Isothiazolinone biocides enhanced by zinc ions

Abstract

The present invention relates to an antimicrobial composition comprising an isothiazolinone, such as 1,2-benzisothiazolin-3-one, and a zinc compound selected from zinc salts, zinc oxides, zinc hydroxides or combinations thereof. Useful zinc salts include for example, oxides, sulfates, chlorides, and combinations thereof. In use, the zinc from the zinc compound enhances the antimicrobial activity to the isothiazolin-containing composition. This enhancement permits achieving the desired antimicrobial activity at a lower usage rate than is achieved using the isothiazolinone in the absence of the zinc compound. The antimicrobial composition can also contain co-biocides, such as pyrithiones, including zinc pyrithione or copper pyrithione.

Claims

1. An antimicrobial composition concentrate comprising: (a) at least one isothiazolin-3-one comprising 1,2-benzisothiazolin-3-one, N-(n-butyl)-1,2-benzisothiazolin-3-one, or a mixture thereof; and (b) at least one zinc compound comprising a zinc oxide, wherein the isothiazolin-3-one is present in an amount of from about 1% to about 95% w/w based on the total weight of the concentrate, wherein the zinc compound is present in an amount from about 1% to about 50% w/w based on the total weight of the concentrate, and wherein the weight ratio of the isothiazolin-3-one to the zinc compound is from 1:100 to 100:1.

2. The antimicrobial composition concentrate of claim 1, wherein the isothiazolin-3-one comprises 1,2-benzisothiazolin-3-one.

3. The antimicrobial composition concentrate of claim 1, wherein the isothiazolin-3-one comprises N-(n-butyl)-1,2-benzisothiazolin-3-one.

4. The antimicrobial composition concentrate of claim 1, further comprising a co-biocide comprising a pyrithione compound comprising a pyrithione salt, a pyrithione acid, or a combination thereof, wherein the co-biocide is present in the composition concentrate in an amount of from about 1% to about 50% w/w based on the total weight of the composition concentrate.

5. The antimicrobial composition concentrate of claim 4, wherein the pyrithione compound comprises zinc pyrithione.

6. The antimicrobial composition concentrate of claim 4, wherein the isothiazolin-3-one comprises 1,2-benzisothiazolin-3-one.

7. The antimicrobial composition concentrate of claim 4, wherein the pyrithione compound comprises copper pyrithione, sodium pyrithione, or a mixture thereof.

8. The antimicrobial composition concentrate of claim 4, wherein the isothiazolin-3-one comprises N-(n-butyl)-1,2-benzisothiazolin-3-one.

9. The antimicrobial composition concentrate of claim 1, wherein the zinc compound is present in an amount greater than the isothiazolin-3-one.

10. An antimicrobial composition concentrate comprising: (a) 1,2-benzisothiazolin-3-one, (b) zinc oxide, and (c) zinc pyrithione, wherein the 1,2-benzisothiazolin-3-one is present in an amount of from about 1% to about 95% w/w based on the total weight of the concentrate, wherein the zinc oxide is present in an amount from about 1% to about 50% w/w based on the total weight of the concentrate, and wherein the zinc pyrithione is present in an amount from about 1% to about 50% w/w based on the total weight of the concentrate, wherein the weight ratio of the isothiazolin-3-one to the zinc compound is from 1:100 to 100:1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 provides a graphical depiction of an Isobologram showing Antimicrobial Activity of Mixtures of 1,2-Benzisothiazolin-3-one (“BIT”) and Zinc Chloride against Ps. aeruginosa based on Fractional Inhibitory Concentration values.

(2) FIG. 2 provides a graphical depiction of an Isobologram showing Antimicrobial Activity of Mixtures of Kathon® (a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one (“CIMIT”) and 2-methyl-4-isothiazolin-3-one (“MIT”)) and Zinc Chloride against Ps. aeruginosa based on Fractional Inhibitory Concentration values.

