Evaluation of a rapid method for the detection of streptococcal group A antigen directly from throat swabs

Development of species-specific primers for detection of Streptococcus pyogenes from throat swabs

A species-specific molecular marker has been developed to detect the human pathogen Streptococcus pyogenes from throat swabs. Streptococcus pyogenes is an important pathogen among Gram-positive organisms. A rapid and simple diagnostic tool is of utmost importance for the identification of this pathogen. The random amplified polymorphic DNA (RAPD) technique was used to differentiate the S. pyogenes strains. A differentially amplified fragment obtained from RAPD profiles was sequenced and characterized, which was developed into a sequence characterized amplified region (SCAR) marker to evaluate the specificity of S. pyogenes from other species of Streptococcus. The sensitivity of the SCAR primers was studied by qualitative PCR and the detection limit was found to be 10(-1) ng of genomic DNA or one to two cells of S. pyogenes. The specificity of the primers was assayed in 270 clinical throat swabs wherein 23 samples turned to be positive, which was highly significant over culture-based methods. This species-specific primer enables accurate detection of S. pyogenes from clinical samples and will be a useful tool in epidemiological studies.

Cell wall carbohydrate compositions of strains from the Bacillus cereus group of species correlate with phylogenetic relatedness

Members of the Bacillus cereus group contain cell wall carbohydrates that vary in their glycosyl compositions. Recent multilocus sequence typing (MLST) refined the relatedness of B. cereus group members by separating them into clades and lineages. Based on MLST, we selected several B. anthracis, B. cereus, and B. thuringiensis strains and compared their cell wall carbohydrates. The cell walls of different B. anthracis strains (clade 1/Anthracis) were composed of glucose (Glc), galactose (Gal), N-acetyl mannosamine (ManNAc), and N-acetylglucosamine (GlcNAc). In contrast, the cell walls from clade 2 strains (B. cereus type strain ATCC 14579 and B. thuringiensis strains) lacked Gal and contained N-acetylgalactosamine (GalNAc). The B. cereus clade 1 strains had cell walls that were similar in composition to B. anthracis in that they all contained Gal. However, the cell walls from some clade 1 strains also contained GalNAc, which was not present in B. anthracis cell walls. Three recently identified clade 1 strains of B. cereus that caused severe pneumonia, i.e., strains 03BB102, 03BB87, and G9241, had cell wall compositions that closely resembled those of the B. anthracis strains. It was also observed that B. anthracis strains cell wall glycosyl compositions differed from one another in a plasmid-dependent manner. When plasmid pXO2 was absent, the ManNAc/Gal ratio decreased, while the Glc/Gal ratio increased. Also, deletion of atxA, a global regulatory gene, from a pXO2- strain resulted in cell walls with an even greater level of Glc.

Doubly branched hexasaccharide epitope on the cell wall polysaccharide of group A streptococci recognized by human and rabbit antisera

A number of epitope specificities associated with the cell wall polysaccharide antigen of group A streptococci were identified in a polyclonal rabbit antiserum induced in rabbits by whole group A streptococci and in polyclonal convalescent human antisera from children that had recovered from streptococcal A infections. The identification was achieved by using a series of synthetic oligosaccharides, glycoconjugates, and bacterial polysaccharide inhibitors to inhibit the binding of the group A helical polysaccharide to the polyclonal antisera. The exclusively dominant epitope expressed in the convalescent human antisera was the doubly branched extended helical hexasaccharide with the structure alpha-L-Rhap(1-->2)[beta-D-GlcpNAc(1-->3)]alpha-L-Rhap(1-->3)alpha-L-Rhap(1-->2)[beta-D-GlcpNAc(1-->3)]alpha-L-Rhap. The hexasaccharide epitope also bound with the highest immunoreactivity to the rabbit antiserum. In contrast, the human antisera did not show significant binding to the singly branched pentasaccharide with the structure alpha-L-Rhap(1-->2)alpha-L-Rhap(1-->3)alpha-L-Rhap(1-->2)[beta-D-GlcpNAc(1-->3)]alpha-L-Rhap or the branched trisaccharide alpha-L-Rhap(1-->2)[beta-D-GlcpNAc(1-->3)]alpha-l-Rhap, although both these haptens bound significantly to the same rabbit antiserum, albeit with less immunoreactivity than the hexasaccharide. Inhibition studies using streptococcal group A and B rabbit antisera and the inhibitors indicated above also suggested that the group A carbohydrate, unlike the group B streptococcal polysaccharide, does not contain the disaccharide alpha-L-Rhap(1-->2)alpha-L-Rhap motif at its nonreducing chain terminus, stressing the importance of mapping the determinant specificities of these two important streptococcal subcapsular group polysaccharides to fully understand the serological relationships between group A and group B streptococci.

Evaluation of the patient with sore throat, earache, and sinusitis: an evidence based approach

Sore throat, earache, and sinusitis are common presenting complaints in the emergency department, and all fall within the top ten in the United States. These complaints usually have a benign course but rarely can be a symptom of a serious disease process. This article provides an evidence-based review of the literature regarding the diagnosis of pharyngitis, otitis media, and sinusitis.

