Registration Dossier

Administrative data

Endpoint:
in vitro gene mutation study in bacteria
Type of information:
experimental study
Adequacy of study:
key study
Study period:
04 August - 01 September 2017
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Remarks:
GLP study conducted according to OECD Guideline 471 without any deviation

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
2017

Materials and methods

Test guidelineopen allclose all
Qualifier:
according to
Guideline:
OECD Guideline 471 (Bacterial Reverse Mutation Assay)
Version / remarks:
dated 21 July 1997
Deviations:
no
Qualifier:
according to
Guideline:
EU Method B.13/14 (Mutagenicity - Reverse Mutation Test Using Bacteria)
Version / remarks:
dated 30 May 2008
Deviations:
no
Qualifier:
according to
Guideline:
EPA OPPTS 870.5100 - Bacterial Reverse Mutation Test (August 1998)
Version / remarks:
dated August 1998
Deviations:
no
Principles of method if other than guideline:
Not applicable
GLP compliance:
yes (incl. certificate)
Remarks:
28 October 2016
Type of assay:
bacterial reverse mutation assay

Test material

Reference
Name:
Unnamed
Type:
Constituent
Test material form:
liquid: viscous
Specific details on test material used for the study:
STABILITY AND STORAGE CONDITIONS OF TEST MATERIAL
- Storage condition of test material: Room temperature in the dark

TREATMENT OF TEST MATERIAL PRIOR TO TESTING
- The test item was accurately weighed and, on the day of each experiment, approximate half-log dilutions were prepared in pre-dried tetrahydrofuran by mixing on a vortex mixer. No purity correction was required. Tetrahydrofuran is toxic to the bacterial cells at and above 50 µL (0.05 mL), therefore all of the formulations were prepared at concentrations four times greater than required on Vogel-Bonner agar plates. To compensate, each formulation was dosed using 25 µL (0.025 mL) aliquots. Tetrahydrofuran is considered an acceptable vehicle for use in this test system (Maron et al., 1981). All formulations were used within four hours of preparation and were assumed to be stable for this period.

Method

Target gene:
histidine locus for Salmonella strains and tryptophan for E. coli strain
Species / strain
Species / strain / cell type:
S. typhimurium TA 1535, TA 1537, TA 98, TA 100 and E. coli WP2
Details on mammalian cell type (if applicable):
Not applicable
Additional strain / cell type characteristics:
not applicable
Cytokinesis block (if used):
Not applicable
Metabolic activation:
with and without
Metabolic activation system:
rat liver homogenate metabolizing system (10% liver S9 in standard co-factors)
Test concentrations with justification for top dose:
Experiment 1 - Plate Incorporation Method: 1.5, 5, 15, 50, 150, 500, 1500 and 5000 µg/plate, with and without S9-mix
Experiment 2 - Pre-Incubation Method: 15, 50, 150, 500, 1500 and 5000 µg/plate, with and without S9-mix
Vehicle / solvent:
- Vehicle(s)/solvent(s) used: Tetrahydrofuran (THF)
- Justification for choice of solvent/vehicle: In solubility checks performed in house, the test item was noted as insoluble in dimethyl sulphoxide and dimethyl formamide at 50 mg/mL and acetone at 100 mg/mL but fully soluble in tetrahydrofuran at 200 mg/mL. Tetrahydrofuran was selected as the vehicle. Following solubility information provided by the Sponsor, sterile distilled water was not evaluated as a potential vehicle in this test system.
Controlsopen allclose all
Untreated negative controls:
yes
Remarks:
untreated
Negative solvent / vehicle controls:
yes
True negative controls:
no
Positive controls:
yes
Positive control substance:
4-nitroquinoline-N-oxide
9-aminoacridine
N-ethyl-N-nitro-N-nitrosoguanidine
Remarks:
without metabolic activation
Untreated negative controls:
yes
Remarks:
untreated
Negative solvent / vehicle controls:
yes
True negative controls:
no
Positive controls:
yes
Positive control substance:
benzo(a)pyrene
other: 2-Aminoanthracene
Remarks:
with metabolic activation
Details on test system and experimental conditions:
SOURCE OF TEST SYSTEM: The bacteria used in the test were obtained from:
- University of California, Berkeley, on culture discs, on 04 August 1995
- British Industrial Biological Research Association, on a nutrient agar plate, on 17 August 1987