(3) FIG. 3 provides a graphical depiction of an Isobologram showing Antimicrobial Activity of Mixtures of MIT and Zinc Chloride against Ps. aeruginosa based on Fractional Inhibitory Concentration values.

(4) FIG. 4 provides a graphical depiction of an Isobologram showing Antimicrobial Activity of Mixtures of N-(n-butyl)-1,2-benzisothiazolin-3-one (“BBIT”) and Zinc Chloride against Ps. aeruginosa based on Fractional Inhibitory Concentration values.

(5) FIG. 5 provides a graphical depiction of an Isobologram showing Antimicrobial Activity of Mixtures of BIT and Sodium Chloride against Ps. aeruginosa based on Fractional Inhibitory Concentration values.

DETAILED DESCRIPTION OF THE INVENTION

(6) It has now been surprisingly found in accordance with the present invention that an antimicrobial composition containing an isothiazolin-3-one, plus a zinc compound selected from the group consisting of zinc salts, zinc oxides, zinc hydroxides, and combinations thereof, exhibits enhanced antimicrobial efficacy, as compared to isothiazolin-containing compositions that do not contain the metal compound. Enhanced efficacy are suitably provided in antimicrobial compositions over a wide range of pHs of from 3 to 12.

(7) The antimicrobial composition of the present invention is suitably incorporated into a functional fluid. The functional fluid suitably comprises a base medium plus (a) at least one isothiazolin-3-one and (b) at least one zinc compound selected from the group consisting of zinc salts of organic acids, zinc salts of inorganic acids, zinc oxides, zinc hydroxides, and combinations thereof. Illustrative zinc salts include zinc chloride, zinc sulfide, zinc carbonate, basic zinc carbonate (also known as “hydroxy-containing zinc carbonate”, also know as “zinc hydroxy carbonate” which is further identified by the empirical formula Zn.sub.5(OH).sub.6(CO.sub.3).sub.2), and combinations thereof.

(8) The isothiazolinone useful in the present invention is preferably an isothiazolin-3-one that is selected from: 1,2-benzisothiazolin-3-one (“BIT”), N-(n-butyl)-1,2-benzisothiazolin-3-one (“BBIT”), 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (“DCOIT”), 2-methyl-4-isothiazolin-3-one (“MIT”), mixtures of 5-chloro-2-methyl-4-isothiazolin-3-one (“CIMIT”) plus MIT (available from Rohm and Haas Company, Philadelphia, Pa. under the tradename Kathon®) dithio-2,2′-bis(benzmethylamide), and combinations thereof. Particularly preferred isothiazolins are BIT, MIT and BBIT, and combinations thereof.

(9) In the composition, the isothiazolin-3-one is present in an amount of from 1 to 500 ppm (preferably from 5 to 500 ppm), the zinc compound is present in an amount of from 5 to 200,000 ppm (preferably from 5 to 500 ppm), and the weight ratio of the isothiazolin-3-one to the zinc compound is from 1:2000 to 100:1 (preferably from 100:1 to 1:100).

(10) Optionally, the antimicrobial composition additionally comprises a pyrithione salt or pyrithione acid. Useful pyrithione salts include copper pyrithione, zinc pyrithione, sodium pyrithione. If used, the pyrithione salt or pyrithione acid is suitably present in the composition in an amount of from about 0.001% w/w to about 1.0% w/w.

(11) In one embodiment, the combination of antimicrobial components for the antimicrobial composition can be provided in the form of an antimicrobial composition concentrate that, upon dilution with water, provides antimicrobial efficacy in a functional fluid. The concentrate comprises (a) at least one isothiazolin-3-one and (b) at least one zinc compound selected from the group consisting of zinc salts, zinc oxides, zinc hydroxides, and combinations thereof. In the concentrate, component (a) is present in an amount of from 1 to 95% w/w, and component (b) is present in an amount of from 1 to 50% w/w.

