A mutagen assay detecting forward mutations in an arabinose-sensitive strain of Salmonella typhimurium

Bacterial worth in genotoxicity assessment studies

Bacterial-based genotoxicity test systems play a significant role in the detection and evaluation of genotoxicity in vitro and have gained importance due to attributes like wide applicability, speed, high sensitivity, good reproducibility, and simplicity. The Salmonella microsomal mutagenicity assay was created by Ames and colleagues at the beginning of the 1970s, and it was based on the fundamental notion that in auxotrophic bacterial strains with inhibited growth, a mutant gene would revert to its original state on exposure to genotoxicants. This is the most successful and widely used in vitro genotoxicity test. Later, a number of additional test systems that incorporated DNA repair mechanisms including the bacterial SOS response were created. Genetic engineering has further provided significant advancement in these test systems with the development of highly sophisticated bacterial tester strains with significantly increased sensitivity to evaluate the chemical nature of hazardous substances and pollutants. These bacterial bioassays render an opportunity to detect the defined effects of compounds at the molecular level. In this review, all the aspects related to the bacterial system in genotoxicity assessment have been summarized and their role is elaborated concerning real-time requirements and future perspectives.

Current Status of Short-Term Tests for Evaluation of Genotoxicity, Mutagenicity, and Carcinogenicity of Environmental Chemicals and NCEs

The advent of the industrial revolution has seen a significant increase in the number of new chemical entities (NCEs) released in the environment. It becomes imperative to check the toxic potential of NCEs to nontarget species before they are released for commercial purposes because some of these may exert genotoxicity, mutagenicity, or carcinogenicity. Exposure to such compounds produces chemical changes in DNA, which are generally repaired by the DNA repair enzymes. However, DNA damage and its fixation may occur in the form of gene mutations, chromosomal damage, and numerical chromosomal changes and recombination. This may affect the incidence of heritable mutations in man and may be transferred to the progeny or lead to the development of cancer. Hence, adequate tests on NCEs have to be undertaken for the risk assessment and hazard prediction. Compounds that are positive in tests that detect such damages have the potential to be human mutagens/carcinogens. Only long-term animal bioassays, involving lifetime studies on animals, were used earlier to classify substances as mutagens/carcinogens. These tests were cumbersome and time consuming and required a lot of facilities and personnel. Short-term tests, therefore, were brought into practice. A "battery" of three to four of these short-term tests has been proposed now by a number of regulatory authorities for the classification of compounds as mutagenic or carcinogenic. This review deals with the current status of these short-term tests.

Isolation and prevalidation of an Escherichia coli tester strain for the use in mechanistic and metabolic studies of genotoxins

We have isolated an Escherichia coli tester strain for the use in mechanistic and metabolic studies of genotoxins. We started with one of the more used and better characterized E. coli K-12 laboratory strains, AB1157. We isolated a lipopolysaccharide defective mutant of strain AB1886 which is an excision repair deficient derivative of AB1157 and introduced a newly constructed plasmid pKR11, encoding mucAB, resulting in strain MR2101/pKR11. A genotoxicity assay was designed, monitoring the reversion to arginine prototrophy and a preliminary validation was carried out against Ames tester strain TA100 with a set of diagnostic compounds. The results seem to indicate that strain MR2101/pKR11 is an adequate tester strain which can be a useful tool in mechanistic studies. Moreover, this strain can serve as mother strain to isolate improved and more specialized tester strains.

Influence of S9 mix on the expression of mutants in the L-arabinose resistance test of Salmonella typhimurium

S9 mix produces an effect similar to that of D-glucose in the L-arabinose resistance test of Salmonella typhimurium, releasing the growth inhibition exerted by L-arabinose. Two elements are responsible for this effect: the glucose-6-phosphate present in the cofactors of the S9 mix and the S9 fraction itself. UV light was used as a mutagen to compare the efficiency of S9 mix and D-glucose in allowing phenotypic expression of mutants in selective plates with L-arabinose; 0.5 ml of S9 mix per plate showed and efficacy similar to that of 0.5 mg of D-glucose per plate. To verify that the S9 mix is equivalent to D-glucose traces in selective plates with respect to the number of induced mutants in compounds requiring metabolic activation, we utilized 2 direct-acting nitrofurans. Our conclusion is that activation of agents could be erroneously attributed to the S9 mix, when plates with 0.5 mg of D-glucose are compared to plates with 0.5 ml of S9 mix plus 0.5 mg of D-glucose. Our results suggest that D-glucose traces be omitted in experiments requiring the presence of the S9 mixture.