METHOD OF APPLICATION: in agar (plate incorporation) and preincubation

DURATION
- Preincubation period: 20 minutes in Experiment 2 at 34 to 40°C
- Exposure duration: ca. 48 hours for both experiments

CONTROLS:
- Vehicle/solvent control: THF
- Negative (untreated) controls were performed to assess the spontaneous revertant colony rate.
- Positive control items used demonstrated a direct and indirect acting mutagenic effect depending on the presence or absence of metabolic activation.
- Sterility controls were performed in triplicate as follows:
Top agar and histidine/biotin or tryptophan in the absence of S9-mix;
Top agar and histidine/biotin or tryptophan in the presence of S9-mix; and
The maximum dosing solution of the test item in the absence of S9-mix only (test in singular only).

NUMBER OF REPLICATIONS: Triplicate

- OTHER: All of the plates were incubated at 37 ± 3 °C for approximately 48 hours and scored for the presence of revertant colonies using an automated colony counting system. The plates were viewed microscopically for evidence of thinning (toxicity). Several manual counts were required due to revertant colonies spreading slightly, thus distorting the actual plate count.
Rationale for test conditions:
The dose range for Experiment 1 was predetermined and was 1.5 to 5000 µg/plate (i.e. maximum recommended dose level). The experiment was repeated on a separate day (pre-incubation method) using fresh cultures of the bacterial strains and fresh test item formulations. The dose range was amended, following the results of Experiment 1, and was 15 to 5000 µg/plate. Six test item dose levels per bacterial strain were selected in the second mutation test in order to achieve both a minimum of four non-toxic dose levels and the potential toxic limit of the test item following the change in test methodology from plate incorporation to pre-incubation.
Evaluation criteria:
Criteria for determining a positive result:
- A dose-related increase in mutant frequency over the dose range tested (De Serres and Shelby, 1979).
- A reproducible increase at one or more concentrations.
- Biological relevance against in-house historical control ranges.
- If exposure to a test item produces a reproducible increase in mean revertant colony numbers of at least twice (three times in the case of strains TA1535 and TA1537, which have relatively low spontaneous reversion rates) that of the concurrent vehicle controls, with some evidence of a positive concentration-response relationship, it will be considered to exhibit mutagenic activity in this test system. (Cariello and Piegorsch, 1996)).

A test item will be considered non-mutagenic (negative) in the test system if the above criteria are not met.

Although most experiments will give clear positive or negative results, in some instances the data generated will prohibit making a definite judgment about test item activity. Results of this type will be reported as equivocal.
Statistics:
None

Results and discussion

Test results
Key result
Species / strain:
other: S. typhimurium TA 1535, TA 1537, TA 98, TA 100 and E. coli WP2
Metabolic activation:
with and without
Genotoxicity:
negative
Cytotoxicity / choice of top concentrations:
no cytotoxicity
Vehicle controls validity:
valid
Untreated negative controls validity:
valid
Positive controls validity:
valid
Additional information on results:
TEST-SPECIFIC CONFOUNDING FACTORS
- Precipitation: No test item precipitate was observed on the plates at any of the doses tested in either the presence or absence of S9-mix.

MUTAGENICITY
- The vehicle (tetrahydrofuran (THF)) control plates gave counts of revertant colonies within the normal range. All of the positive control chemicals used in the test induced marked increases in the frequency of revertant colonies, both with or without metabolic activation. Thus, the sensitivity of the assay and the efficacy of the S9-mix were validated.
- No visible reduction in the growth of the bacterial background lawn was noted at any dose level, either in the presence or absence of metabolic activation (S9-mix), in Experiment 1 (plate incorporation method). In Experiment 2 (pre incubation method), the test item caused a visible reduction in all of the Salmonella bacterial lawns in the absence of S9-mix, initially from 1500 µg/plate. In the presence of S9-mix, toxicity was noted to just one bacterial strain (Salmonella strain TA1535), initially from 1500 µg/plate.
- No significant increases in the frequency of revertant colonies were recorded for any of the bacterial strains, with any dose of the test item, either with or without metabolic activation (S9-mix) in Experiment 1 (plate incorporation method) and Experiment 2 (pre incubation method).
- Refer Tables 7.6.1/1 to 7.6.1/5 for more details.

