Changes in Aerobic Pharyngeal Flora Related to Antibiotic Use and the Emergence of Gram-negative Bacilli

Abstract

Pharyngeal flora were evaluated semi-quantitatively by counting and differentiating the colonies growing on blood agar plates into four major bacterial groups. In children they consisted mainly of α-streptococci, followed by smaller portions of Neisseria, and of gram-positive organisms other than α-streptococci. Gram-negative bacilli did not occur under normal conditions.

Oral ampicillin in a daily dose of 50 mg/kg caused suppression of the α-streptococci, and promoted emergence of gram-negative bacilli. Oral cephalexin in the same dose caused changes in the same direction, but less extensively than ampicillin. Parenteral aminoglycoside antibiotics in usual dosage did not change the pharyngeal flora.

Differential counting of the growth is a simple and useful tool to evaluate the ecology of pharyngeal flora. It serves to monitor overgrowth of resistant organisms in the respiratory tract.