Simple method for precise determination of chemical lethality in the L-arabinose resistance test of Salmonella typhimurium

A simple method is described for the determination of the lethal effects of chemicals assayed with the L-arabinose resistance test of Salmonella typhimurium. The method uses a mixed culture of 2 isogenic bacterial strains which are distinguished on the basis of their different nutritional requirements: sensitivity or resistance to L-arabinose, auxotrophy or prototrophy to histidine and leucine. BA13 (the mutation indicator strain) is the strain for routine screening of mutagens and allows the selection of forward mutation from L-arabinose sensitivity to L-arabinose resistance. BAL13 (the survival indicator strain) is a derivative of BA13. Both bacterial strains are found to be equally sensitive to the lethal effects of mutagens. The method described permits the measurement of cell survival at the same high cell concentration as used in the measurement of the mutant yield and in the same type of minimal medium with L-arabinose and glycerol, except for the histidine supplement in the mutant plates or the leucine in the survival plates.

Response of the L-arabinose forward mutation assay of Salmonella typhimurium to frameshift-type mutagens

The present study was designed to evaluate the capacity of the L-arabinose resistance test of Salmonella typhimurium in the detection of frameshift-type mutagens. To this end the response of the Ara test was examined with respect to 15 chemicals which had been previously described as able to revert the Ames tester strain TA97. The mutagenicity of each compound was determined by the liquid test under experimental conditions which optimize the mutagenic response of the Ara test with the tester strain BA9. Strain TA97 was used simultaneously with BA9. The Ara forward-mutation assay efficiently detected the mutagenic activity of 14 out of the 15 chemicals assayed. PR toxin was the only compound which gave a weak dose response without doubling the spontaneous mutant level. In comparison with the Ara test, a total of 3 chemicals (HZ, PE and PR toxin) were not found to be mutagenic with strain TA97. In most cases (11/15) the mutagenic response of the Ara test was comparatively greater than that of strain TA97. Three chemicals (DEO, PRF and 9-AA) were detected with quite similar degrees of sensitivity by both mutation assays. ICR-191, which seems highly specific in reverting frameshift mutations with added cytosines in a run of cytosines, was the only chemical with a lower mutagenic activity in the Ara test than in strain TA97. The results enhance the interest of the L-arabinose forward-mutation assay as an alternative to the set of specific tester strains used by the histidine reverse-mutation assay in massive, general and primary screening for genotoxic agents.

Preliminary evaluation of treatment and selection conditions which affect expression of anthracycline mutagenicity in Salmonella typhimurium and a diploid human lymphoblast cell line

Mutagenic potency in the Ames Salmonella test is an important endpoint that can be influenced by biological and technical factors. The ranking of mutagenic activity of a series of anthracyclines was measured using different conditions of exposure and mutation selection. A 20 min preincubation treatment version of the Ames test using a 0.2-2.0 microgram/ml (0.36-3.60 nM/ml) dose range of each of the anthracyclines Adriamycin, Daunomycin, Carminomycin, 4'-O-methyldoxorubicin and 4-demethoxydoxorubicin confirmed the order of mutagenic potency seen with the same compounds under direct plating conditions. Preincubation results also confirm direct-plating results by showing the greater sensitivity of selection to His+ reversion over 8-azaguanine resistance to anthracycline mutagenicity. However, the order of mutagenic potency was changed by lengthening the preincubation treatment time to 2 h or reducing the population density of the treated cell inoculum by ten fold. These results suggest that certain treatment conditions enable the treated cells to diminish the phenotypic expression of anthracycline mutagenicity. For comparative purposes, daunomycin and Adriamycin mutagenicity in response to 0.1-0.2 nM/ml and 0.1-0.3 nM/ml dose ranges, respectively, were assessed in a human cell culture system with 6-thioguanine and 5-trifluorothymidine forward mutation selection. A daunomycin dose of 0.1 nM/ml generated approximately 25-fold and 20-fold increases in mutant fraction with 6-thioguanine and 5-trifluorothymidine selections, respectively. Equivalent dosing with Adriamycin generated approximately a 4-fold increase in mutant fraction with 6-thioguanine selection and little or no increase with 5-trifluothymidine selection.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutagenicity study on pyrazole, seven pyrazole derivatives, and two nitroimidazoles with the L-arabinose resistance test of Salmonella typhimurium