(12) The antimicrobial compositions of the present invention are suitably used in functional fluids such as polymer emulsions, or other coating compositions, to impart both wet state and dry film preservation. The base medium can be, for example, a polymer useful in polymer emulsions wherein the polymer is selected from the group consisting of acrylic and substituted (meth)acrylates, styrene/butadiene, ethylene vinyl acetate, polyvinyl acetate, styrene/butadiene/N-methylol acrylamide, nitrile and copolymers of the aforementioned. Typical functional fluids include coating compositions, such as paints, adhesives, sealants, caulks, mineral and pigment slurries, printing inks, agricultural pesticide formulations, household products, personal care, metal working fluids and other aqueous based systems.

(13) The invention is further described in the Examples given below. All percentages given herein are weight percents based on the total weight of the composition, unless otherwise stated. All patents referred to in this application are incorporated herein by reference in their entirety.

Example 1

(14) In this example, samples of a commercial water based acrylic latex, trade name REVACRYL 1A, supplied by the Harlow Chemical Company, pH 8.1, containing antimicrobial additives in amounts as identified in Table I below, were challenged with a bacterial suspension consisting of

(15) Pseudomonas aeruginosa, NCIMB 8295

(16) Providencia rettgeri, NCIMB 10842

(17) Serratia marcescens, NCIMB 9523

(18) Aeromonas hydrophila, NCIMB 9233

(19) Alcaligenes spp. (Wet State Preservation isolate)

(20) Burkholderia cepacia, ATCC 25416

(21) Pseudomonas putida, NCIMB9494

(22) All test species were cultured onto nutrient agar and incubated for 24 hours at 30° C. After this period individual suspensions of each test organism were prepared at a concentration of 10.sup.6 colony forming units per ml in saline solution by means of a Thoma counting chamber, achieved by mixing equal volumes of the individual suspensions. The concentration of bacteria in each aliquot after each challenge was 2×10.sup.6 per ml.

(23) The antimicrobial additive-containing paint samples were stored at 40° C. for 7 days prior to commencing the test. The paint samples were challenged with 1% v/v of bacterial suspension as described above. All test samples were incubated at 30° C. for the duration of the test and examined for viable bacteria after 1, 3 and 7 days after each challenge. Viable bacteria were detected by streaking aliquots onto nutrient agar followed by incubation at 30° C. for 2 days.

(24) Table I presents the results obtained after the third inoculation days 1, 3 and 7.

(25) TABLE-US-00001 TABLE I 3.sup.rd inoculation Sample BIT/ppm Day 1 Day 3 Day 7 BIT/ZnO 50 C C C 100 1+ (92) 1+ (27) − 150 1+ (95) S (7) − 200 S (10) T (4) − 250 1+ (89) 1+ (22) − 300 1+ (77) 1+ (12) − BIT/ZnCl2 50 C C C 100 ++ 1+ (30) − 150 1+ (34) 1+ (21) − 200 1+ (51) 1+ (14) − 250 ++ (185) 1+ (25) − 300 1+ (58) 1+ (17) − PROXEL 50 C C C BD20 100 C C C 150 C C C 200 C C C 250 1+ (75) − − 300 1+ (54) − − 400 1+ (47) − − Assessments are rated as: − no growth detected T 0-5 colonies present S 5-10 colonies 1+ 20-100 colonies + light growth - no specified number of colonies ++ moderate growth - colonies visible, some coalescence +++ heavy growth - coalescing colonies visible throughout C confluent heavy growth

(26) Results demonstrate a potentiating effect of BIT in combination with a zinc containing salt when compared against a commercial product, namely PROXEL BD20 a product of Arch Chemicals, Inc. containing only BIT for the preservation of a commercial latex in that only 100 ppm of BIT is required in the presence of a zinc containing salt compared to 250 ppm of BIT for sample containing only BIT.