HISTORICAL CONTROL DATA (with ranges, means and standard deviation and confidence interval (e.g. 95%)
- Positive historical control data: Refer Table 7.6.1/6
- Negative (solvent/vehicle) historical control data: Refer Table 7.6.1/6

OTHERS:
- Prior to use, the master strains were checked for characteristics, viability and spontaneous reversion rate (all were found to be satisfactory). The amino acid supplemented top agar and the S9-mix used in both experiments was shown to be sterile. The test item formulation was also shown to be sterile.
- Results for the negative controls (spontaneous mutation rates) are presented in 7.6.1/1 and were considered to be acceptable. These data are for concurrent untreated control plates performed on the same day as the Mutation Test.

Any other information on results incl. tables

Table 7.6.1/1:Spontaneous Mutation Rates (Concurrent Negative Controls)

 

Number of revertants (mean number of colonies per plate)

Base-pair substitution type

Frameshift type

Experiment 1

TA100

TA1535

WP2uvrA

TA98

TA1537

79

 

29

 

26

 

34

 

8

 

90

(87)

24

(27)

31

(30)

26

(29)

17

(11)

91

 

27

 

32

 

27

 

9

 

Experiment 2

TA100

TA1535

WP2uvrA

TA98

TA1537

84

 

19

 

22

 

24

 

18

 

77

(80)

17

(18)

19

(22)

33

(29)

13

(13)

78

 

18

 

24

 

29

 

8

 

 

 

Table 7.6.1/2:Test Results: Experiment 1 – Without Metabolic Activation

 

Test Period

From: 17 August 2017

To: 20 August 2017

S9-Mix

(-)

Dose Level

Per Plate

Number of revertants (mean) +/- SD

Base-pair substitution strains

Frameshift strains

TA100

TA1535

WP2uvrA

TA98

TA1537

Solvent Control

(THF)

101

77

95

(91)

12.5#

38

34

35

(36)

2.1

27

30

31

(29)

2.1

30

33

21

(28)

6.2

12

11

11

(11)

0.6

1.5 µg

85

98

93

(92)

6.6

33

36

35

(35)

1.5

35

23

25

(28)

6.4

21

34

27

(27)

6.5

12

12

13

(12)

0.6

5 µg

84

88

77

(83)

5.6

31

31

38

(33)

4.0

43

36

19

(33)

12.3

22

25

26

(24)

2.1

7

10

13

(10)

3.0

15 µg

93

90

84

(89)

4.6

34

37

32

(34)

2.5

32

35

33

(33)

1.5

27

25

30

(27)

2.5

8

9

10

(9)

1.0

50 µg

79

70

102

(84)

16.5

29

29

39

(32)

5.8

28

31

35

(31)

3.5

22

23

26

(24)

2.1

10

8

14

(11)

3.1

150 µg

86

82

79

(82)

3.5

35

38

32

(35)

3.0

32

32

28

(31)

2.3

18

28

26

(24)

5.3

8

5

15

(9)

5.1

500 µg

82

71

90

(81)

9.5

29

30

35

(31)

3.2

16

29

34

(26)

9.3

32

26

20

(26)

6.0

9

10

9

(9)

0.6

1500 µg

71

67

90

(76)

12.3

37

32

36

(35)

2.6

48

26

33

(36)

11.2

34

28

31

(31)

3.0

10

6

11

(9)

2.6

5000 µg

90

78

88

(85)

6.4

38

35

39

(37)

2.1

30

38

28

(32)

5.3

29

28

36

(31)

4.4

7

8

10

(8)

1.5

Positive controls

S9-Mix

(-)

Name

Dose Level

No. of Revertants

ENNG

ENNG

ENNG

4NQO

9AA

3 µg

5 µg

2 µg

0.2 µg

80 µg

675

710

713

(699)

21.1

736

731

733

(733)

2.5

956

1058

969

(994)

55.5

190

182

183

(185)

4.4

316

322

344

(327)

