1. Introduction
===============

Marine bacteria are a rich source of natural products with interesting biological and pharmacological activities \[[@b1-marinedrugs-08-02976],[@b2-marinedrugs-08-02976]\]. While most research on marine bacterial metabolites focusses on compounds with moderate polarity isolable from fermentation experiments, relatively few volatile compounds from marine as well as other bacteria are known \[[@b3-marinedrugs-08-02976]\]. Many bacteria, marine or terrestrial, produce volatiles, but the specific function of these compounds is known only in few cases. It seems likely that the volatiles are used either in intra- or inter-specific communication and/or in the chemical defence against other microorganisms. Several studies have shown that bacterial volatiles can indeed influence the growth of other organisms \[[@b4-marinedrugs-08-02976]\]. Bacterial volatiles can impact, most often inhibit, the growth of fungi \[[@b4-marinedrugs-08-02976]--[@b9-marinedrugs-08-02976]\], but only few studies describe the activities of certain compounds. Compounds identified from fungistatic soil bacteria inhibiting spore germination of certain fungi were acetamide, trimethylamine, 3-methyl-2-pentanone, benzaldehyde, *N*,*N*-dimethyloctylamine, benzothiazole, 1-butanamine, methanamine, phenylacetaldehyde, and 1-decene \[[@b10-marinedrugs-08-02976],[@b11-marinedrugs-08-02976]\]. Benzothiazole, cyclohexanol, nonanal, decanal, dimethyl trisulfide, and 2-ethyl-1-hexanol, the latter most likely an anthropogenic artefact, identified from several *Pseudomonas* strains, were active in inhibiting growth of the plant pathogen, *Sclerotinia sclerotiorum* \[[@b12-marinedrugs-08-02976]\]. 1-Undecene and dimethyl disulfide (**2**), released by several bacteria, were also active \[[@b4-marinedrugs-08-02976]\], despite the fact that the latter proved to be inactive in other studies \[[@b10-marinedrugs-08-02976]--[@b12-marinedrugs-08-02976]\]. Effects on plants have been described for 2,3-butanediol (**47**) and acetoin (**48**) which improve plant growth in *Arabidopsis thaliana*, and also effects on animals have been described \[[@b4-marinedrugs-08-02976]\]. On the other hand, reports on interbacterial effects of volatile compounds are rare. Indole acts as a signalling compound in *Escherichia coli* \[[@b13-marinedrugs-08-02976]\] and shows effects on other bacteria \[[@b14-marinedrugs-08-02976]\] while volatile carboxylic acids inhibit spore formation in several pathogenic bacteria \[[@b15-marinedrugs-08-02976]\].

More or less all studies performed so far concentrated on the evaluation of the effects of readily available compounds. While these studies showed activity in some cases, questions remain about the activity potential of other components. The volatiles identified so far can be roughly divided into different groups depending on their occurrence. While there are common compounds produced by many different, often unrelated bacteria, e.g., dimethyl disulfide (**52**), or at least by certain groups as geosmin (**49**) and 2-methylisoborneol (**50**) found in actinobacteria, myxobacteria, and cyanobacteria, some are unique for certain strains as the lactones **4**--**11** \[[@b3-marinedrugs-08-02976]\].

In the present study 52 compounds were tested for their biological effects, covering strain or species specific compounds (**1**--**44**) and commonly occurring ones (**45**--**52**). In a first set of experiments we established a general profile of the biological activities of these compounds. Therefore, the antiproliferative activity with bacteria, fungi, and murine fibroblasts was tested. The antimicrobial assays were focussed on medically important strains. In a second set of experiments the possibility of interference with well known bacterial chemical communication channels was probed. In this context, the compounds were investigated in an assay on bacterial crosstalk \[[@b16-marinedrugs-08-02976]\] to demonstrate whether they can influence the known bacterial quorum-sensing signalling pathway via *N*-acylhomoserine lactones (AHLs).

The compounds tested ([Figure 1](#f1-marinedrugs-08-02976){ref-type="fig"}) were identified from different marine bacteria (see [Table 1](#t1-marinedrugs-08-02976){ref-type="table"} for references). Several compounds originally known from terrestrial bacteria are also included to compare activities between compounds originating from terrestrial or marine sources. Nevertheless, there is no clear cut difference in chemistry between terrestrial and bacterial microorganisms. Common compounds usually occur in both areas.

