A new alkaline pH-adjusted medium enhances detection of beta-hemolytic streptococci by minimizing bacterial interference due to Streptococcus salivarius

A new selective medium (CNA-P) that reduces or eliminates the inhibitory ac tivity of bacteriocin-producing Streptococcus salivarius against beta-hemol ytic streptococci has been developed and compared with sheep blood agar (SB A) for the sensitive detection of small numbers of beta-hemolytic streptoco cci in clinical: specimens. CNA-P has as its basis a commercial medium (Dif co Columbia CNA agar) supplemented with 5% (vol/vol) sheep blood, and the C NA is further modified by addition of 100 mM PIPES buffer [piperazine-N,N'- bis(2-ethanesulfonic acid)] (pH 7.5) to maintain cultures at an alkaline pH during incubation. CNA-P was shown to inhibit the production and/or releas e of four different types of S. salivarius bacteriocins or bacteriocin-like inhibitory molecules. The efficacies of CNA-P and SEA for detection of bet a-hemolytic streptococci in 1,352 pharyngeal samples from 376 children were compared. The beta-hemolytic streptococcal isolates recovered from the sam ples included 314 group A (S. pyogenes), 61 group G, 33 group B, and 5 grou p C streptococci. Of 314 samples that yielded S. pyogenes, 300 were positiv e on CNA-P (96%) and 264 (86%) were positive on SEA. A significantly greate r number of S. pyogenes isolates from these samples were recovered only on CNA-P (50 of 314) compared with the number of isolates recovered only on SE A (14 of 314). In addition, the degree of positivity, a measure of the tota l numbers of S. pyogenes isolates on the plate, was significantly higher on CNA-P than on SEA (2.40 versus 2.07; P < 0.001). Interestingly, CNA-P was also found to enhance the hemolytic activity of streptolysin O, allo,ring d etection of streptolysin S-deficient S. pyogenes strains which might otherw ise go undetected on SEA and other isolation media.