Example 2

Investigation of Potentiation Between 1,2-Benzisothiazolin-3-one (BIT) and Zinc Chloride (ZnCl2) Against Pseudomonas aeruginosa

(27) Bacteria

(28) Pseudomonas aeruginosa (NCIB 10421) maintained on nutrient agar, were used to study the synergy between BIT and ZnCl.sub.2.

(29) Calculation of Minimum Inhibitory Concentrations Against Mono-Cultures

(30) Bacteria were grown to stationary phase (approximately 24 hours) in nutrient broth (approximately 10.sup.9 organisms per ml). A 0.1% (v/v) inoculum was used to seed fresh medium and 100 μl of the inoculum was then added to each well of a microtitre plate, except for the first well which contained 200 μl.

(31) Using doubling dilutions, the concentration of the compounds under investigation were varied in each well along the ordinate axis. The presence or absence of growth was recorded by visual inspection after 24 hours incubation at 37° C.

(32) Calculation of Antimicrobial Activity Against Mono-Cultures

(33) Microtitre plates were used for this assay. A simple matrix was constructed with varied concentrations of the two compounds from 2×MIC (minimum inhibitory concentration) down to zero concentration in a 10×10 array. As the microtitre plate has only 96 wells, the combinations of the two compounds that made up the extreme concentrations (highest and lowest) were omitted. Solutions were made up in broth at two times the final concentrations after pre-dissolving the compounds in distilled water.

(34) The mixture (100 μl) was added to the plate so that the total volume in each well was 200 μl. Nutrient broth was used for Ps. aeruginosa. Plates were incubated for 16-24 hours at 37°. The presence or absence of growth was determined by visual inspection.

(35) Results for Example 2

(36) TABLE-US-00002 TABLE 2 Table 2: Antimicrobial Activity of Compounds Under Investigation MIC (ppm) against Ps. aeruginosa COMPOUND 1 2 3 4 BIT 30 42 42 30 ZnCl.sub.2 300 450 375 450 BIT is 1,2-benzisothiazolin-3-one available from Arch Limited PROXEL ® GXL.

(37) Calculation of Activity Against Mono-Cultures

(38) The Minimum Inhibitory Concentration (MIC) is the lowest concentration of biocide which showed growth inhibition when used alone. For the purpose of Fractional Inhibitory Concentration (FIC) calculations, if a single biocide did not control growth, the MIC was taken as the highest concentration used. A Fractional Inhibitory Concentration is the concentration of biocide which controlled growth in the mixture divided by the MIC of that biocide. FIC values for both compounds in the mixture were calculated and the results are shown in Table 3. The sum of these two figures gives an indication of the action of the two biocides. A value less than one indicates an enhanced effect, if the total is unity or greater the action is additive and if the value is greater than two the biocides are antagonistic.

(39) TABLE-US-00003 TABLE 3 Fractional Inhibitory Concentrations for 1,2-Benzisothtazolin-3-one (BIT) and Zinc Chloride (ZnCl.sub.2) against Ps. aeruginosa Table 3 FIC VALUES COMPOUND 1 2 3 4 BIT 1 0.20 0.20 0.40 0.14 0.14 0.29 0.29 0.43 0.14 0.20 0.40 0 ZnCl.sub.2 0 0.25 0.50 0.25 0.33 0.50 0.17 0.33 0.17 0.60 0.33 0.17 1 Total 1 0.45 0.70 0.65 0.47 0.64 0.46 0.62 0.60 0.74 0.53 0.57 1

(40) If a graph with the axes representing the biocide Fractional Inhibitory Concentrations for the two biocides on linear scales is constructed, when the combination is additive the isobole (i.e. the line joining the points that represent all combinations with the same effect including the equally effective concentrations of the biocides used alone) is straight, enhanced combinations give concave isoboles and antagonistic combinations give convex isoboles. As shown in FIG. 1, the combination of BIT and ZnCl.sub.2 has an enhanced effect against Ps. aeruginosa as confirmed by the concave isobole on the graph of FIG. 1.