14.7

ENNG:N-ethyl-N'-nitro-N-nitrosoguanidine

4NQO:4-Nitroquinoline-1-oxide

9AA:   9-Aminoacridine

#:       Standard deviation

 

Table 7.6.1/3:Test Results: Experiment 1 – With Metabolic Activation

 

Test Period

From: 17 August 2017

To: 20 August 2017

S9-Mix

(+)

Dose Level

Per Plate

Number of revertants (mean) +/- SD

Base-pair substitution strains

Frameshift strains

TA100

TA1535

WP2uvrA

TA98

TA1537

Solvent Control

(THF)

84

90

84

(86)

3.5#

34

29

21

(28)

6.6

46

40

40

(42)

3.5

28

24

30

(27)

3.1

11

14

11

(12)

1.7

1.5 µg

86

97

81

(88)

8.2

25

24

32

(27)

4.4

24

33

39

(32)

7.5

26

24

29

(26)

2.5

11

12

15

(13)

2.1

5 µg

67

78

79

(75)

6.7

26

22

25

(24)

2.1

42

39

39

(40)

1.7

29

26

23

(26)

3.0

13

12

8

(11)

2.6

15 µg

76

83

88

(82)

6.0

30

27

26

(28)

2.1

35

41

36

(37)

3.2

24

22

24

(23)

1.2

12

14

11

(12)

1.5

50 µg

69

98

73

(80)

15.7

29

21

23

(24)

4.2

35

33

33

(34)

1.2

28

28

30

(29)

1.2

10

8

12

(10)

2.0

150 µg

91

70

75

(79)

11.0

28

22

22

(24)

3.5

28

34

30

(31)

3.1

22

29

23

(25)

3.8

9

13

11

(11)

2.0

500 µg

74

72

75

(74)

1.5

25

31

26

(27)

3.2

40

39

37

(39)

1.5

34

21

24

(26)

6.8

9

8

13

(10)

2.6

1500 µg

88

71

70

(76)

10.1

24

20

29

(24)

4.5

39

44

38

(40)

3.2

25

24

28

(26)

2.1

10

8

9

(9)

1.0

5000 µg

76

73

79

(76)

3.0

26

15

27

(23)

6.7

31

45

39

(38)

7.0

27

28

27

(27)

0.6

10

10

12

(11)

1.2

Positive controls

S9-Mix

(+)

Name

Dose Level

No. of Revertants

2AA

2AA

2AA

BP

2AA

1 µg

2 µg

10 µg

5 µg

2 µg

931

487

1113

(844)

322.0

325

317

371

(338)

29.1

158

183

170

(170)

12.5

193

229

234

(219)

22.4

694

643

648

(662)

28.1

2AA:   2-Aminoanthracene

BP:     Benzo(a)pyrene

#:       Standard deviation

 

 

Table 7.6.1/4:Test Results: Experiment 2 – Without Metabolic Activation

 

Test Period

From: 29 August 2017

To: 01 September 2017

S9-Mix

(-)

Dose Level

Per Plate

Number of revertants (mean) +/- SD

Base-pair substitution strains

Frameshift strains

TA100

TA1535

WP2uvrA

TA98

TA1537

Solvent Control

(THF)

117

110

98

(108)

9.6#

13

12

17

(14)

2.6

21

22

24

(22)

1.5

35

31

30

(32)

2.6

11

12

7

(10)

2.6

15 µg

97

85

81

(88)

8.3

14

15

9

(13)

3.2

23

22

21

(22)

1.0

28

20

29

(26)

4.9

9

10

11

(10)

1.0

50 µg

107

98

115

(107)

8.5

16

11

16

(14)

2.9

17

22

18

(19)

2.6

15

32

21

(23)

8.6

6

9

9

(8)

1.7

150 µg

116

114

108

(113)

4.2

11

14

12

(12)

1.5

24

16

32

(24)

8.0

15

18

19

(17)

2.1

15

10

10

(12)

2.9

500 µg

129

104

100

(111)

15.7

12

11

14

(12)

1.5

35

12

18

(22)

11.9

12

13

17

(14)

2.6

8

10

14

(11)

3.1

1500 µg

111

104

88

(101)

