NAS agar is more suitable than McKay agar for primary culture of Streptococcus milleri group (SMG) fastidious bacteria, S. intermedius in particular

Abstract

Streptococcus milleri group (SMG) is a group of three streptococcal species (S. anginosus, intermedius and constellatus) that act as opportunist pathogens, among others in cystic fibrosis. Due to their fastidious character, they are both difficult to cultivate and to differentiate from less pathogenic streptococcal species, therefore being most probably underdiagnosed. Semi-selective McKay agar and NAS agar were developed to facilitate SMG recovery from clinical samples; however, direct comparison of recovery rates has not been published yet. We tested the performance of both media on 123 patient samples and demonstrated general superiority of NAS agar for SMG recovery during primary cultivation convincingly. This observation was also confirmed by quantitative drop tests during subculture. Despite the undisputed overall superiority of NAS agar over McKay agar, a smaller fraction of strains grew better on McKay agar. Inter-strain differences were the most probable explanation. Therefore, when economic conditions are not limiting and maximum recovery rate is desirable, both plates are advised to be used in parallel for primary cultivation of clinical samples.

Autoaggregation inStreptococcus intermediusis driven by the Pel polysaccharide

“…The Streptococcus milleri group (SMG) is a part of this larger group and consists of Streptococcus intermedius , Streptococcus constellatus and Streptococcus anginosus . These species are normally considered commensal bacteria that commonly reside in the oral, gastrointestinal, and urogenital tracts of healthy individuals (37) but with the advent of better detection and diagnostic techniques, their relevance as opportunistic pathogens is increasingly being recognized (38, 39). These bacteria cause purulent infections at sterile body sites (40, 41) and were found to be the most common cause of invasive pyogenic streptococcal infections in the Calgary Health Region between 1999 and 2004 (42).…”