Example 3

Investigation of Potentiation Between Kathon® (CIMIT/MIT) and Zinc Chloride (ZnCl2) Against Pseudomonas aeruginosa

(41) Bacteria

(42) Pseudomonas aeruginosa (NCIB 10421) maintained on nutrient agar, were used to study the synergy of Kathon® and ZnCl.sub.2.

(43) Calculation of Minimum Inhibitory Concentrations Against Mono-Cultures

(44) Bacteria were grown to stationary phase (approximately 24 hours) in nutrient broth (approximately 10.sup.9 organisms per ml). A 0.1% (v/v) inoculum was used to seed fresh medium and 100 μl of the inoculum was then added to each well of a microtitre plate, except for the first well which contained 200 μl.

(45) Using doubling dilutions, the concentration of the compounds under investigation were varied in each well along the ordinate axis. The presence or absence of growth was recorded by visual inspection after 24 hours incubation at 37° C.

(46) Calculation of Antimicrobial Activity Against Mono-Cultures

(47) Microtitre plates were used for this assay. A simple matrix was constructed with varied concentrations of the two compounds from 2×MIC (minimum inhibitory concentration) down to zero concentration in a 10×10 array. As the microtitre plate has only 96 wells, the combinations of the two compounds that made up the extreme concentrations (highest and lowest) were omitted. Solutions were made up in broth at two times the final concentrations after pre-dissolving the compounds in distilled water.

(48) The mixture (100 μl) was added to the plate so that the total volume in each well was 200 μl Nutrient broth was used for Ps. aeruginosa. Plates were incubated for 16-24 hours at 37°. The presence or absence of growth was determined by visual inspection.

(49) Results of Example 3

(50) TABLE-US-00004 TABLE 4 Table 4: Antimicrobial Activity of Compounds Under Investigation MIC (ppm) against Ps. aeruginosa COMPOUND 1 2 3 4 Kathon 0.9 0.9 0.9 0.53 ZnCl.sub.2 294 294 210 600

(51) The mixture of CIMIT and MIT is available from Rohm and Haas Company, Philadelphia, Pa. under the tradename Kathon®.

(52) Calculation of Synergy Against Mono-Cultures

(53) The Minimum Inhibitory Concentration (MIC) is the lowest concentration of biocide which showed growth inhibition when used alone. For the purpose of Fractional Inhibitory Concentration (FIC) calculations, if a single biocide did not control growth, the MIC was taken as the highest concentration used. A Fractional Inhibitory Concentration is the concentration of biocide which controlled growth in the mixture divided by the MIC of that biocide. FIC values for both compounds in the mixture were calculated and the results are shown in Table 5. The sum of these two figures gives an indication of the action of the two biocides. A value less than one indicates an enhanced effect, if the total is unity or greater the action is additive and if the value is greater than two the biocides are antagonistic.

(54) TABLE-US-00005 TABLE 5 Fractional Inhibitory Concentrations for Kathon (CIMIT/MIT) and Zinc Chloride (ZnCl.sub.2) against Ps. aeruginosa Table 5 FIC VALUES COMPOUND 1 2 3 4 Kathon 1 0.17 0.17 0.33 0.33 0.17 0.33 0.33 0.14 0.28 0.28 0 ZnCl.sub.2 0 0.14 0.29 0.14 0.29 0.29 0.14 0.29 0.50 0.38 0.50 1 Total 1 0.31 0.46 0.47 0.62 0.46 0.47 0.62 0.64 0.66 0.78 1

(55) If a graph with the axes representing the biocide Fractional Inhibitory Concentrations for the two biocides on linear scales is constructed, when the combination is additive the isobole (i.e. the line joining the points that represent all combinations with the same effect including the equally effective concentrations of the biocides used alone) is straight, enhanced combinations give concave isoboles and antagonistic combinations give convex isoboles. As shown in FIG. 2, the combination of Kathon® and ZnCl.sub.2 has an enhanced effect against Ps. aeruginosa as confirmed by the concave isobole in FIG. 2.

