Susceptibility of <i>Streptococcus mutans </i>and <i>Streptococcus sobrinus</i> to Cell Wall Inhibitors and Development of a Novel Selective Medium for <i>S. sobrinus</i>

Abstract

Representative strains of <i>Streptococcus mutans</i> and <i>Streptococcus sobrinus </i>showed differences in susceptibility to members of the monobactam group of β-lactam antibiotics: <i>S. sobrinus</i> was less sensitive than <i>S. mutans</i>. The minimum inhibitory concentrations of aztreonam (AZT) and carumonam, both of which belong to this group, were 2,000 µg/ml for <i>S. sobrinus</i> and 125 µg/ml for <i>S. mutans</i>. Further addition of fosfomycin, bacitracin and sodium chloride to Mitis Salivarius agar (MS) supplemented with AZT resulted in growth inhibition of <i>S. mutans</i> and oral streptococci other than <i>S. sobrinus,</i> and was therefore used as a selective medium for <i>S. sobrinus</i> (MS-SOB medium). The average growth recovery of laboratory and clinically isolated strains of <i>S. sobrinus</i> on MS-SOB medium was 74.1% compared to that on MS medium. Seventy-eight percent of clinical samples in which <i>S. sobrinus</i> was detected yielded pure growth of <i>S. sobrinus </i>on MS-SOB medium.