Stainless steel surfaces coated with paints containing a silver-and zinc-containing zeolite (AgION antimicrobial) were assayed in comparison to uncoated stainless steel for antimicrobial activity against vegetative cells and spores of three Bacillus species, namely, B. anthracis Sterne, B. cereus T, and B. subtilis 168. Under the test conditions (25°C and 80% relative humidity), the zeolite coating produced approximately 3 log 10 inactivation of vegetative cells within a 5-to 24-h period, but viability of spores of the three species was not significantly affected.Metals such as silver and zinc have long been recognized for their broad-spectrum antimicrobial properties, and a number of preparations containing silver and/or zinc have been formulated for reducing infections of indwelling catheters and stainless steel orthopedic devices (reviewed in references 1, 2, 7, 9, 11, and 13). An interesting advance in technology has been to trap silver and zinc ions within zeolites (inorganic ceramics) and to apply these compounds to various materials as longlasting antimicrobial treatments (8,14). Commercial concerns have claimed that application of silver-zinc zeolite coatings to stainless steel surfaces such as air ducts, countertops, or food preparation areas can reduce the bacterial load, hence lowering the risk of contamination by pathogenic or food spoilage microorganisms. However, controlled studies testing the efficacy of such preparations are scarce. Therefore, the present study was undertaken to test claims of the antibacterial properties of a particular silver-zinc zeolite preparation, the AgION antimicrobial, applied to stainless steel sheets. The challenge organisms used were species of the genus Bacillus, chosen for their environmental ubiquity, their ability to form resistant spores, and their known roles as agents of food poisoning (B. cereus, B. anthracis), food spoilage (B. subtilis), and bioterrorism (B. anthracis).The bacterial strains used, namely, B. cereus strain T (WN129), B. subtilis strain 168 (WN131), and B. anthracis Sterne (WN742), were from the communicating author's laboratory collection. Vegetative cells of all strains were produced as follows. Overnight cultures grown in liquid Luria-Bertani (LB) medium (5) were diluted to an optical density of 10 Klett units (Klett-Summerson colorimeter, no. 66 red filter) into 10 ml of fresh LB medium and incubated in a rotary shaker (New Brunswick G76; 300 rpm, 37°C) to 300 Klett units (ϳ10 8 CFU/ ml). (Note that to prevent sporulation during the course of experiments, B. anthracis Sterne vegetative cells were grown to only 160 Klett units [ϳ5 ϫ 10 7 CFU/ml] in LB.) Spores of all strains were produced by incubation in liquid Schaeffer's sporulation medium (12) at 37°C for 72 h in a rotary shaker with vigorous aeration. Spores were harvested by centrifugation (10,000 ϫ g for 10 min at 25°C), purified using the lysozyme and buffer-washing technique described by Nicholson and Setlow (6), heat shocked (80°C, 10 min), and stored at 4°C in phosphate-buffered s...