2. Results and Discussion
=========================

Classical agar diffusion assays were performed using the fungi *Aspergillus fumigatus*, *Botrytis cinerea*, and *Pythium debaryanum* as well as the yeasts *Hansenula anomala*, *Saccharomyces cerevisiae*, and *Candida albicans*. Furthermore, the bacteria *Pseudomonas aeruginosa*, *Klebsellia pneumoniae*, *Staphylococcus aureus*, *Micrococcus luteus*, *Mycobacterium phlei* and a mutant of *Escherichia coli* which is defective in the *tolC* gene \[[@b17-marinedrugs-08-02976]\] were used as test organisms. The results are shown in [Table 1](#t1-marinedrugs-08-02976){ref-type="table"}. Octanoic acid (**1**) is active against hyphal fungi and yeasts, as are the amides **22** and **23**. *γ*-Butyrolactones **4**--**8** are active against fungi, yeasts, and bacteria, with a large influence of a double bond in the ring. Expansion of the ring to *δ*- (**9**) or *∈*-lactones (**10** and **11**) reduces the activity. 9-Methyl-3-decanol (**2**), the characteristic component of the headspace volatiles of *Myxococcus xanthus*, shows activity against Gram-positive bacteria and the *tolC* mutant of *Escherichia coli*, while alcohol **3** only inhibits the sensitive *E. coli* mutant. Pyrazines (**25**--**32**) as well as ketones (**12**--**19**) are largely inactive with the exception of (*Z*)-15-methylhexadec-12-en-2-one (**19**), showing broad activity. *S*-Methyl benzothioate (**35**) is the only sulfur containing compound with activity against fungi and yeast, while all others (**32**--**40**) are inactive. 2-Pentylpyridine (**41**), produced by a *Streptomyces* strain \[[@b18-marinedrugs-08-02976]\], is slightly active against *S. cerevisiae*, *E. coli*, and *K. pneumoniae*. In contrast, the latter bacterium and *C. albicans* are the only microorganisms furfuryl isovalerate (**42**) is not active against.

The results discussed so far were obtained with compounds produced only by specific bacterial strains. In contrast, the compounds common to many bacteria showed no inhibitory activity. This is even true for components as dimethyl disulfide (**52**) which showed activity in other assays \[[@b4-marinedrugs-08-02976]\].

Additionally, the cytotoxic activity against L-929 mouse fibroblasts was investigated in concentration dependant assays. The obtained minimal inhibition concentration (MIC) values indicated moderate or no activity. The most active compounds were furfuryl isovalerate (**42**, MIC 62 *μ*mol/L), the ketone **19** (MIC 73 *μ*mol/L), the lactones **10** and **11** (MIC 260 and 280 *μ*mol/L), as well as the pyrazine **28** (MIC 320 *μ*mol/L). All these compounds were also active in at least some antimicrobial assays except **28**.

All compounds were also investigated for their activity in *N*-acylhomoserine lactone (AHL) mediated bacterial communication systems (quorum-sensing). These test were performed using green fluorescent protein (gfp) equipped reporter strains capable of sensing AHLs with certain side chain specificity. The used *E. coli* MT102 (pJBA132) reporter shows the highest sensitivity for *N*-(3-oxohexanoyl)homoserine lactone (3-oxo-C6-AHL) \[[@b32-marinedrugs-08-02976]\], while the *Pseudomonas putida* F117 (pKR-C12) reporter is very sensitive to *N*-(dodecanoyl)homoserine lactone (C12-AHL) \[[@b33-marinedrugs-08-02976]\]. The corresponding AHLs were added to the sensor strains together with the test compound and the resulting reduction or increase of fluorescence was recorded ([Table 2](#t2-marinedrugs-08-02976){ref-type="table"}). Values of more than 50% were considered significant.

