The determinants of the lung clearance of Streptococcus pneumoniae, Klebsiella pneumoniae, Escherichia coli, and Staphylococcus aureus were studied in normal mice after exposure to an aerosol of viable bacteria and 99Tc(m) labeled dead bacteria. The fraction of bacteria in the lungs that remained viable 4 h after exposure were: S. pneumoniae, 7.3%; K. pneumoniae, 121%; E. coli, 88.5%; S. aureus, 27.6%. The rate of physical removal of bacterial particles (kmc) was determined from the change in lung 99Tc(m) counts with time: kmc ranged between 7 and 12%/h and was similar in all species. The rate of mucociliarly clearance and of intrapulmonary bacterial killing (kk + kmc) was calculated from the change in bacterial counts with time in animals that had received tetracycline to inhibit bacterial multiplication. kk, the rate of intrapulmonary killing, was obtained by subtraction of kmc from (kk + kmc). The calculated values for kk were: S. pneumoniae, -87%/h; K. pneumoniae, -17%/h; E. coli, -18%/h; S. aureus, -22%/h. The rate of intrapulmonary bacterial multiplication (kg) was estimated from the relationship of bacterial counts in tetracycline and nontetracycline treated animals, assuming that tetracycline altered only kg. kg, expressed as the doubling time, was: S. pneumoniae, 310 min; K. pneumoniae, 217 min; E. coli, 212 min; S. aureus, no multiplication. The data indicate that the marked differences in the clearance of these species from the normal mouse lung result from the interaction of differing rates of in vivo bacterial multiplication and killing.
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