The influence of different concentrations of melatonin on the cell surface hydrophobic characteristics of Neisseria meningitidis J. Uberos, A. Molina, J. Lie  bana 1 , M.C. Augustin and A. Mun Ä oz Grupo de Investigacio  n CTS-190, Departamento de Paediatrõ Âa, Hospital Clõ Ânico Universitario `San Cecilio' and 1 Departamento de Microbiologõ Âa, Facultad de Medicina, Granada, Spain 180/2000: received 9 May 2000 and accepted 20 June 2000 J. UBEROS, A. MOLINA, J. LIE  BANA, M.C. AUGUSTIN AND A. MUN Ä O Z . 2000. The cell surface hydrophobicity of micro-organisms is a characteristic that has been associated with the colonization of mammalian epithelia and with their capacity to induce diseases. Melatonin is a hormone produced by the pineal gland that affects the immune response mechanism. This study investigated, as an expression of the virulence of Neisseria meningitidis, how its hydrophobic characteristics were affected by exposure to increasing concentrations of melatonin. An increase in the cell surface hydrophobicity of N. meningitidis was found at concentrations of 1 mmol l 1 , while lower concentrations of melatonin did not signi®cantly affect this particular cell surface characteristic of the micro-organism. It may be concluded that melatonin clearly in¯uences the cell surface hydrophobicity of N. meningitidis,a circumstance that should be taken into account in future studies to determine whether this hormone plays a role in the variable pathogenicity of the bacteria in different hosts. INTRODUCTION The cell surface hydrophobicity of bacteria is considered to be a characteristic that is related to the colonization of epithelia in mammals (Stendahl et al. 1973; Rosenberg et al. 1980; Rosenberg 1984a) and to their susceptibility to phagocytosis (Stendahl et al. 1973; Ferreiros et al. 1989; Svanborg et al. 1984). Nevertheless, despite its theoretical importance in the virulence of Neisseria meningitidis, no clear parallel has yet been found between this cell surface characteristic and the clinical evidence of the micro-organ- ism, which may remain in the oropharynx of asymptomatic subjects or which may become evident as a disease with systemic extent and a fatal sepsis (Ferreiros et al. 1986). We are led to conclude that other factors inherent to the host, and not just the micro-organism, may be involved in the varying patterns of N. meningitidis. In recent years there have been important advances in our knowledge of the immune-affecting role of melatonin (Attia et al. 1991; Lissoni et al. 1991) and of its participa- tion in the stabilization processes of oxidative stress (Hardeland et al. 1993; Reiter et al. 1993; Reiter 1994; Reiter et al. 1997) and membrane stabilization (Reiter et al. 1995; Garcia et al. 1998). Melatonin is a hormone in the pineal gland which has been related to the immune response mechanism (Rodrõ Âguez et al. 1997). The in¯uence of melatonin on the non-speci®c immune response mechanism and its repercus- sions on the virulence of certain bacteria have not yet been clearly identi®ed. Therefore, our aim in this study was to evaluate the possible relationship between melatonin and the cell surface characteristics of N. meningitidis. In addi- tion, as iron is a factor that has been widely linked to the virulence of N. meningitidis and other micro-organisms (Criado et al. 1993; Lachica and Zink 1984) we tested whether the different concentrations of iron in the culture medium also modi®ed the cell surface characteristics of this particular micro-organism. MATERIALS AND METHODS Bacterial strains and culture conditions We studied six strains of N. meningitidis, two of which were isolated from the nasopharynx of healthy subjects (P391, ungroupable, autoagglutinable, serotype 15 and P000, serogroup B, serotype 15) and four (V002, serogroup B, serotype 15; V021, serogroup B, non typable; V019, sero- group B, serotype 1 and V111, serogroup C) which were Correspondence to: J. Uberos, Grupo de Investigacio Ân CTS-190, Departamento de Paediatrõ Âa, Hospital Clõ Ânico Universitario `San Cecilio', Granada, Spain. Letters in Applied Microbiology 2000, 31, 294298 = 2000 The Society for Applied Microbiology