Invasion of a host colonized with another species. Established populations were inoculated into groups of 10-22 three-day-old neonatal rats 48 hours prior to pulsing 105 cfu of a different species or PBS. The 25th to 75th percentiles of nasal wash and epithelium samples taken 48 hours after bacterial challenge are represented by the box plots, with the bold horizontal bar indicating the median value, circles outlying values and dotted error bars. T-test P values < 0.005 are represented by **. Resident bacterial density was not significantly different from un-invaded rats in any combination of species.