Example 4

Investigation of Potentiation Between MIT and Zinc Chloride (ZnCl2) Against Pseudomonas aeruginosa

(56) Bacteria

(57) Pseudomonas aeruginosa (NCIB 10421) maintained on nutrient agar, were used to study the synergy between MIT and ZnCl.sub.2.

(58) Calculation of Minimum Inhibitory Concentrations Against Mono-Cultures

(59) Bacteria were grown to stationary phase (approximately 24 hours) in nutrient broth (approximately 10.sup.9 organisms per ml). A 0.1% (v/v) inoculum was used to seed fresh medium and 100 μl of the inoculum was then added to each well of a microtitre plate, except for the first well which contained 200 μl.

(60) Using doubling dilutions, the concentration of the compounds under investigation were varied in each well along the ordinate axis. The presence or absence of growth was recorded by visual inspection after 24 hours incubation at 37° C.

(61) Calculation of Antimicrobial Activity Against Mono-Cultures

(62) Microtitre plates were used for this assay. A simple matrix was constructed with varied concentrations of the two compounds from 2×MIC (minimum inhibitory concentration) down to zero concentration in a 10×10 array. As the microtitre plate has only 96 wells, the combinations of the two compounds that made up the extreme concentrations (highest and lowest) were omitted. Solutions were made up in broth at two times the final concentrations after pre-dissolving the compounds in distilled water.

(63) The mixture (100 μl) was added to the plate so that the total volume in each well was 200 μl. Nutrient broth was used for Ps. aeruginosa. Plates were incubated for 16-24 hours at 37°. The presence or absence of growth was determined by visual inspection.

(64) Results of Example 4

(65) TABLE-US-00006 TABLE 6 Table 6: Antimicrobial Activity of Compounds Under investigation MIC (ppm) against Ps. aeruginosa COMPOUND 1 2 3 4 MIT 9 6 10.5 8 ZnCl.sub.2 210 210 450 450

(66) Calculation of Synergy Against Mono-Cultures

(67) The Minimum Inhibitory Concentration (MIC) is the lowest concentration of biocide which showed growth inhibition when used alone. For the purpose of Fractional Inhibitory Concentration (FIC) calculations, if a single biocide did not control growth, the MIC was taken as the highest concentration used. A Fractional Inhibitory Concentration is the concentration of biocide which controlled growth in the mixture divided by the MIC of that biocide. FIC values for both compounds in the mixture were calculated and the results are shown in Table 7. The sum of these two figures gives an indication of the action of the two biocides. A value less than one indicates an enhanced effect, if the total is unity or greater the action is additive and if the value is greater than two the biocides are antagonistic.

(68) TABLE-US-00007 TABLE 7 Fractional Inhibitory Concentrations for (MIT) and Zinc Chloride (ZnCl.sub.2) against Ps. aeruginosa Table 7 FIC Values COMPOUND 1 2 3 4 MIT 1 0.17 0.33 0.33 0.50 0.25 0.50 0.75 0.14 0.29 0.17 0 ZnCl.sub.2 0 0.57 0.29 0.43 0.14 0.43 0.29 0.14 0.33 0.17 0.50 1 Total 1 0.74 0.62 0.76 0.64 0.68 0.79 0.89 0.48 0.46 0.67 1

(69) If a graph with the axes representing the biocide Fractional Inhibitory Concentrations for the two biocides on linear scales is constructed, when the combination is additive the isobole (i.e. the line joining the points that represent all combinations with the same effect including the equally effective concentrations of the biocides used alone) is straight, enhanced combinations give concave isoboles and antagonistic combinations give convex isoboles. As shown in FIG. 3, the combination of MIT and ZnCl.sub.2 has an enhanced effect against Ps. aeruginosa as confirmed by the concave isobole in FIG. 3.

