Impact of dirithromycin on the normal oral and intestinal microflora.

Twenty healthy volunteers received 500 mg of dirithromycin orally once daily for seven days. The concentrations of dirithromycin in serum and saliva were low (less than or equal to 1.5 mg/l), while the faecal concentrations were high (greater than or equal to 12 mg/kg). The numbers of streptococci, Haemophilus and Neisseria increased in the aerobic oral microflora during dirithromycin treatment. In the aerobic intestinal microflora, the numbers of enterobacteria decreased significantly, while streptococci and staphylococci increased. New colonizing dirithromycin resistant enterobacteria were isolated during and after treatment. The anaerobic intestinal microflora was also affected; thus the numbers of gram-positive cocci, bifidobacteria, eubacteria and bacteroides decreased, while the numbers of clostridia and lactobacilli increased. Dirithromycin has an ecological impact on the oral and intestinal microflora.