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 . 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 5 ( oh ) 6 ( co 3 ) 2 ), and combinations thereof . 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 . 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 ). 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 . 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 . 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 . 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 . 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 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 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 6 per ml . 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 . table i presents the results obtained after the third inoculation days 1 , 3 and 7 . 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 . investigation of potentiation between 1 , 2 - benzisothiazolin - 3 - one ( bit ) and zinc chloride ( zncl 2 ) against pseudomonas aeruginosa pseudomonas aeruginosa ( ncib 10421 ) maintained on nutrient agar , were used to study the synergy between bit and zncl 2 . calculation of minimum inhibitory concentrations against mono - cultures bacteria were grown to stationary phase ( approximately 24 hours ) in nutrient broth ( approximately 10 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 . 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 . 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 . 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 . 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 . 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 fig1 , the combination of bit and zncl 2 has an enhanced effect against ps . aeruginosa as confirmed by the concave isobole on the graph of fig1 . investigation of potentiation between kathon ® ( cimit / mit ) and zinc chloride ( zncl 2 ) against pseudomonas aeruginosa pseudomonas aeruginosa ( ncib 10421 ) maintained on nutrient agar , were used to study the synergy of kathon ® and zncl 2 . bacteria were grown to stationary phase ( approximately 24 hours ) in nutrient broth ( approximately 10 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 . 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 . 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 . 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 . 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 . 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 fig2 , the combination of kathon ® and zncl 2 has an enhanced effect against ps . aeruginosa as confirmed by the concave isobole in fig2 . investigation of potentiation between mit and zinc chloride ( zncl 2 ) against pseudomonas aeruginosa pseudomonas aeruginosa ( ncib 10421 ) maintained on nutrient agar , were used to study the synergy between mit and zncl 2 . bacteria were grown to stationary phase ( approximately 24 hours ) in nutrient broth ( approximately 10 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 . 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 . 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 . 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 . 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 . 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 fig3 , the combination of mit and zncl 2 has an enhanced effect against ps . aeruginosa as confirmed by the concave isobole in fig3 . investigation of potentiation between bbit and zinc chloride ( zncl 2 ) against pseudomonas aeruginosa pseudomonas aeruginosa ( ncib 10421 ) maintained on nutrient agar , were used to study the synergy between bbit and zncl 2 . bacteria were grown to stationary phase ( approximately 24 hours ) in nutrient broth ( approximately 10 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 . 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 . 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 . 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 . 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 . 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 fig4 , the combination of bbit and zncl 2 has an enhanced effect against ps . aeruginosa as confirmed by the concave isobole shown in fig4 . investigation of potentiation between 1 , 2 - benzisothiazolin - 3 - one ( bit ) and sodium chloride ( nacl 2 ) against pseudomonas aeruginosa as compared to the synergy between 1 , 2 - benzisothiazolin - 3 - one ( bit ) and zinc chloride ( zncl 2 ) against pseudomonas aeruginosa ( example 2 ) pseudomonas aeruginosa ( ncib 10421 ) maintained on nutrient agar , were used to study the efficacy of the combination of bit with sodium chloride . bacteria were grown to stationary phase ( approximately 24 hours ) in nutrient broth ( approximately 10 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 . 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 . 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 . 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 . 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 . 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 fig5 , the combination of bit and nacl 2 is additive against ps . aeruginosa as confirmed by the straight isobole in fig5 . 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 2 , as shown in example 2 , show unexpected potentiation results against ps . aeruginosa .
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 .