Comparison of TestPack Plus Strep A with selective and nonselective culture media for detection of group-A streptococci

The TestPack Plus Strep A (TPPSA), an immunoassay method, was compared with conventional culture methods including nonselective trypticase soy agar with 5% sheep blood and selective SXT blood agar for detection of group-A streptococci (GAS). A total of 452 throat swabs was evaluated, of which 261 (57.7%) and 191 (42.3%) were compared with culture using nonselective and selective media, respectively. Of 261 specimens evaluated on nonselective media, 71 (27.1%) were culture positive for GAS. TPPSA demonstrated a sensitivity of 91.6% and a specificity of 94.2% with positive and negative predictive values of 85.5% and 96.8%, respectively. Of 191 specimens evaluated on selective media, 44 (23.0%) were culture positive for GAS. TPPSA demonstrated a sensitivity of 93.2% and a specificity of 98.0% with positive and negative predictive values of 93.2% and 98.0%, respectively. The performance of TPPSA when compared with nonselective and selective media demonstrated a similar sensitivity, but a higher specificity was seen when compared to selective media. Overall, TPPSA was extremely easy to perform, had built-in procedural controls, required minimal technologist time, and was easy to interpret. With an accuracy of 93.5% when compared with nonselective media and 96.9% when compared with selective media, TPPSA could be recommended as a reliable method for detection of GAS.

Managing sore throat: theory versus practice

The authors address the reliability of clinical impression in the management of sore throat. Five primary care family physicians in rural Ontario examined 222 patients with sore throats. The clinical impression of either Group A β-hemolytic streptococcal (GABHS) or non-GABHS pharyngitis was recorded and throat cultures were ordered in each case. The physicians predicted that 50% of the cultures would be positive for GABHS, whereas only 13.5% actually had positive results. The initial clinical diagnosis of "strep throat" was correct only one in five times. Without cultures, at least 112 patients would have been treated with antibiotics, 87 unnecessarily. The authors conclude that the clinical prediction of GABHS is inaccurate and can lead to unnecessary use of antibiotics.

Performance of an enzyme immunoassay test and anaerobic culture for detection of group A streptococci in a pediatric practice versus a hospital laboratory

The ability of pediatricians and hospital laboratory personnel to detect group A streptococci in patients with suspected streptococcal pharyngitis was evaluated using the TestPack Strep A and anaerobic culture. Duplicate throat specimens (for similar processing by both the pediatricians and laboratory technologists) were simultaneously collected on rayon-tipped swabs from patients with symptoms of pharyngitis. Each swab was first inoculated to a 5% sheep blood agar plate, then tested for group A streptococcus antigen using the TestPack Strep A according to the manufacturer's instructions. Cultures were incubated anaerobically at 35 degrees C for 2 nights unless positive after 1 night. Group A streptococci were identified using specific antisera. Pediatric office or laboratory cultures from 112 (31.3%) of the 358 patients contained group A streptococci. Of the patients with positive cultures, 96 (85.7%) and 107 (95.5%) were detected by the pediatricians and laboratory, respectively. Respective findings with the TestPack Strep A by the pediatricians and laboratory were sensitivity 68.8% and 74.8%, specificity 94.3% and 95.6%, predictive value of a positive result 81.5% and 87.9%, and predictive value of a negative result 89.2% and 89.9%. Anaerobic culture was significantly more sensitive than the TestPack Strep A for detection of group A streptococci by both the pediatricians (P less than 0.005) and laboratory personnel (P less than 0.05).

Rapid detection methods in microbiology: are they right for your office?

This article discusses some of the direct test methods for diagnosing Group A streptococcal pharyngitis, urinary tract infection, and the three most common sexually transmitted diseases: gonorrhea, Chlamydia, and Herpes simplex virus infection. Emphasis is placed on their practicality for the office setting as well as the limitations of the test methods in various patient populations.

Detection of group A streptococcal antigen from throat swabs by use of a latex agglutination test kit in general practice

In 11 general practice offices, with a total of 29 general practitioners, throat swabs from 468 patients with acute pharyngotonsillitis were assayed for group A streptococci with a commercial antigen detection test kit (Culturette Brand Ten-Minute Group A Strep ID; Marion Scientific, Div. of Marion Laboratories, Inc., Kansas City, MO 64114, USA). Compared to aerobic cultures carried out in our laboratory, the sensitivity of the antigen detection test was 73%; the specificity, 98%; the positive predictive value, 96%; the negative predictive value, 84%; and the overall agreement with laboratory findings, 88%. The prevalence of group A streptococci was 42%. Tests performed by assistants were significantly less sensitive (52%) than tests carried out by the physicians themselves (77%). The sensitivity of office cultures was 83% for blood agar plates supplemented with bacitracin discs, and 77% for Streptocult. We conclude that antigen detection tests should not presently be substituted for office cultures in the diagnosis of group A streptococcal throat infections.