11.8

14

11

9

(11)

2.5

15

19

33

(22)

9.5

14

20

29

(21)

7.5

4

14

4

(7)

5.8

5000 µg

120

113

98

(110)

11.2

13

15

13

(14)

1.2

19

16

24

(20)

4.0

22

14

21

(19)

4.4

11

4

15

(10)

5.6

Positive controls

S9-Mix

(-)

Name

Dose Level

No. of Revertants

ENNG

ENNG

ENNG

4NQO

9AA

3 µg

5 µg

2 µg

0.2 µg

80 µg

810

1092

1251

(1051)

223.3

509

474

590

(524)

59.5

758

748

804

(770)

29.9

327

352

375

(351)

24.0

187

183

258

(209)

42.2

ENNG:N-ethyl-N'-nitro-N-nitrosoguanidine

4NQO:4-Nitroquinoline-1-oxide

9AA:   9-Aminoacridine

#:       Standard deviation

 

 

Table 7.6.1/5:Test Results: Experiment 2 – With Metabolic Activation

 

Test Period

From: 29 August 2017

To: 01 September 2017

S9-Mix

(+)

Dose Level

Per Plate

Number of revertants (mean) +/- SD

Base-pair substitution strains

Frameshift strains

TA100

TA1535

WP2uvrA

TA98

TA1537

Solvent Control

(THF)

85

89

88

(87)

2.1#

24

25

19

(23)

3.2

29

26

37

(31)

5.7

32

32

40

(35)

4.6

16

17

6

(13)

6.1

15 µg

88

111

82

(94)

15.3

28

27

21

(25)

3.8

28

14

22

(21)

7.0

37

35

42

(38)

3.6

7

23

10

(13)

8.5

50 µg

76

79

90

(82)

7.4

17

22

26

(22)

4.5

15

24

31

(23)

8.0

29

39

37

(35)

5.3

16

11

14

(14)

2.5

150 µg

88

77

76

(80)

6.7

19

22

19

(20)

1.7

19

21

15

(18)

3.1

28

31

38

(32)

5.1

15

11

11

(12)

2.3

500 µg

80

65

81

(75)

9.0

21

28

18

(22)

5.1

27

29

26

(27)

1.5

29

31

24

(28)

3.6

17

7

13

(12)

5.0

1500 µg

68

78

100

(82)

16.4

22

20

22

(21)

1.2

25

36

26

(29)

6.1

46

30

25

(34)

11.0

15

9

14

(13)

3.2

5000 µg

68

70

68

(69)

1.2

24

18

23

(22)

3.2

33

36

36

(35)

1.7

41

37

30

(36)

5.6

7

11

10

(9)

2.1

Positive controls

S9-Mix

(+)

Name

Dose Level

No. of Revertants

2AA

2AA

2AA

BP

2AA

1 µg

2 µg

10 µg

5 µg

2 µg

887

967

1141

(998)

129.9

208

216

232

(219)

12.2

214

229

236

(226)

11.2

148

152

183

(161)

19.2

433

296

492

(407)

100.6

2AA:   2-Aminoanthracene

BP:     Benzo(a)pyrene

#:       Standard deviation

 

 

Table 7.6.1/6:History Profile of Vehicle and Positive Control Values

 

COMBINED VEHICLE AND UNTREATED CONTROL VALUES 2015

Strain

S9-Mix

TA100

TA1535

TA102

WP2uvrA

TA98

TA1537

WP2uvrA

pKM101

WP2pKM101

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

Values†

274

278

504

285

26

13

461

229

526

299

506

282

42

51

39

49

Min

60

61

7

7

222

278

10

12

11

10

4

6

87

98

89

93

Max

166

175

31

29

376

388

58

43

45

46

27

27

237

254

174

177

Mean

91

95

16

14

286

333

24

27

21

24

12

13

156

164

123

137

SD

19.3

19.1

4.5

4.0

48.7

37.6

5.6

5.9

6.2

6.1

3.8

3.4

42.2

35.6

23.1

21.2

POSITIVE CONTROL VALUES 2015

 

Strain

S9-Mix

TA100

TA1535

TA102

WP2uvrA

TA98

TA1537

WP2uvrA

pKM101

WP2pKM101

 

