[Code of Federal Regulations]
[Title 40, Volume 31]
[Revised as of July 1, 2006]
From the U.S. Government Printing Office via GPO Access
[CITE: 40CFR798.5500]
[Page 207-210]
TITLE 40--PROTECTION OF ENVIRONMENT
CHAPTER I--ENVIRONMENTAL PROTECTION AGENCY (CONTINUED)
PART 798_HEALTH EFFECTS TESTING GUIDELINES--Table of Contents
Subpart F_Genetic Toxicity
Sec. 798.5500 Differential growth inhibition of repair proficient and
repair deficient bacteria: ``Bacterial DNA damage or repair tests.''
(a) Purpose. Bacterial DNA damage or repair tests measure DNA damage
which is expressed as differential cell killing or growth inhibition of
repair deficient bacteria in a set of repair proficient and deficient
strains. These tests do not measure mutagenic events per se. They are
used as an indication of the interaction of a chemical with genetic
material implying the potential for genotoxicity.
(b) Definition. Test for differential growth inhibition of repair
proficient and repair deficient bacteria measure differences in
chemically induced cell killing between wild-type strains with full
repair capacity and mutant strains deficient in one or more of the
enzymes which govern repair of damaged DNA.
(c) Reference substances. These may include, but need not be limited
to, chloramphenicol or methyl methanesulfonate.
(d) Test method--(1) Principle. The tests detect agents that
interact with cellular DNA to produce growth inhibition or killing. This
interaction is recognized by specific cellular repair systems. The
assays are based upon the use of paired bacterial strains that differ by
the presence of absence of specific DNA repair genes. The response is
expressed in the preferential inhibition of growth or the preferential
killing of the DNA repair deficient strain since it is incapable of
removing certain chemical lesions from its DNA.
(2) Description. Several methods for performing the test have been
described. Those described here are:
(i) Tests performed on solid medium (diffusion tests).
(ii) Tests performed in liquid culture (suspension tests).
(3) Strain selection--(i) Designation. At the present time,
Escherichia coli polA (W3110/p3478) or Bacillus subtilis rec (H17/M45)
pairs are recommended. Other pairs may be utilized when appropriate.
(ii) Preparation and storage. Stock culture preparation and storage,
growth requirements, method of strain identification and demonstration
of appropriate phenotypic requirements should be performed using good
microbiological techniques and should be documented.
(4) Bacterial growth. Good microbiological techniques should be used
to grow fresh cultures of bacteria. The phase of growth and cell density
should be documented and should be adequate for the experimental design.
(5) Metabolic activation. Bacteria should be exposed to the test
substance both in the presence and absence of an appropriate metabolic
activation system. The most commonly used system
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is a cofactor supplemented postmitochondrial fraction prepared from the
livers of rodents treated with enzyme inducing agents. The use of other
species, tissues or techniques may also be appropriate.
(6) Control groups--(i) Concurrent controls. Concurrent positive,
negative, and vehicle controls should be included in each assay.
(ii) Negative controls. The negative control should show
nonpreferential growth inhibition (i.e., should affect both strains
equally). Chloramphenicol is an example of a negative control.
(iii) Genotype specific controls. Examples of genotype specific
positive controls are methyl methanesulfonate for polA strains and
mitomycin C for rec strains.
(iv) Positive controls to ensure the efficacy of the activation
system. The positive control reference substance for tests including a
metabolic activation system should be selected on the basis of the type
of activation system used in the test.
(v) Other positive controls. Other positive control reference
substances may be used.
(7) Test chemicals--(i) Vehicle. Test chemicals and positive and
negative control reference substances should be dissolved in an
appropriate vehicle and then further diluted in vehicle for use in the
assay.
(ii) Exposure concentrations. The test should initially be performed
over a broad range of concentrations. Among the criteria to be taken
into consideration for determining the upper limits of test chemical
concentration are cytotoxicity and solubility. Cytotoxicity of the test
chemical may be altered in the presence of metabolic activation systems.
For freely soluble nontoxic chemicals, the upper test chemical
concentration should be determined on a case by case basis. Because
results are expressed as diameters of zones of growth inhibition in the
diffusion test, it is most important that the amounts of chemical on the
disc (or in the wells) are exact replicates. When appropriate, a
positive response should be confirmed by testing over a narrow range of
concentrations.
(e) Test performance--(1) Diffusion assay--(i) Disc diffusion
assays. Disc diffusion assays. may be performed in two ways:
(A) A single strain of bacteria may be added to an agar overlay or
spread on the surface of the agar and the test chemical placed on a
filter disc on the surface of the agar or;
(B) DNA repair proficient and DNA repair deficient bacteria may be
streaked in a line on the surface of the agar of the same plate and a
disc saturated with test chemical placed on the surface of the agar in
contact with the streaks.
(ii) Well diffusion assays. In well diffusion assays, bacteria may
be either added to the agar overlay or spread onto the surface of the
agar. A solution of the test chemical is then placed into a well in the
agar.
