Background: Colonization of the nasopharyngeal flora by Streptococcus pneumoniae increases in winter. However, in vitro studies suggest that bacterial adherence decreases in low temperatures. These contras may be related to the difference between in vitro and physiological conditions. Objectives: We investigated whether pneumococcal adherence to buccal epithelial cells of rats increases in cold temperature. Methods: The adherence of S. pneumonia ( ATCC 49619) was studied in six groups of rats. Basal pneumococcal adherence was checked after 1 h both at room temperature and in a cold environment ( at 10 degrees C) for the evaluation of changes in basal flora in the first two groups ( basal control). In the remaining four groups, the effect of cold treatment before or after inoculation of S. pneumoniae, or cold treatment during the whole study period was investigated. Results: The adherence of inoculated pneumococci was found to be significantly decreased in rats exposed to cold air during the whole study period ( group 3). However, the decrease was not significant in rats exposed to cold air after bacterial inoculation ( group 6). In contrast, pneumococcal adherence was found to be significantly increased in rats staying at 24 degrees C room temperature ( control, group 5), during the whole pre- and postincubation period. A slight increase was also detected in rats pretreated in cold air and then kept at room temperature ( group 4) ( p >0.05). Conclusion: In contrast to our expectations, pneumococcal adherence was found to be decreased in cold environment in vivo. Copyright (C) 2006 S. Karger AG, Basel.