Letters in Applied Microbiology 1998, 27, 189–192 Antimicrobial effect of tetramethyldithiooxamide and its application as a biocide S.V. Tabakova 1 , T.V. Kantardjiev 2 and V.V. Mircheva 3 1 Institute of Molecular Biology, The Bulgarian Academy of Sciences, 2 National Centre of Infectious and Parasitic Diseases, and 3 Institute of Polymers, The Bulgarian Academy of Sciences, Sofia, Bulgaria 1850/98: received and accepted 15 July 1998 S.V. TABAKOVA, T.V. KANTARDJIEV AND V.V. MIRCHEVA. 1998. Tetramethyldithiooxamide (TMDTOA) is a stable and effective inhibitor of metal corrosion. The antimicrobial effects of TMDTOA and its metal compounds were investigated with regard to their use as biocides for water treatment. Growth of a variety of strains of bacteria and yeasts was completely inhibited at 400 mmol 1 -1 TMDTOA or its metal complexes. At 100 mg 1 -1 there was a 99·99% reduction in the number of viable micro-organisms; this activity persisted for 1–3 months. TMDTOA can be produced cheaply at 98% purity by a novel method, representing an alternative cost-effective water treatment agent combining corrosion-inhibiting and biocide characteristics. INTRODUCTION Microbial growth in circulating water heat exchange systems causes clogging, sludging, and enhances metal corrosion (Gla- zunov and Ilyaletidinov 1984). To prevent equipment damage resulting from the activity of micro-organisms and the accretion of microbial biomass, biocides are added to the circulating solutions. Chromate compounds acting both as inhibitors of corrosion and as broad spectrum biocides were widely used until concerns about their toxicity necessitated the termination of their application (Al-Hajjar and Riad 1990). Biocidal agents currently available for water treatment have a number of disadvantages. Tiram, for example, is rap- idly deactivated under operating conditions (Monga and Grover 1990), while the quinolones display a narrow anti- microbial spectrum (Alberts 1976), meaning that a com- bination of antimicrobial agents needs to be used (Grim et al. 1988; Morpeth and Austin 1989) The high human toxicity of certain biocides precludes their use (Petersen 1992) and furthermore, necessitates the development of new biocides. Ideally, a water treatment agent with biocide activity should possess both antifungal and antibacterial effects, per- sist for long periods of time under the operating conditions of the system, should not be aggressive to the materials into which it comes in contact, and should not release toxic com- pounds into the working environment. The search for a novel water biocide revealed the potential of the dithiooxamides (DTOAs). Tetramethyldithiooxamide Correspondence to: Dr S.V. Tabakova, PO Box 689, 1000 Sofia, Bulgaria. © 1998 The Society for Applied Microbiology (TMDTOA) and its metal complexes inhibit the corrosion of steel (Raitschevsky et al. 1995) and copper (Monticelli et al. 1991), and are stable at high temperatures and in aqueous solutions (Raitschevsky et al. 1995). However, as far as is known the antimicrobial effect of TMDTOA has not been investigated. The reported tuber- culostatic effect of bis-substituted DTOAs (Mollin et al. 1986) and the well established antifungal effect of some thiocarbamates provide evidence that TMDTOA may be growth inhibitory for bacteria and fungi. The aim of this work was to study the antimicrobial effect of TMDTOA and its metal complexes with regard to their applicability as water biocides. Accordingly, in vitro minimal inhibitory concentrations (MICs) of the compounds for a number of strains of bacteria and yeasts, and the reduction of the number of viable micro-organisms in water and cooling solution at different TMDTOA concentrations, have been determined. MATERIALS AND METHODS Chemicals TMDTOA was synthesized by a method originally described by Mircheva (1982) which afforded the product 98% purity. For MIC determinations and laboratory studies, the com- pound was purified to 99·5% by twofold recrystallization from ethanol. The NiSO 4 , Fe 2 SO 4 , CuSO 4 and ZnSO 4 were of analytical grade. EPEPE, where E denotes a polyethylene