Streptococcus pneumoniae in the nasal cavity of mice causes lower respiratory tract infection after airway obstruction

A model of persistent colonisation in the nasal cavity of mice by Streptococcus pneumoniae has been established. S. pneumoniae NNP-4 was introduced to the lungs of CBA/J mice at a density of c. 10(4) cfu/lung by an aerosol method and a high dose of ampicillin was administered 1 h after infection. This antibiotic eliminated bacteria from the lungs and trachea, but did not affect the bacterial counts in the nasal cavity. In mice given ampicillin, the bacteria were recovered from the nasal cavity only more than 2 weeks after infection, but IgG antibody against the colonising organisms was produced in sera around day 8 after infection. Airway obstruction was induced by intratracheal injection of formalin 2% into mice. Organisms appeared in the lungs in greater numbers when formalin was injected before the antibody production than when the immunity was established. In the early stages of infection, 10(3)-10(4) cfu appeared in the lungs 6 h after the formalin injection and the bacterial counts increased to c. 10(6) cfu within 24 h. When ampicillin was administered again 1 h after formalin was given, no bacteria were recovered from lungs 6 h later. However, in some of the mice given ampicillin after formalin, bacteria appeared in the lungs on the next day and the bacterial counts increased thereafter. These results suggest that S. pneumoniae in the nasal cavity invade the lower respiratory tract and that these organisms can localise and proliferate in lungs in the event of damage to the airway.