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 activity of bacteriocin-producing Streptococcus salivarius against beta-hemolytic streptococci has been developed and compared with sheep blood agar (SBA) for the sensitive detection of small numbers of beta-hemolytic streptococci in clinical specimens. CNA-P has as its basis a commercial medium (Difco Columbia CNA agar) supplemented with 5% (vol/vol) sheep blood, and the CNA 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 release of four different types of S. salivarius bacteriocins or bacteriocin-like inhibitory molecules. The efficacies of CNA-P and SBA for detection of beta-hemolytic streptococci in 1,352 pharyngeal samples from 376 children were compared. The beta-hemolytic streptococcal isolates recovered from the samples included 314 group A (S. pyogenes), 61 group G, 33 group B, and 5 group C streptococci. Of 314 samples that yielded S. pyogenes, 300 were positive on CNA-P (96%) and 264 (86%) were positive on SBA. A significantly greater 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 SBA (14 of 314). In addition, the degree of positivity, a measure of the total numbers of S. pyogenes isolates on the plate, was significantly higher on CNA-P than on SBA (2.40 versus 2.07; P < 0.001). Interestingly, CNA-P was also found to enhance the hemolytic activity of streptolysin O, allowing detection of streptolysin S-deficient S. pyogenes strains which might otherwise go undetected on SBA and other isolation media.
Dierksen
K P
KP
Department of Microbiology, University of Otago, Dunedin, New Zealand.
Ragland
N L
NL
Tagg
J R
JR
D000362:Agar; D000898:Antibiosis; D001431:Bacteriological Techniques; D001769:Blood; D002021:Buffers; D002648:Child; D002675:Child, Preschool; D003470:Culture Media; D006801:Humans; D006863:Hydrogen-Ion Concentration; D010612:Pharyngitis / Q000175:diagnosis / Q000382:microbiology; D010614:Pharynx / Q000382:microbiology; D013290:Streptococcal Infections / Q000175:diagnosis* / Q000382:microbiology; D013291:Streptococcus / Q000254:growth & development / Q000302:isolation & purification* / Q000502:physiology*; D013297:Streptococcus pyogenes / Q000254:growth & development / Q000302:isolation & purification*; D014059:Tongue / Q000382:microbiology
D002021:Buffers; D003470:Culture Media; D000362:Agar