278 Biofilms of Salmonella typhimurium DT104 © 2006 Journal of Microbiology, Immunology and Infection Characterization of biofilm-forming abilities of antibiotic-resistant Salmonella typhimurium DT104 on hydrophobic abiotic surfaces Yakubu B. Ngwai 1 , Yoshikazu Adachi 2 , Yasuki Ogawa 2 , Hiromichi Hara 3 1 Department of Microbiology, Human Virology and Biotechnology, National Institute for Pharmaceutical Research and Development, Abuja, Nigeria; and 2 Animal Health Laboratory and 3 Department of Plant Production Science, School of Agriculture, Ibaraki University, Ibaraki-ken, Japan Received: October 31, 2005 Revised: December 1, 2005 Accepted: December 26, 2005 Corresponding author: Dr. Yakubu B. Ngwai, Department of Microbiology, Human Virology and Biotechnology, National Institute for Pharmaceutical Research and Development, P.M.B. 21 Garki, Abuja, Nigeria. E-mail: ybngwai@yahoo.com Background and Purpose: Salmonella typhimurium DT104 strain has emerged as a global human and veterinary public health concern because of its antibiotic resistance and extensive host range. Although it is thought to be more virulent, to date, factors relevant to its virulence have not been fully elucidated. Thus, understanding how this strain forms biofilms on hydrophobic surfaces will add to current knowledge on its possible virulence mechanism. Methods: Biofilm-forming abilities of clinical isolates of S. typhimurium DT104 from human and animal sources on hydrophobic inanimate surfaces were assessed by absorbance at 600 nm of crystal violet-bound cells recovered from 96-well tissue culture plates after growth in a nutrient-rich growth medium and various adjusted media; and scanning electron microscopy based on standard procedures. Results: In the nutrient-rich growth medium, Luria-Bertani (LB), biofilms were formed in small quantities, preferentially on polystyrene (p<0.05), and followed different time courses. Significantly lower amounts of biofilms were formed on polystyrene when a nutrient-deficient growth medium (adherence test medium) was used. Inclusion of D-(+)-mannose in LB at a concentration of 100 mM significantly (p<0.05) inhibited biofilm formation on polystyrene. D-(+)-glucose relatively enhanced biofilm formation but D-(-)-mannitol only insignificantly influenced the process. The action of mannose on polyvinly chloride (PVC) was insignificant, suggesting that its action may be surface-dependent. Additionally, glucose significantly reduced biofilm growths of 2 of the isolates and only that of the PVC-loving strain T980021 on polystyrene and PVC, respectively. At the concentration tested, unlike xylose, both D-mannose and D-glucose significantly (p<0.05) inhibited bacterial growth, providing a possible mechanism for their inhibitory action on biofilm formation by S. typhimurium. While stress of starvation resulted in significant reduction in biofilm formation on polystyrene in all but the PVC-loving strain T980021, high osmolarity had little effect on the quantity of biofilm formed on polystyrene. The extent of primary attachment to polystyrene as well as their capacity to form biofilm did not correlate with their cell surface hydrophobicity and exopolysaccharide production. Conclusions: D-(+)-mannose inhibits biofilm formation by S. typhimurium DT104 on polystyrene but not on PVC. There was also a general lack of correlation between the ability of S. typhimurium DT104 to form biofilm and its physicochemical surface characteristics. Key words: Biofilms, Salmonella typhimurium DT104, surface properties, two-dimensional gel electrophoresis Original Article J Microbiol Immunol Infect 2006;39:278-291 remains the most frequently isolated in human, swine, avian, and bovine salmonellosis [3,4]. An emergent strain, Salmonella typhimurium DT104, is of particular concern because of its antibiotic resistance and wide host range [5- 7]; the strain is also thought to be more virulent than other Typhimurium strains [8,9]. However, virulence assays based on stress response, tissue invasion, macrophage survival and mortality rate in animals, have indicated otherwise [10-12]. Introduction Non-typhoid salmonellae are a common cause of bacterial gastroenteritis worldwide [1,2]. The serotype Typhimurium