Example 5

Investigation of Potentiation Between BBIT and Zinc Chloride (ZnCl2) Against Pseudomonas aeruginosa

(70) Bacteria

(71) Pseudomonas aeruginosa (NCIB 10421) maintained on nutrient agar, were used to study the synergy between BBIT and ZnCl.sub.2.

(72) Calculation of Minimum Inhibitory Concentrations Against Mono-Cultures

(73) Bacteria were grown to stationary phase (approximately 24 hours) in nutrient broth (approximately 10.sup.9 organisms per ml). A 0.1% (v/v) inoculum was used to seed fresh medium and 100 μl of the inoculum was then added to each well of a microtitre plate, except for the first well which contained 200 μl.

(74) Using doubling dilutions, the concentration of the compounds under investigation were varied in each well along the ordinate axis. The presence or absence of growth was recorded by visual inspection after 24 hours incubation at 37° C.

(75) Calculation of Antimicrobial Activity Against Mono-Cultures

(76) Microtitre plates were used for this assay. A simple matrix was constructed with varied concentrations of the two compounds from 2×MIC (minimum inhibitory concentration) down to zero concentration in a 10×10 array. As the microtitre plate has only 96 wells, the combinations of the two compounds that made up the extreme concentrations (highest and lowest) were omitted. Solutions were made up in broth at two times the final concentrations after pre-dissolving the compounds in distilled water.

(77) The mixture (100 μl) was added to the plate so that the total volume in each well was 200 μl. Nutrient broth was used for Ps. aeruginosa. Plates were incubated for 16-24 hours at 37°. The presence or absence of growth was determined by visual inspection.

(78) Results of Example 5

(79) TABLE-US-00008 TABLE 8 Table 8: Antimicrobial Activity of Compounds Under Investigation MIC (ppm) against Ps. aeruginosa COMPOUND 1 2 3 4 BBIT 315 360 42 180 ZnCl.sub.2 525 525 600 600 BBIT is N-(n-butyl)-1,2-benzisothiazolin-3-one.

(80) Calculation of Synergy Against Mono-Cultures

(81) The Minimum Inhibitory Concentration (MIC) is the lowest concentration of biocide which showed growth inhibition when used alone. For the purpose of Fractional Inhibitory Concentration (FIC) calculations, if a single biocide did not control growth, the MIC was taken as the highest concentration used. A Fractional Inhibitory Concentration is the concentration of biocide which controlled growth in the mixture divided by the MIC of that biocide. FIC values for both compounds in the mixture were calculated and the results are shown in Table 9. The sum of these two figures gives an indication of the action of the two biocides. A value less than one indicates an enhanced effect, if the total is unity or greater the action is additive and if the value is greater than two the biocides are antagonistic.

(82) TABLE-US-00009 TABLE 9 Fractional Inhibitory Concentrations for BBIT and Zinc Chloride (ZnCl.sub.2) against Ps. aeruginosa Table 9 FIC values COMPOUND 1 2 3 4 BBIT 1 0.29 0.25 0.14 0.71 0.25 0 ZnCl.sub.2 0 0.43 0.43 0.50 0.38 0.38 1 Total 1 0.71 0.68 0.64 1.09 0.63 1

(83) If a graph with the axes representing the biocide Fractional Inhibitory Concentrations for the two biocides on linear scales is constructed, when the combination is additive the isobole (i.e. the line joining the points that represent all combinations with the same effect including the equally effective concentrations of the biocides used alone) is straight, enhanced combinations give concave isoboles and antagonistic combinations give convex isoboles. As shown in FIG. 4, the combination of BBIT and ZnCl.sub.2 has an enhanced effect against Ps. aeruginosa as confirmed by the concave isobole shown in FIG. 4.