Several compounds were able to modify the sensor response. The lactones **5**--**9** inhibited the response of the C12-AHL sensor. Especially the *δ*-lactone **9** was highly active. In contrast, the 3-oxo-C6-AHL sensor showed a different behavior. In particular **4** and **7** stimulated this sensor. This influence may be due to the structural similarity between the lactones and the AHLs. Ketones also proved to be active, with **15**, **16**, **18**, **19**, and the related alcohol **3** reducing activity of the C12-AHL sensor, while **17** did so with the 3-oxo-C6-AHL sensor, as did the pyridine derivative **41**. The sulfur containing ester **34** and furfuryl isovalerate (**42**), also one of the most active compounds in the antimicrobial assays, were the only compounds able to reduce the activity of both sensors. In this assay also compounds common to many different bacteria showed some activity, reducing the effectivity of the 3-oxo-C6-AHL sensor. These compounds were the alcohols **45** and **46**, and both enantiomers of methylisoborneol (**50** and **51**). In summary, 25% of the compounds were able to reduce activity of the C12-AHL sensor, while 19% show inhibitory activity against 3-oxo-C6-AHL and 13% enhanced its activity.

The concentration of the compounds used in the sensor tests was quite high and dilution experiments usually led to reduced activity (see [Table 2](#t2-marinedrugs-08-02976){ref-type="table"}). The concentration of compounds released by bacteria can be quite high close to the bacterial membrane, their point of release. Therefore, nearby other bacteria might well experience relatively high concentrations of such metabolites, pointing to the possibility that these compounds might have ecological relevance for known bacterial communication channels.

In summary, the results show that bacterial volatiles can, despite their relatively small size, influence different biological traits. Often it is not clear why bacteria produce volatiles. One interesting observation is that some special compounds produced by only few organisms can have antimicrobial activity and therefore might ensure survival of the bacterium when under competition with other bacteria. Contrary to polar and larger compounds they are transported by air and are useful for bacteria under certain conditions, but can also travel in water with high velocity because of their small size. The volatiles can also interfere with bacterial communication channels and are ideally suited for being signalling compounds by themselves. Their potential receptors are not known because only few bacterial communication mechanisms have been established so far (see Introduction). Finally, in some cases they may also serve as a means of detoxification, as recently proposed for some pyrazines\[[@b34-marinedrugs-08-02976]\]. Interestingly, the pyrazines show almost no antimicrobial activity. The activity of the volatiles against proliferative cells can be regarded as quite low with the exception of furfuryl isovalerate, a surprisingly active compound in several aspects.

Finally, there is no clear difference in activity between volatiles produced by marine or terrestrial bacteria. The current known origin of the these compounds from bacteria either of terrestrial or marine habitats is probably pure chance. Most likely terrestrial strains will be found in the future that produce compounds only known from marine sources so far, e.g., sulfur compounds **37**--**40**, and *vice versa*.

In summary, we have shown that bacterial volatiles can have a multitude of activities. Their production has to be taken into account in future studies, especially when researching aspects of the bacterial chemical communication systems and their chemical defence.

3. Experimental Section
=======================

3.1. Antimicrobial assay
------------------------

Antimicrobial activities were determined by agar diffusion assays using paper discs of 6 mm diameter soaked with 20 *μ*L of methanolic solution of the test compound. Microorganisms from the HZI collection were grown on standard media and seeded into liquid agar medium to a final O.D. of 0.01 (bacteria) or 0.1 (yeasts). Spores of fungi were seeded according to experience. Plates were incubated at 30 °C and the diameter of resulting inhibition zones were measured after 1 or 2 days.

3.2. Cell proliferation assay
-----------------------------

L-929 mouse fibroblasts were obtained from the German Collection of Microorganisms and Cell Cultures (DSMZ) and cultivated at 37 °C and 10% CO~2~ in DME medium (high glucose) supplemented with 10% fetal calf serum. Inhibition of proliferation was measured in microtiter plates. 60 *μ*L of serial dilutions of the test compounds were given to 120 *μ*L of the suspended cells (50,000/mL) in wells of 96-well plates. After 5 days growth each well was judged visually under the microscope. The lowest concentration of a compound that led to a clearly reduced growth is given as minimal inhibitory concentration (MIC).

