Both 4,5-dichloro-2-N-octyl-4-isothiazolin-3-one, referred to herein as DOI, and the combination of a chlorinated isocyanurate and a bromide compound, referred to herein as Towerbrom. RTM.,  are known individually as antimicrobial and/or disinfecting agents.  The unexpected finding of the present invention is that they are synergistic when used in combination.  As used herein, the terms "synergy" and "synergistic" refer to instances where the effectiveness of a composition comprising two or more biocides, such as DOI and Towerbrom. RTM.,  exceeds the sum of the efficacies of the individual components taken alone.  Thus, using a synergistic biocidal combination may allow for use of a lower overall concentration of biocide or the realization of an enhanced antimicrobial effect at a comparable dosage. 
Both Towerbrom. RTM.,  and related compounds, and DOI, and related compounds, are known in the art as antimicrobial agents, both alone and in conjunction with other biocides.  The synergistic combination of Towerbrom. RTM.  and DOI, however, is not taught or suggested in the art.  For example, chlorinated isocyanurates, alone or in combination with sodium bromide and/or potassium bromide, have been reported as disinfecting agents in PCT Application Number WO 93/04987, and U.S. Pat.  Nos.  4,557,926 and 5,015,643.  In addition, U.S. Pat.  No.  5,254,526 discloses a method of inhibiting the growth of algae by introducing to the body of water being treated a chlorine-containing oxidizer and a water soluble bromide which has been premixed with an alkali metal, alkaline earth metal or ammonium polyphosphate. 
Using microbicidal compositions of DOI either alone or in conjunction with other compounds, as well as the use of other isothiazoline compounds as microbicides, is reported, for example, in EP 608 912; EP 615 688; EP 611 522; EP 476 943; and U.S. Pat.  Nos.  5,756,526; 5,730,907; 5,703,105; 5,648,086; and 5,559,083.  European Patent 490 567 reports the use of halogen-containing organic stabalizers for 3-isothiazolines, including trichloroisocyanuric acid or other halogenated compounds.  Methods of preventing or inhibiting the growth of microorganisms is also reported.  The patent does not appear to teach the use of a chlorinated isocyanurate in conjunction with a bromide compound in a synergistic mixture with isothiazolines. 
As used herein, the phrases "antimicrobial", "biocide", and "inhibiting microbial growth" refer to the killing of, the inhibition of, or the control of the growth of bacteria, yeast, mold, and/or algae.  A number of important industries have experienced serious adverse effects from the activity of such biological growth on the raw materials which they employ, in their process waters, on various components of their manufacturing processes, and in the finished products which they produce.  Such industries include the paint, wood, textile, cosmetic and personal care, leather, tobacco, fur, rope, paper, pulp, plastics, fuel, oil, rubber, and machine industries. 
Systems which utilize circulating water or aqueous media become infected with microorganisms and experience substantial impairment of their efficiency when deposits of the microorganisms build up in the system.  The deposits coat the walls of tanks and other vessels and any machinery or processing equipment which is employed and create blockages in pipes and valves.  The deposits also create discolorations and other imperfections in the products being produced, forcing costly shutdowns.  Control of microorganisms is particularly important in aqueous media in which there are dispersed particles or fines in the aqueous media, for example, dispersed cellulosic fibers and dispersed fillers and pigments in papermaking, and dispersed pigments in paint manufacture. 
It is contemplated that the synergistic admixture of Towerbrom. RTM.  and DOI as disclosed herein, and the methods for using the same, will be useful in virtually any aqueous system or on any article or product of manufacture in which inhibition of microbial growth is desired, absent compatibility problems.  Important applications of the synergistic antimicrobial combinations of the present invention include, for example: inhibiting the growth of bacteria and fungi, including yeast and mold, in aqueous paints, adhesives, latex emulsions, inks and joint cements; preserving wood; preserving cutting oils and metal working fluids; controlling slime-producing bacteria and fungi, including yeast and mold, in pulp and paper mills and cooling towers; as a spray or dip treatment for textiles and leather to prevent mold growth; as a component of anti-fouling paints to prevent adherence of fouling organisms; protecting paint films, especially exterior paints, from attack by fungi which occurs during weathering of the paint film; protecting processing equipment from slime deposits during manufacture of cane and beet sugar, foods, foodstuffs and food additives; preventing microorganism buildup and deposits in air washer or scrubber systems and in industrial fresh water supply systems; controlling microorganism contamination in closed loop and recirculating water cooling systems; controlling microorganism contamination and deposits in oil field drilling fluids and muds, and in secondary petroleum recovery processes; preventing bacterial and fungal growth in paper coating processes which might adversely affect the quality of the paper coating; controlling bacterial and fungal growth and deposits during the manufacture of various specialty boards, e.g., cardboard and particle board; preventing sap stain discoloration on freshly cut wood of various kinds; controlling bacterial and fungal growth in clay and pigment slurries of various types which are manufactured for later use in paper coating and paint manufacturing and which are susceptible to degradation by microorganisms during storage and transport; as a hard surface disinfectant to prevent growth of bacteria and fungi on walls, floors, etc.;  and in swimming pools to prevent algal growth. 
Slime control in papermaking processes is also of particular importance.  The control of bacteria and fungi in pulp and paper mill water systems which contain aqueous dispersions of papermaking fibers in various consistencies is especially critical.  The uncontrolled buildup of slime produced by the accumulation of bacteria and fungi may cause off-grade production, decreased production due to down-time and greater cleanup frequency, increased raw material usage, and increased maintenance costs.  The problem of slime deposits is especially critical in light of the widespread use of closed white water systems in the paper industry. 
Another important area that requires the use of good antimicrobial compositions to control bacterial and fungal growth is in clay and pigment slurries.  These slurries comprise various clays (e.g., kaolin) and pigments (e.g., calcium carbonate and titanium dioxide) and usually are manufactured at a location separate from the end use application.  This means that they are generally transported and stored for later use at the application site.  Because of high quality standards for the paper and paint products in which such slurries are used, it is essential that these clay or pigment slurries have a very low microorganism count per gram of sample. 
There remains a very real and substantial need for antimicrobial compositions capable of effectively controlling and/or inhibiting microbial growth in industrial aqueous systems and on articles of manufacture.  Because of increasing environmental regulations, there is still a further need to provide biocidal compositions having enhanced antimicrobial effect which are effective in lower doses than historically used.  Use of lower amounts of biocides has a favorable impact on the environment, and allows users to realize significant cost savings.