The mutagenicity of pyrazole and seven pyrazole derivatives (4-nitropyrazole, 4-bromopyrazole, 1-methyl-4-nitropyrazole, 3,5-dimethyl-4-nitropyrazole, 1-methyl-4-bromopyrazole, 4,4'-dinitro-1, 1'-methylene-dipyrazole and 4,4'-dibromo-1,1'-methylene-dipyrazole) has been investigated with the L-arabinose forward mutation assay of Salmonella typhimurium. Two nitroimidazoles (1-methyl-5-nitroimidazole and metronidazole) were included as reference drugs. The mutagenicity of each chemical was determined by both preincubation and liquid tests, in the presence or absence of S9 microsomal fraction. The mutagenic response was expressed as the absolute number of L-arabinose resistant mutants growing in selective plates, supplemented with traces of D-glucose. Strain BA13 with a wildtype lipopolysaccharide barrier was used as a comparison to the deep rough derivative BA9. No mutagenic effect was detected with pyrazole and two of its derivatives, 1-methyl-4-bromopyrazole and 4,4'-dibromo-1,1'-methylene-dipyrazole. The other five pyrazole derivatives were mutagenic to different degrees, although their mutagenic potencies were always considerably lower than those of the two nitroimidazoles. The results suggest that 4-nitropyrazoles, as well as 4,4'-dinitro-1, 1'-methylene-dipyrazoles, should be investigated further as alternatives to, or even substitutes for, the currently used nitroimidazoles.

A simple method for the extraction of mutagens from airborne particles

Organic materials were extracted with acetone from filters of airborne particles by soaking, shaking, soxhletion, and sonication. These extracts were tested with and without S9 for mutagenicity using Ames assay and arabinose-resistant assay of Salmonella typhimurium. Among the extraction methods, soaking extract had the highest mutagenic activity followed by sonication, shaking, and soxhletion in both the assays. With the samples studied, it was concluded that soaking with acetone for 1/2 hr is the simplest and an efficient procedure for the extraction of mutagens from airborne particles.

Oxidative mutagens specific for A-T base pairs induce forward mutations to L-arabinose resistance in Salmonella typhimurium

Strain TA102 of S. typhimurium is a new histidine-requiring mutant, particularly suited to the detection of oxidative mutagens acting at A.T base pairs. 10 oxidizing chemicals, previously tested in strain TA102, were used to evaluate the mutagenic sensitivity of the L-arabinose forward mutation assay of S. typhimurium with respect to those types of mutagens. The mutagenicity of each compound was determined by liquid test, measuring both the frequency of mutants among the survivors and the absolute number of mutants growing in selective plates with traces of D-glucose. Strain BA13 with a wild-type lipopolysaccharide barrier was used as compared to the deep rough derivative strain BA9. The chemicals studied were: bleomycin, t-butyl hydroperoxide, chromium trioxide, cumene hydroperoxide, formaldehyde, glyoxal, glutaraldehyde, hydrogen peroxide, paraquat, and phenylhydrazine. Additionally, ultrasonic oscillation was used as a presumable non-mutagenic lethal control treatment. The L-arabinose forward mutation assay detected the mutagenic activity of all the chemicals under study with a high degree of sensitivity, including paraquat which is unable to revert strain TA102. Positive responses were obtained at doses equivalent to or 10 times lower than the doses detected by strain TA102. The results support the idea that the L-arabinose forward mutation assay could replace the set of specific tester strains used by the histidine reverse mutation assay in general screening for genetic toxins.