Comparative Example A

Investigation of Potentiation Between 1,2-Benzisothiazolin-3-one (BIT) and Sodium Chloride (NaCl2) Against Pseudomonas aeruginosa as Compared to the Synergy Between 1,2-Benzisothiazolin-3-one (BIT) and Zinc Chloride (ZnCl2) Against Pseudomonas aeruginosa (Example 2)

(84) Bacteria

(85) Pseudomonas aeruginosa (NCIB 10421) maintained on nutrient agar, were used to study the efficacy of the combination of BIT with sodium chloride.

(86) Calculation of Minimum Inhibitory Concentrations Against Mono-Cultures

(87) Bacteria were grown to stationary phase (approximately 24 hours) in nutrient broth (approximately 10.sup.9 organisms per ml). A 0.1% (v/v) inoculum was used to seed fresh medium and 100 μl of the inoculum was then added to each well of a microtitre plate, except for the first well which contained 200 μl.

(88) Using doubling dilutions, the concentration of the compounds under investigation were varied in each well along the ordinate axis. The presence or absence of growth was recorded by visual inspection after 24 hours incubation at 37° C.

(89) Calculation of Antimicrobial Activity Against Mono-Cultures

(90) Microtitre plates were used for this assay. A simple matrix was constructed with varied concentrations of the two compounds from 2×MIC (minimum inhibitory concentration) down to zero concentration in a 10×10 array. As the microtitre plate has only 96 wells, the combinations of the two compounds that made up the extreme concentrations (highest and lowest) were omitted. Solutions were made up in broth at two times the final concentrations after pre-dissolving the compounds in distilled water.

(91) The mixture (100 μl) was added to the plate so that the total volume in each well was 200 μl. Nutrient broth was used for Ps. aeruginosa. Plates were incubated for 16-24 hours at 37°. The presence or absence of growth was determined by visual inspection.

(92) Results of Comparative Example A

(93) TABLE-US-00010 TABLE 10 Table 10: Antimicrobial Activity of Compounds Under Investigation COMPOUND MIC (ppm) against Ps. aeruginosa BIT 30 NaCl.sub.2 None detected (greater than 675) BIT is 1,2-benzisothiazolin-3-one available from Arch Limited as PROXEL.sup.RTM GXL.

(94) Calculation of Potentiation Against Mono-Cultures

(95) The Minimum Inhibitory Concentration (MIC) is the lowest concentration of biocide which showed growth inhibition when used alone. For the purpose of Fractional Inhibitory Concentration (FIC) calculations, if a single biocide did not control growth, the MIC was taken as the highest concentration used. A Fractional Inhibitory Concentration is the concentration of biocide which controlled growth in the mixture divided by the MIC of that biocide. FIC values for both compounds in the mixture were calculated and the results are shown in Table 11. The sum of these two figures gives an indication of the action of the two biocides. A value less than one indicates an enhanced effect, if the total is unity or greater the action is additive and if the value is greater than two the biocides are antagonistic.

(96) TABLE-US-00011 TABLE 11 Table 11: Fractional Inhibitory Concentrations for 1,2-Benzisothiazolin-3- one (BIT) and Sodium Chloride (NaCl.sub.2) against Ps. aeruginosa COMPOUND FIC VALUES BIT 1 1.00 0 NaCl.sub.2 0 0.33 1 Total 1 1.33 1

(97) If a graph with the axes representing the biocide Fractional Inhibitory Concentrations for the two biocides on linear scales is constructed, when the combination is additive the isobole (i.e. the line joining the points that represent all combinations with the same effect including the equally effective concentrations of the biocides used alone) is straight, enhanced combinations give concave isoboles and antagonistic combinations give convex isoboles. As shown in FIG. 5, the combination of BIT and NaCl.sub.2 is additive against Ps. aeruginosa as confirmed by the straight isobole in FIG. 5. Additionally, the results of this Example further illustrate that the combination of an isothiazolin-3-one such as BIT and a zinc compound such as ZnCl.sub.2, as shown in Example 2, show unexpected potentiation results against Ps. aeruginosa.