3.3. Acylhomoserine lactone sensors
-----------------------------------

The AHL biosensor strains *E. coli* MT102 (pJBA132) and *Pseudomonas putida* F117 (pRK-C12) were grown in Luria-Bertani (LB) medium with 25 *μ*g/mL tetracycline at 37 °C and with 20 *μ*g/mL gentamycin at 30 °C, respectively. Overnight cultures (50 mL) of the strains were diluted with the same volume of fresh medium, incubated for about one more hour and adjusted to an OD620 of 1.0. Aliquots (100 *μ*L) of these cultures were transferred by pipet into 96-well microtiter plates which contained 90 *μ*L of LB medium, and 5 *μ*L of the test compounds in the concentrations reported in [Table 2](#t2-marinedrugs-08-02976){ref-type="table"}. To test the compounds for inhibitory activity on quorum-sensing, the sample wells contained 5 *μ*L of the corresponding AHL *N*-3-oxohexanoylhomoserine lactone for *E. coli* MT102, *N*-dodecanoylhomoserine lactone for *P. putida* F117) at a concentration of 0.25 *μ*g/mL. Samples containing only the corresponding AHL without test compound were used as positive control and samples containing solvent (10 *μ*L) alone were used as negative control. The microtiter plates were incubated at 30 °C with shaking. After 0, 4, 8, and 24 h growth at an OD620 nm, fluorescence (excitation wavelength 485 nm, detection wavelength of 535 nm) was determined in a Victor 1420 Multilabel Counter (Perkin Elmer). Fold induction of fluorescence in the test samples was calculated by dividing their specific fluorescence (gfp535/OD620) by the specific fluorescence of the negative control. Relative activity was calculated by comparing the fold induction of positive control samples (only homoserine lactones) to samples containing samples with test compounds and the corresponding homoserine lactone.

We thank the Deutsche Forschungsgemeinschaft for financial support.

###### 

Compounds used in this study. Acids, alcohols, lactones, ketones, amides, pyrazines, sulfur compounds, aromatic compounds, and substances commonly occurring as bacterial volatiles (**45**--**52**).

![](marinedrugs-08-02976f1a)

![](marinedrugs-08-02976f1b)

###### 

Agar diffusion inhibition assay. Asf: *Aspergillus fumigatus*; Boc: *Botrytis cinerea*; Caa: *Candida albicans*; Pyd: *Pythium debaryanum*; Hna: *Hansenula anomala*; Scc: *Saccharomyces cerevisiae*; tolC: *E. coli tolC* mutant; Psa: *Pseudomonas aeruginosa*; Kbp: *Klebsellia pneumoniae*; Sta: *Staphylococcus aureus*; Mcl: *Micrococcus luteus*; Myp: *Mycobacterium phlei*; Amount: amount of compound used in the agar diffusion assay; mm: diameter of inhibition zone; i: incomplete inhibition; MIC: cytotoxic activity against L-929 mouse fibroblasts, minimal inhibition concentration; Oc: source.

