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This paper presents results of a collaborative experiment between six laboratories which examined the yields of unstable chromosomal aberrations in human lymphocytes induced in vitro by X-rays over the dose range 0-300 mGy. The work included data points of nominal doses of 0, 3, 5, 6, 10, 20, 30, 50 and 300 mGy. Cells from 24 donors were examined and a total of about 300,000 metaphases were scored. The work was undertaken to determine the limits of sensitivity of the system taking into account variations in scoring data due to inter-donor sample and inter-laboratory effects. Despite the existence of these effects, aberration yields significantly in excess of control values were seen at doses greater than 20 mGy and these were consistent with a linear extrapolation from higher doses. Below 20 mGy the observed dicentric yields were generally lower than background, but not significantly so. Excess acentric aberrations, on the other hand, and centric rings, were higher than the controls but the increase was usually not significant. It is concluded that the statistical uncertainties are such that below 20 mGy this technique cannot distinguish between a linear or a threshold model.

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Damage from occupational or accidental exposure to ionising radiation is often assessed by monitoring chromosome aberrations in peripheral blood lymphocytes, and these procedures have, in several cases, assisted physicians in the management of irradiated persons. Thereby, circulating lymphocytes, which are in the G0 stage of the cell cycle are stimulated with a mitogenic agent, usually phytohaemagglutinin, to replicate in vitro their DNA and enter cell division, and are then observed for abnormalities. Comparison with dose-response relationships obtained in vitro allows an estimate of exposure based on scoring: Unstable aberrations by the conventional, well-established analysis of metaphases for chromosome abnormalities or for micronuclei; So-called stable aberrations by the classical G-banding (Giemsa-Stain-banding) technique or by the more recently developed fluorescent in situ hybridisation (FISH) method using fluorescent-labelled probes for centromeres and chromosomes. Three factors need to be considered in applying such biological dosimetry: (1) Radiation doses in the body are often inhomogeneous. A comparison of the distribution of the observed aberrations among cells with that expected from a normal poisson distribution can allow conclusions to be made with regard to the inhomogeneity of exposure by means of the so-called contaminated poisson distribution method; however, its application requires a sufficiently large number of aberrations, i.e. an exposure to a rather large dose at a high dose rate. (2) Exposure can occur at a low dose rate (e.g. from spread or lost radioactive sources) rendering a comparison with in vitro exposure hazardous. Dose-effect relationships of most aberrations that were scored, such as translocations, follow a square law. Repair intervening during exposure reduces the quadratic component with decreasing dose rate as exposure is spread over a longer period of time. No valid solution for this problem has yet been developed, although, in theory, both deterministic damage and aberrations might be repaired to a similar degree; a comparison of aberrations following a linear dose relationship might also help when the doses have been sufficiently large. (3) Investigations might have been possible only a certain time after the exposure. The relatively rapid disappearance of lymphocytes carrying unstable aberrations limits their use in retrospective dosimetry, years after exposure. Scoring stable aberrations, thought to persist in the circulating lymphocytes, might appear more appropriate in such situations. However, the examination of a representative number of cells by G-banding is extremely laborious, and the FISH method is not only expensive but has not yet been fully validated in different laboratories. In conclusion, biological dosimetry has serious limitations exactly for situations where the need for information is most urgent. It renders its most useful results when an individual has been exposed to a rather homogeneous high-level radiation over a short time interval, i.e. accide...

To characterize the functional cell surface, the zeta potentials and elemental surface composition of Streptococcus salivarius HB and a range of mutants with known molecular surface structures were determined. Zeta potentials of fully hydrated cells were measured as a function of pH in dilute potassium phosphate solutions, yielding isoelectric points of the strains. Elemental composition (0, C, N, and P) of the outer 2 to 5 nm of the freeze-dried cell surfaces were measured by X-ray photoelectron spectroscopy. An increasing loss of proteinaceous fibrillar surface antigens of the mutants was found to be accompanied by a progressive decrease in the N/C ratio from 0.104 in the parent strain HB to 0.053 in mutant HBC12. Simultaneously, the value of the isoelectric point shifted from 3.0 to 1.3. In a previous study (A. H. Weerkamp, H. C. van der Mei, and J. W. Slot, Infect. Immun. 55:438-455, 1987) on the cell surfaces of the same strains, it was shown that removal of fibrils led to increased exposure of (lipo)teichoic acid at the surface, which explains the low isoelectric point caused by the low pKa of the phosphate groups.The outer surface of bacterial cells mediates many crucial interactions of the cells with their environment, such as adherence, flocculation, antigenicity, susceptibility to phagocytosis, and binding of macromolecules. Understanding these complex functions requires detailed knowledge of the elemental, molecular, and structural composition of the cell surface, which is obviously different from the bulk of the cell wall (3, 18). Generally such knowledge is obtained by electron microscopic methods, by probing the cell surface with specific molecules such as antibodies or lectins, by functional tests such as hydrophobicity assays, or by indirect techniques involving the removal of surface components (8,12).Only a few techniques provide information on the chemical composition on the outer surface of intact cells. An elemental analysis over the thickness of the outer 2 to 5 nm can be obtained by X-ray irradiation of surfaces, causing ejection of electrons, which can be discriminated by their binding energy in a specific chemical element. X-ray photoelectron spectroscopy (XPS) is most frequently used for nonbiological surfaces but has been used in a few cases for the study of bacterial and eucaryotic cells (2, 7, 13). Compared with other surface techniques (1), XPS has the advantage of being nondestructive and highly surface sensitive; it gives a high information content and yields quantitative data on elemental composition and chemical bonding. A disadvantage, common to most analytical surface techniques, is that samples must be measured under high vacuum (10-6 to 10-9 Pa).Previously, we studied the structure and molecular composition of the cell surface of Streptococcus salivarius HB and a range of mutants devoid of specific fibrillar appendages and lacking adhesive abilities (11,17). No information is * Corresponding author. available on the elemental surface composition of these cells and how it w...

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