential murine leukemia and overt leukemia cells in various organs at different phases of leukemogenesis was demonstrated by transplantation experiments and sensitivity to bacteriocin (colicin HSC10). Such correlation was found in three experimental models: AKR mice developing spontaneous T-cell leukemia and BL/6 mice infected with radiation leukemia virus variants inducing a high or low overt T-cell leukemia incidence. The sensitivity to bacteriocin was evaluated by testing the cell cycle perturbation following in-vitro incubation of lymphoid cells with colicin (or Tris buffer as controls) monitored by flow-cytometry. The analysis was based on measuring relative differences in fluorescence intensity of propidium iodide stained DNA in the individual cells. The interaction with colicin of leukemic cells and lymphoid cells containing potential leukemic cells (PLC) resulted in a reduction in the cell number of the G0/G1 and SG2M phases while cells accumulated in the "pre-G1" channels. In contrast, normal lymphoid cells exposed to bacteriocin did not show such changes in the DNA histograms. The distribution pattern of PLC in the thymus and spleen (in the models tested) obtained by transplantation studies coincided with sensitivity of spleen and thymus cells to colicin. However, in most instances, the PLC in the bone marrow were not recognized by colicin, but their leukemogenic potential was reduced following interaction with colicin as shown by PLC transplantation studies. It is thus suggested that colicin might be used for identification and eradication of transformed cells.

Colicinogeny in chronic inflammatory bowel disease

There are several indirect arguments for a possible role of colicins in chronic inflammatory bowel disease (IBD). Colicinogeny was therefore investigated in 54 patients with ulcerative colitis, 39 patients with Crohn's disease, and 160 clinically healthy controls. No significant difference was found among the examined groups. The leukocyte migration inhibition test (with colicins as antigens) was performed to estimate cellular hypersensitivity to colicins. Migration indices not exceeding the normal range in controls contrasted with abnormal values found in 36% of ulcerative colitis and 80% of Crohn's disease patients. The results are believed to be proof of cellular hypersensitivity of IBD patients to colicins of their own Escherichia coli strains. The importance of this finding must be further clarified.