  No   Compound                                      Amount   Asf   Boc   Pyd   Caa   Hna   Scc   tolC   Psa   Kbp   Sta   Mcl   Myp   MIC        Oc
  ---- --------------------------------------------- -------- ----- ----- ----- ----- ----- ----- ------ ----- ----- ----- ----- ----- ---------- -----------------------------------------------------------------------------------------
  1    Octanoic acid                                 112      10i   20          11i         11i   0      0     0     0     0     0     480        [(1)](#tfn1-marinedrugs-08-02976){ref-type="table-fn"}
  2    9-Methyl-3-decanol                            156      0     0           0           0     12     0     0     8     13    20    560        \[[@b19-marinedrugs-08-02976]\]
  3    10-Methylundecan-2-ol                         382      0     0           0           0     8      0     0     0     0     0     420        \[[@b20-marinedrugs-08-02976]\]
  4    10-Methylundecan-4-olide                      416      11    14          0           0     14     0     0     8i    10i   19    430        \[[@b21-marinedrugs-08-02976]\]
  5    10-Methylundec-2-en-4-olide                   346      15    14          9i          14    17     11i   0     9     8i    11    363        \[[@b21-marinedrugs-08-02976]\]
  6    10-Methylundec-3-en-4-olide                   218      9i    9           0           10    13     13i   0     8i    8i    12    230        \[[@b21-marinedrugs-08-02976]\]
  7    10-Methyldodecan-4-olide                      428      8i    10i         0           0     15     12i   0     8i    8i    13    415        \[[@b21-marinedrugs-08-02976]\]
  8    10-Methyldodec-2-en-4-olide                   140      8i    8           0           9     20     0     0     8     8     10    135        \[[@b21-marinedrugs-08-02976]\]
  9    10-Methylundecan-5-olide                      250      0     0           0           0     0      0     0     0     0     0     780        \[[@b21-marinedrugs-08-02976]\]
  10   9-Methylundecan-6-olide                       28       0     0           0           0     8i     0     0     0     0     18i   260        \[[@b22-marinedrugs-08-02976]\]
  11   10-Methylundecan-6-olide                      30       0     0           0           0     0      0     0     0     0     0     280        \[[@b22-marinedrugs-08-02976]\]
  12   6-Ethyl-3-octanone                            304      0     0           0           0     0      0     0     0     0     0     *\>*1000   [(2)](#tfn2-marinedrugs-08-02976){ref-type="table-fn"}
  13   9-Methyl-3-decanone                           162      0     0           0           0     9      0     0     0     0     0     590        \[[@b19-marinedrugs-08-02976]\]
  14   9-Methyl-1-phenyldecan-1-one                  200      0     0     0           0     0     0      0     0     0     0     0     500        \[[@b20-marinedrugs-08-02976]\]
  15   3-Methyltridecan-4-one                        528      0     0           0           0     0      0     0     0     0     0     510        \[[@b20-marinedrugs-08-02976]\]
  16   2-Methyltridec-2-en-4-one                     414      0     0           0           8i    0      0     0     0     0     0     405        \[[@b20-marinedrugs-08-02976]\]
  17   2-Methyl-4-tetradecanone                      488      0     0           0           0     0      0     0     0     0     0     443        \[[@b20-marinedrugs-08-02976]\]
  18   (*Z*)-13-Methyltetradec-8-en-2-one            280      0     0           0           0     0      0     0     0     0     0     257        \[[@b23-marinedrugs-08-02976]\]
  19   (*Z*)-15-Methylhexadec-12-en-2-one            280      8i    9i          0           9     9      12i   0     13i   10    13    73         \[[@b23-marinedrugs-08-02976]\]
  20   Isopentyl formamide                           524      0     0           0           0     0      0     0     0     0     0     *\>*1000   \[[@b20-marinedrugs-08-02976]\]
  21   Isopentyl acetamide                           386      0     0           0           0     0      0     0     0     0     0     *\>*1000   \[[@b20-marinedrugs-08-02976]\]
  22   3-Methyl-*N*-(2-phenylethyl)pentanamide       290      12i   13i         15i         0     11     0     0     0     0     0     815        \[[@b24-marinedrugs-08-02976]\]
  23   4-Methyl-*N*-(2-phenylethyl)pentanamide       130      10i   0           10i         0     10i    0     0     0     0     0     365        \[[@b24-marinedrugs-08-02976]\]
  24   *N*-(2-phenylethyl)benzamide                  282      0     0           0           0     0      0     0     0     0     0     773        \[[@b24-marinedrugs-08-02976]\]