Specificity study of kits for detection of group A streptococci directly from throat swabs

A total of 78 Streptococcus strains, 15 Staphylococcus strains, and 2 Stomatococcus strains were used to test the specificity of 18 different antigen detection systems for group A streptococci and five products that detect a specific enzyme associated with group A streptococci. All streptococcal strains possessing the group A antigen were correctly identified with 31 different lots of reagents in the 18 antigen detection systems. The specificities of the 31 different lots of reagents ranged between 88.5 and 100%. A limited number of nonspecific reactions were observed with Enterococcus gallinarium, group C Streptococcus strain C23, and Staphylococcus aureus F49 and Cowan 1. The antigen detection kits that used enzymes as the extraction reagent gave slightly more specific results than did the kits that used chemical extraction reagents. The reagents in the five kits designed to detect the enzyme pyroglutamic acid arylamidase in Streptococcus pyogenes reacted positively with S. pyogenes (group A streptococcus); however, the reagents also reacted positively with all group D enterococcal streptococci and with about half of the staphylococcal strains treated. The nonspecificity of tests based on pyroglutamic acid arylamidase detection would seem to limit the usefulness of these kits with mixed cultures.

Comparison of TestPack Strep A test kit with culture technique for detection of group A streptococci

Results obtained with Abbott Laboratories TestPack Strep A (TPSA), a 7-min enzyme immunoassay method, were compared with culture results to measure the ability of this assay to detect group A streptococci directly from 365 throat swabs. Our study demonstrated a sensitivity of 90.0% and a specificity of 97.4% for TPSA compared with cultures incubated for 48 h. The positive and negative predictive values of this assay versus the culture method were 92.8 and 96.3%, respectively. If specimens that provided fewer than 10 colonies per plate of group A streptococci are eliminated from the data, the sensitivity is increased to 95.6%. Additionally, 10 group A and 40 non-group A streptococcal isolates were tested directly with TPSA for the ability to distinguish group A from non-group A streptococci. All 50 isolates were correctly identified (100% accuracy). TPSA is a rapid, accurate, and easy-to-interpret method for detection and confirmation of group A streptococcal antigen directly from throat swabs and pure culture isolates.

Comparative evaluation of PathoDx Strep A test and culture for the detection of group A streptococci in pharyngeal specimens

The PathoDx Strep A kit, a 10-min acid extraction and latex agglutination test, was compared with routine culture for the direct detection of group A beta-hemolytic streptococci (BHS) in 414 pharyngeal specimens collected from children with pharyngitis. The results showed that the latex test compared favorably with culture for detecting group A BHS in pharyngeal specimens (sensitivity 96.7%, specificity 97.9%, and positive and negative predictive values of 97.2% and 97.4%, respectively). The comparable number of false-positive (five) and false-negative (six) latex tests along with review of patient histories suggest that these discrepant results were attributable to sampling error during specimen procurement rather than deficiencies in the latex kit. In addition, clear-cut, agglutination reactions were obtained in over 96% of positive latex tests regardless of the amount of group A BHS that was recovered by culture. The PathoDx Strep A test is a rapid, reliable noncultural alternative for the detection of group A BHS in pharyngeal specimens.

Role of screening tests in rapid testing

Screening tests have had a major role in the diagnostic microbiology laboratory during recent years. Many factors have contributed to the increased use of these methods. The primary factor is the need for rapid detection and identification of the etiologic agent in life-threatening infections, which include bacterial meningitis, infective endocarditis and septicemia, and pneumonia. Various direct screening methods are also available for eliminating negative specimens rapidly. These methods have had an impact on laboratory work flow and the outpatient population by eliminating the possibility of bacterial infections of the urinary, upper respiratory, and gastrointestinal tracts. Screening methods have also been applied to isolated colonies for the rapid differentiation and identification of certain groups of organisms. Finally, many of these screening methods have contributed to cost-savings for both the laboratory and the patient. In this era of prospective pricing, and until the entrance of biotechnology into the diagnostic microbiology laboratory, the microbiologist must rely upon these rapid, inexpensive screening procedures. The role of these screening tests in the diagnostic microbiology laboratory and their impact on patient care and cost are reviewed herein.

Comparison of a latex agglutination test and four culture methods for identification of group A streptococci in a pediatric office laboratory

A latex agglutination test and four culture methods for the detection of group A hemolytic streptococci were compared in a pediatric office laboratory. The anaerobic (GasPak) and Detekta-Kit methods produced the highest recovery rates, but aerobic incubation of 5% blood agar plates gave very acceptable results (sensitivity 92%, or 98% if the 1+ positive cultures were eliminated; specificity 100%), and had fewer disadvantages. Delaying inoculation of plates for 2 to 6 hours resulted in significantly greater numbers of strongly positive (3+ and 4+) cultures. The group A selective sheep blood agar media as used in this study offered no advantage. The Culturette test had a sensitivity of 72% and specificity of 98%, compared with the GasPak method, and 77% and 97%, respectively, compared with standard aerobic cultures. Thus it appears too insensitive to be used alone, but might complement culture methods in selected patients.