Validation of the Salmonella (SV50)/arabinose-resistant forward mutation assay system with 26 compounds

Mutagenic sensitivity of the Salmonella/arabinose-resistant (Arar) assay system using the tester strain SV50 was evaluated with 26 compounds both by the preincubation and the standard plate incorporation tests. The mutagenic activity of all 26 compounds was also tested with TA98 and/or TA100 of the Ames Salmonella/microsome assay system. The results indicate that 13 and 10 of 26 compounds were mutagenic and nonmutagenic, respectively, in both assay systems. PR toxin and hydrogen peroxide were mutagenic only in the Arar assay, while 2-nitrofluorene was mutagenic only in the Ames assay. The results also show that the mutagenic response of SV50 to 13 of 15 mutagenic compounds was much higher (2.1-154-fold) if the compounds were tested with the preincubation rather than the plate incorporation test. The mutagenic activity of 4 compounds (diethyl sulfate, niridazole, PR toxin and hydrogen peroxide) in the Arar assay was detected only with the preincubation test. Since the Arar assay using tester strain SV50 has similar mutagenic sensitivity as the Ames assay to chemicals with different modes of action and since it requires only one tester strain, we find this assay system to be useful for screening environmental mutagens. Based on the effectiveness of the preincubation test in this study, it is recommended that the preincubation test instead of the plate incorporation test be used for the Arar assay system with tester strain SV50.

Genetic characterization of Salmonella typhimurium LT2 ara mutations

Seventeen independently isolated L-arabinose utilization-deficient mutants of Salmonella typhimurium LT2 were characterized. Four complementation groups (araA, araB, araC, and araD) were identified and were equivalent to the same genes in the ara system in Escherichia coli. The order of the four genes was determined to be araD-araA-araB-araC-leu. Two transcription units were found: the araBAD operon was transcribed counterclockwise, and the araC gene was transcribed clockwise.

Mutagenicity studies of ambient airborne particles. II. Comparison of extraction methods

Organic materials were extracted with acetone from airborne particles by shaking, soxhletion and sonication for varying durations. 4-h, 1-h and 1/8-min extractions by shaking, soxhletion and sonication, respectively gave maximum his+ revertants with the Ames Salmonella/microsome assay. In a comparative study of extraction methods, sonication gave the highest and soxhletion the lowest mutagenic response. It appears that sonication with acetone is the best procedure for the extraction of mutagens from airborne particles as shown by Ames assay and Arar assay systems in Salmonella typhimurium.

Genotoxic properties of 2,4,7-trinitro-9-fluorenone

The genotoxic effects of 2,4,7-trinitro-9-fluorenone (TNF) were studied in assays employing procaryotic (Salmonella typhimurium and Escherichia coli) and eucaryotic (Saccharomyces cerevisiae, mouse lymphoma L5178Y and Chinese hamster ovary) cells. The results show that TNF is a potent mutagen for procaryotes. It causes both frame-shift and base-pair substitution mutations, although frame-shift mutations were predominant. In Saccharomyces cerevisiae, this compound appeared to be too toxic to permit detection of genotoxic effects. TNF was also toxic to mouse lymphoma cells and Chinese hamster ovary cells but the toxic effects were reduced by metabolic activation. TNF induced a clear increase in sister-chromatid exchanges in CHO cells and in mutant frequency in mouse lymphoma cells both in the presence and absence of metabolic activation.

Mutagenic detection of complex environmental mixtures using the Salmonella/arabinose-resistant assay system

The mutagenic sensitivity of SV50, the R-factor plasmid containing tester, of the Salmonella/arabinose-resistant assay system has been evaluated with different environmental complex mixtures, including extracts of airborne and diesel emission particles, oil-shale ash, nitrosated coal dust and water samples. The mutagenicities of all extracts were detectable with this assay. This study indicates that the arabinose-resistant assay with SV50 is useful for the detection of the mutagenic activity of environmental complex mixtures.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

Mutagenicity of oil-shale ash

3 oil-shale ash samples were extracted with solvents and analyzed for mutagenicity with a number of tests systems. In Salmonella typhimurium, the ash extracts were highly mutagenic with the Ames his reversion and the ara-resistant systems. Mutation induction by the ash in Salmonella was independent of metabolic activation and was of the frameshift type. These ash extracts showed a substantial killing effect, but failed to induced ad-3 reversion in Neurospora crassa, gene conversion and mitotic crossing-over in Saccharomyces cerevisiae and TGr mutation in cultured CHO cells.