(2) Suspension assays. (i) A bacterial suspension may be exposed to
the test chemical and the number of surviving bacteria determined (as
colony-forming units) either as a function of time of treatment or as a
function of the concentration of test agent.
(ii) Nonturbid suspensions of bacteria may be exposed to serial
dilutions of the test agent and a minimal inhibitory concentration for
each strain determined, as evidenced by the presence or absence of
visible growth after a period of incubation.
(iii) Paired bacterial suspensions (usually with some initial
turbidity) may be treated with a single dose of the chemical. Positive
results are indicated by a differential inhibition in the rate of
increase of turbidity of the paired cultures.
(3) Number of cultures. When using a plate diffusion procedure, at
least two independent plates should be used at each dilution. In liquid
suspension assays, at least two independent specimens for determination
of the number of viable cells should be plated.
(4) Incubation conditions. All plates in a given test should be
incubated for the same time period. This incubation period should be for
18 to 24 hrs at 37 [deg] C.
(f) Data and report--(1) Treatment of results--(i) Diffusion assays.
Results should be expressed in diameters of zones of growth inhibition
in millimeters or as areas derived therefrom as
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mm\2\. Dose-response data, if available, should be presented using the
same units.
(ii) Liquid suspension assays. (A) Survival data can be presented as
dose responses, preferably as percentage of survivors or fractional
survival of each strain or as a relative survival (ratio) of the two
strains.
(B) Results can also be expressed as the concentrations required to
effect a predetermined survival rate (e.g., D37, the dose
permitting 37 percent survival). These data are derived from the
survival curve. The concentration should be expressed as weight per
volume, as moles, or as molarity.
(C) Similarly, results can be expressed as minimal inhibitory
concentration or as minimal lethal dose. The former is determined by the
absence of visible growth in liquid medium and the latter is determined
by plating dilutions onto semisolid media.
(iii) In all tests, concentrations must be given as the final
concentrations during the treatment. Raw data, prior to transformation,
should be provided. These should include actual quantities measured,
e.g., neat numbers. For measurement of diffusion, the diameters of the
discs and/or well should be indicated and the measurements should
indicate whether the diameter of the discs and/or well was subtracted.
Moreover, mention should be made as to whether the test chemical gave a
sharp, diffuse, or double-zone of growth inhibition. If it is the
latter, the investigator should indicate whether the inner or the outer
zone was measured.
(iv) Viability data should be given as the actual plate counts with
an indication of the dilution used and the volume plated or as derived
titers (cells per ml). Transformed data alone in the absence of
experimental data are not acceptable (i.e, ratios, differences, survival
fraction).
(2) Statistical evaluation. Data should be evaluated by appropriate
statistical methods.
(3) Interpretation of results. (i) There are several criteria for
determining a positive result, one of which is a statistically
significant dose-related preferential inhibition or killing of the
repair deficient strain. Another criterion may be based upon detection
of a reproducible and statistically significant positive response for at
least one of the test points.
(ii) A test substance which does not produce either a statistically
significant dose-related preferential inhibition or killing of the
repair deficient strain or a statistically significant and reproducible
positive response at any one of the test points is considered not to
interact with the genetic material of the organisms used in assay.
(iii) Both biological and statistical significance should be
considered together in the evaluation.
(4) Test evaluation. DNA damage tests in bacteria do not measure DNA
repair per se nor do they measure mutations. They measure DNA damage
which is expressed as cell killing or growth inhibition. A positive
result in a DNA damage test in the absence of a positive result in
another system is difficult to evaluate in the absence of a better data
base.
(5) Test report. In addition to the reporting recommendations as
specified under 40 CFR part 792, subpart J the following specific
information should be reported:
(i) Bacterial strains used.
(ii) Phase of bacterial cell growth at time of use in the assay.
(iii) Media composition.
(iv) Details of both the protocol used to prepare the metabolic
activation system and its use in the assay.
(v) Treatment protocol, including doses used and rationale for dose
selection, positive and negative controls.
(vi) Method used for determination of degree of cell kill.
(vii) Dose-response relationship, if applicable.
(g) References. For additional background information on this test
guideline the following references should be consulted:
(1) Ames, B.N., McCann, J., Yamasaki, E. ``Methods for detecting
carcinogens and mutagens with the Salmonella/mammalian-microsome
mutagenicity test,'' Mutation Research, 31:347-364 (1975).
(2) Kada, T., Sadie, Y., Tutikawa, K. ``In vitro and host-mediated
``rec-
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assay'' procedures for screening chemical mutagens; and phloxine, a
mutagenic red dye detected,'' Mutation Research, 16:165-174 (1972).
(3) Leifer, Z., Kada, T., Mandel, M., Zeiger, E., Stafford, R.,
Rosenkranz, H.S. ``An evaluation of bacterial DNA repair tests for
predicting genotoxicity and carcinogenicity: A report of the U.S. EPA's
Gene-Tox Program,'' Mutation Research, 87:211-297 (1981).
(4) Slater, E.E., Anderson, M.D., Rosenkranz, H.S. ``Rapid detection
of mutagens and carcinogens.'' Cancer Research, 31:970-973 (1971).