Human oral defenses against gram-negative rods

Abstract

Gram-negative rods are unusual pharyngeal isolates in normal man, which suggests the presence of effective oral defense mechanisms against these organisms. To understand this protection better, we studied the elimination of gram-negative rods from the mouth after gargle challenge. Suspensions of Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis (108 organisms per ml) were gargled by normal volunteers, and pharyngeal swabs for serial quantitative cultures were taken for 3 hours. Initial swabs yielded approximately 10(5) organisms. Bacterial counts rapidly decreased, and less than 1 per cent of the original inoculum was still recoverable 3 hours later. Studies of tongue, pharynx, and buccal mucosa samples showed the most rapid decrease in buccal mucosa samples and the slowest decrease in tongue swabs. To evaluate physical clearance, gargle challenges were performed using E. coli radiolabeled with technetium-99m. External counts of the oral cavity decreased rapidly, with 21.8, 14.7, 8.9, and 7.8 percent of the initial counts remaining at 30, 60, 120, and 180 min, respectively. Nonpiliated E. coli adhered poorly to buccal epithelial cells in vitro, in contrast to good adherence when piliated strains were tested. Despite these findings, no differences in radiolabel clearance was noted in vivo using piliated and nonpiliated E. coli for gargle challenges. Normal oral defenses against gram-negative rods are highly effective and seem to involve both physical clearance and local bactericidal activity.