An Escherichia coli mutant refractory to nitrosoguanidine mutagenesis

A newly-isolated Escherichia coli mutant suffers only about 10% as many mutations as normal strains on exposure to nitrosoguanidine. The responsible mutation, inm-1, maps at approximately minute 79 in the current E. coli genetic map. The mutant is normal for overall growth, nitrosoguanidine lethality, spontaneous mutagenesis, ultraviolet light lethality and mutagenesis, ethyl methanesulfonate lethality and mutagenesis, and the adaptive repair induced by alkylating agents. The existence of this mutation proves that nitrosoguanidine mutagenesis is not merely the result of reactions between the chemical and DNA, but requires specific cellular function(s), and underscores the peculiarity of nitrosoguanidine as a mutagen.

The L-arabinose-resistance test with Salmonella typhimurium strain SV3 selects forward mutations at several ara genes

A new assay has been described for mutagenicity testing using an L-arabinose-sensitive strain of Salmonella typhimurium. The test strain SV3 and several L-arabinose-resistant mutants selected therefrom are characterized in the present study by 3 different criteria: inhibition of growth by L-arabinose, accumulation of keto-sugars, and activities of the enzymes involved in L-arabinose catabolism. Strain SV3 (ara-531) shows high levels of inducible L-arabinose isomerase (EC 5.3.1.4) and L-ribulokinase (EC 2.7.1.16) activities, but is deficient in L-ribulose-5-phosphate 4-epimerase (EC 5.1.3.4), the enzyme encoded in Escherichia coli by gene D in the araBAD operon. Addition of L-arabinose to SV3 growing in glycerol or casamino acids stops growth. D-Glucose only partially reverses this inhibition. Reversion of the ara-531 mutation restores different levels of epimerase activity and resistance to L-arabinose. However, the great majority of the L-arabinose-resistant mutants do not utilize L-arabinose. The physiological and enzymatic properties of these L-arabinose non-utilizing mutants suggest that L-arabinose resistance is due to forward mutations in at least 3 other genes, araA, araB and araC, blocking steps prior to L-ribulose 5-phosphate accumulation.

Evaluation of three metabolic activation systems by a forward mutation assay in Salmonella

The strain SV3 of Salmonella typhimurium was used as the indicator bacterium in the intrasanguineous host-mediated mutagenicity assay. Bacterial distribution and spontaneous mutation frequency were determined after intravenous injection of SV3 into CD1 male mice. Bacteria were cleared at an exponential rate from the blood stream and recovered mainly from the liver and in smaller quantities from the lungs and kidneys. No bactericidal effect was observed during incubation within the animal, and bacterial division occurred in the liver and probably in the kidneys. The significance of an increased mutation frequency of bacteria recovered from untreated animals is discussed. Mutation induction was measured in bacteria recovered from liver, lungs and kidneys of CD1 mice and CD rats treated with dimethylnitrosamine (DMN). The sensitivity of the intrasanguineous host-mediated technique was compared with the sensitivity of the assay in vitro with microsomal preparations from each tissue and host. Activation by isolated perfused liver and lungs from CD rats was included for comparison with the results from experiments in vivo and in vitro.

Forward mutations to arabinose resistance in Salmonella typhimurium strains: a sensitive assay for mutagenicity testing

The forward-mutation assay using the L-arabinose-sensitive strain SV3 of Salmonella typhimurium has been calibrated against a selected set of mutagens. Strain SV3 is sensitive to chemicals causing base-pair substitutions, frameshift mutations and deletions. New strains deficient for the excision-repair system or the lipopolysaccharide barrier or both have been selected from strain SV3. The additional mutations do not affect the independence of the assay from experimental artifacts due to physiological or lethal damage or differences in plating density. The new strains are more sensitive than SV3 to certain mutagens. Techniques for using this set of strains are presented and their relative advantages discussed.