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

 

Values

276

280

252

264

13

13

231

227

262

276

253

261

20

35

20

35

 

Min

222

250

79

118

953

673

116

103

100

78

164

97

430

494

745

325

 

Max

2266

2402

2779

457

3140

1655

2769

550

502

705

2318

823

1696

2264

3662

1174

 

Mean

614

927

472

246

2303

1093

792

266

222

218

911

336

761

1461

2257

569

 

SD

260.6

452.5

434.8

55.7

815.2

376.5

342.1

97.7

70.2

107.6

412.4

135.7

350.0

382.0

790.7

220.3

 

COMBINED VEHICLE AND UNTREATED CONTROL VALUES 2016

Strain

S9-Mix

TA100

TA1535

TA102

WP2uvrA

TA98

TA1537

WP2uvrA

pKM101

WP2pKM101

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

Values

399

401

758

393

60

30

690

345

788

415

762

398

32

32

16

24

Min

63

66

8

8

216

221

10

13

8

12

3

4

97

104

78

52

Max

154

156

34

39

340

375

53

53

49

51

24

23

268

243

148

166

Mean

90

93

15

15

268

310

22

27

21

25

12

13

161

159

118

110

SD

14.5

14.3

4.5

5.2

26.4

31.1

5.8

6.3

4.8

5.7

3.5

3.5

39.2

32.3

17.0

29.3

POSITIVE CONTROL VALUES 2016

 

Strain

S9-Mix

TA100

TA1535

TA102

WP2uvrA

TA98

TA1537

WP2uvrA

pKM101

WP2pKM101

 

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

-S9

+S9

 

Values

409

406

381

386

30

28

341

335

388

385

379

381

14

24

8

16

 

Min

221

284

84

92

897

629

107

102

100

96

95

101

445

574

1674

372

 

Max

2222

2863

2994

879

2326

2140

1611

637

449

4357

1413

639

1117

1855

2823

945

 

Mean

724

1264

854

240

1633

950

718

240

186

188

406

290

743

1271

2379

535

 

SD

320.4

562.9

664.9

62.1

564.5

382.7

338.6

98.2

49.8

230.8

227.0

92.7

214.6

326.5

426.2

143.3

 

Applicant's summary and conclusion

Conclusions:
Under the test conditions, the test item is not considered as mutagenic in Salmonella typhimurium strains TA1535, TA1537, TA98 and TA100, and Escherichia coli strain WP2uvrA.
Executive summary:

In a reverse gene mutation assay in bacteria, performed according to the OECD Guideline 471 and in compliance with GLP, Salmonella typhimurium strains TA1535, TA1537, TA98 and TA100, and Escherichia coli strain WP2uvrA were exposed to the test item at the following concentrations:

- Experiment 1 - Plate Incorporation Method: 1.5, 5, 15, 50, 150, 500, 1500 and 5000 µg/plate, with and without S9-mix

- Experiment 2 - Pre-Incubation Method: 15, 50, 150, 500, 1500 and 5000 µg/plate, with and without S9-mix

Rat liver homogenate (10% liver S9 in standard co-factors) was used as a metabolizing system. Vehicle control, negative (untreated) and positive control groups were also included in mutagenicity tests.

The vehicle (tetrahydrofuran (THF)) control plates gave counts of revertant colonies within the normal range.  All of the positive control chemicals used in the test induced marked increases in the frequency of revertant colonies, both with or without metabolic activation.  Thus, the sensitivity of the assay and the efficacy of the S9-mix were validated.

No visible reduction in the growth of the bacterial background lawn was noted at any dose level, either in the presence or absence of metabolic activation (S9-mix), in Experiment 1 (plate incorporation method) and in Experiment 2 (pre incubation method).

No test item precipitate was observed on the plates at any of the doses tested in either the presence or absence of S9-mix.

No significant increases in the frequency of revertant colonies were recorded for any of the bacterial strains, with any dose of the test item, either with or without metabolic activation (S9-mix) in Experiment 1 (plate incorporation method) and Experiment 2 (pre incubation method).

Under the test conditions, the test item is not considered as mutagenic in these bacterial systems.