  25   2,5-Diisobutylpyrazine                        1322     0     0           0           0     20     0     0     0     0     0     *\>*1000   \[[@b25-marinedrugs-08-02976]\]
  26   2-Butyl-3,6-dimethylpyrazine                  644      0     0           0           0     10     0     0     0     0     0     *\>*1000   \[[@b23-marinedrugs-08-02976]\]
  27   2-Isopropenyl-5-isopropylpyrazine             178      0     0           0           0     0      0     0     0     0     0     680        \[[@b26-marinedrugs-08-02976]\]
  28   2-Methoxy-3,6-dimethylpyrazine                217      0     0           0           0     0      0     0     0     0     0     320        \[[@b26-marinedrugs-08-02976]\]
  29   2-Methoxy-3,5-diisobutylpyrazine              354      0     0           0           0     10     0     0     0     0     0     *\>*1000   \[[@b26-marinedrugs-08-02976]\]
  30   2-Methoxy-3,6-diisobutylpyrazine              354      0     0           0           0     9      0     0     0     0     0     327        \[[@b26-marinedrugs-08-02976]\]
  31   2-Methoxy-3,6-di-sec-butylpyrazine            106      0     0           0           0     0      0     0     0     0     0     487        \[[@b26-marinedrugs-08-02976]\]
  32   2-Methylsulfanyl-3,6-dimethylpyrazine         916      0     0           0           0     0      0     0     0     0     0     425        \[[@b26-marinedrugs-08-02976]\]
  33   4-Methylsulfanyl-2-butanol                    220      0     0           0           0     0      0     0     0     0     0     *\>*1000   \[[@b23-marinedrugs-08-02976]\]
  34   *S*-Methyl 3-(Methylsulfanyl)propanethioate   1100     0     0           0           0     0      0     0     0     0     0     *\>*1000   \[[@b27-marinedrugs-08-02976]\]
  35   *S*-Methyl benzothioate                       406      10i   11i         10i         0     8i     0     0     0     0     0     *\>*1000   \[[@b28-marinedrugs-08-02976]\]
  36   2-Ethyl-5-isobutylthiophene                   238      0     0           0           0     0      0     0     0     0     0     *\>*1000   [(2)](#tfn2-marinedrugs-08-02976){ref-type="table-fn"}
  37   3,3,6,6-Tetramethyl-1,2,5-trithiepane         200      0     0     0           0     0     0      0     0     0     0     0     705        \[[@b29-marinedrugs-08-02976]\]
  38   3,3,7,7-Tetramethyl-1,2,5-trithiepane         200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b29-marinedrugs-08-02976]\]
  39   3,3,8,8-Tetramethyl-1,2,5,6-tetrathiocane     200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b29-marinedrugs-08-02976]\]
  40   3,3,7,7-Tetramethyl-1,2,5,6-tetrathiocane     200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b29-marinedrugs-08-02976]\]
  41   2-Pentylpyridine                              1198     0     0           0           8i    8      0     8i    0     0     0     *\>*1000   [(3)](#tfn3-marinedrugs-08-02976){ref-type="table-fn"}
  42   Furfuryl isovalerate                          494      11i   15          0           12    8      13    0     13    9     12    62         \[[@b26-marinedrugs-08-02976]\]
  43   2-Furanmethanol                               200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b30-marinedrugs-08-02976]\]
  44   Tropone                                       200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b27-marinedrugs-08-02976],[@b28-marinedrugs-08-02976]\]
       **Common Compounds**                                                                                                                       
  45   3-Methylbutanol                               200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b3-marinedrugs-08-02976]\]
  46   2-Phenylethanol                               200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b21-marinedrugs-08-02976],[@b23-marinedrugs-08-02976],[@b27-marinedrugs-08-02976]\]
  47   (2*R*,3*R*)-Butanediol                        200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b3-marinedrugs-08-02976]\]
  48   Acetoin                                       200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b3-marinedrugs-08-02976]\]
  49   Geosmin                                       200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b21-marinedrugs-08-02976]\]
  50   (+)-*R*-Methylisoborneol                      200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b21-marinedrugs-08-02976]\]
  51   (−)-*S*-Methylisoborneol                      200      0     0     0           0     0     0      0     0     0     0     0     240        
  52   Dimethyl disulfide                            200      0     0     0           0     0     0      0     0     0     0     0     *\>*1000   \[[@b21-marinedrugs-08-02976]\]

Occurs in unclassified myxobacteria \[[@b31-marinedrugs-08-02976]\].

Occurs in *Myxococcus xanthus* DK 1622 \[[@b31-marinedrugs-08-02976]\].

Occurs in *Streptomyces* sp. F5 \[[@b18-marinedrugs-08-02976]\].

###### 

Acylhomoserine lactone assay using *N*-3-oxohexanoylhomoserine lactone for *E. coli* MT102 (C~6~-AHL) and *N*-dodecanoylhomoserine lactone for *P. putida* F117 (C~12~-AHL). The maximum reduction relative to the native ligand is reported (%). Negative values indicate no inhibition, but increase of the fluorescence induction. Here the maximum induction rate relative to the native ligand (100%) is given.

  No   Compound                                      Conc.   C~12~-AHL   C~6~-AHL
  ---- --------------------------------------------- ------- ----------- ----------
  1    Octanoic acid                                 140     23          15
  2    9-Methyl-3-decanol                            250     6           −198
  3    10-Methylundecan-2-ol                         477     60          3
  4    10-Methylundecan-4-olide                      500     7           −550
  5    10-Methylundec-2-en-4-olide                   432     59          −40
  6    10-Methylundec-3-en-4-olide                   272     52          −16
  7    10-Methyldodecan-4-olide                      535     71          −70
  8    10-Methyldodec-2-en-4-olide                   175     61          −32
  9    10-Methylundecan-5-olide                      312     82          15
  10   9-Methylundecan-6-olide                       35      20          12
  11   10-Methylundecan-6-olide                      37      26          41
  12   6-Ethyl-3-octanone                            250     −15         −39
  13   9-Methyl-3-decanone                           250     10          −115
  14   9-Methyl-1-phenyldecan-1-one                  500     1           −27
  15   3-Methyltridecan-4-one                        660     59          −3
  16   2-Methyltridec-2-en-4-one                     517     52          −9
  17   2-Methyl-4-tetradecanone                      610     46          53
  18   (*Z*)-13-Methyltetradec-8-en-2-one            315     56          −18
  19   (*Z*)-15-Methylhexadec-12-en-2-one            315     82          17
  20   Isopentyl formamide                           655     26          34
  21   Isopentyl acetamide                           482     15          31
  22   3-Methyl-*N*-(2-phenylethyl)pentanamide       362     12          18
  23   4-Methyl-*N*-(2-phenylethyl)pentanamide       162     −13         19
  24   *N*-(2-phenylethyl)benzamide                  350     28          49
  25   2,5-Diisobutylpyrazine                        500     −5          −94
  26   2-Butyl-3,6-dimethylpyrazine                  500     13          40
  27   2-Isopropenyl-5-isopropylpyrazine             222     35          72
  28   2-Methoxy-3,6-dimethylpyrazine                500     40          60
  29   2-Methoxy-3,5-diisobutylpyrazine              250     10          −92
  30   2-Methoxy-3,6-diisobutylpyrazine              500     26          −102
  31   2-Methoxy-3,6-di-sec-butylpyrazine            132     33          36
  32   2-Methylsulfanyl-3,6-dimethylpyrazine         500     5           31
  33   4-Methylsulfanyl-2-butanol                    275     26          32
  34   *S*-Methyl 3-(Methylsulfanyl)propanethioate   1375    54          85
                                                     275                 40
  35   *S*-Methyl benzothioate                       500     −2          46
  36   2-Ethyl-5-isobutylthiophene                   250     −25         −180
  37   3,3,6,6-Tetramethyl-1,2,5-trithiepane         500     −22         51
  38   3,3,7,7-Tetramethyl-1,2,5-trithiepane         500     25          18
  39   3,3,8,8-Tetramethyl-1,2,5,6-tetrathiocane     500     46          39
  40   3,3,7,7-Tetramethyl-1,2,5,6-tetrathiocane     250     30          20
  41   2-Pentylpyridine                              1497    29          78
  42   Furfuryl isovalerate                          617     56          84
  43   2-Furylmethanol                               500     22          46
  44   Tropone                                       500     29          36
       **Common Compounds**                                              
  45   3-Methylbutanol                               500     13          61
                                                     250                 14
  46   2-Phenylethanol                               500     45          69
                                                     250                 23
                                                     50                  31
  47   (2*R*,3*R*)-Butanediol                        500     −14         7
  48   Acetoin                                       500     23          23
  49   Geosmin                                       500     13          21
  50   (+)-*R*-Methylisoborneol                      250     3           69
                                                     25                  17
  51   (−)-*S*-Methylisoborneol                      500     12          75
                                                     250                 24
                                                     50                  10
  52   Dimethyl disulfide                            500     17          26
