Identification of Neisseria gonorrhoeae with the ORTHOProbe DNA probe test

Gonorrhoea diagnostics: An update

Diagnosis of gonorrhoea is an ongoing challenge. The organism is fastidious requiring meticulous collection and transport for successful cultivation. Asymptomatic infections are common which go undetected by conventional methods thereby leading to continued transmission and the risk of complications. The nucleic acid amplification tests, now increasingly used in developed countries, offer improved sensitivity compared to bacterial culture. However, these continue to suffer sequence related problems leading to false positive and false negative results. Further, these cannot be used for generation of data on antibiotic susceptibility because genetic markers of antibiotic resistance to recommended therapies have not been fully characterised. They are unaffordable in a setting like ours where reliance is placed on syndromic approach for sexually transmitted infection (STI) management. The use of syndromic approach has resulted in a considerable decline in the number of Neisseria gonorrhoeae isolates that have been cultured for diagnostic purposes. Many laboratories formerly doing so are no longer performing culture for gonococci, and the basic skills have been lost. There is a need to not only revive this skill but also adopt newer technologies that can aid in accurate diagnosis in a cost-effective manner. There is room for innovation that can facilitate the development of a point-of-care test for this bacterial STI.

Comparative evaluation of AccuProbe culture identification test for Neisseria gonorrhoeae and other rapid methods

The AccuProbe chemiluminescent culture identification test for Neisseria gonorrhoeae (Gen-Probe Inc., San Diego, Calif.) was assessed in a comparative evaluation with other rapid methods by using 269 isolates of oxidase-positive, gram-negative diplococci. Chemiluminescence was read with a PAL luminometer, and results were expressed as PAL light units (PLUs): the cutoff value for a positive identification was 1,500 PLUs. All 200 isolates of gonococci confirmed by carbohydrate utilization and serotyped with monoclonal antibodies were identified correctly by AccuProbe on initial testing. The API Quadferm system (Bio Merieux, Marcy l'Etoile, France) identified 95% (n = 190) of the gonococci correctly on initial testing and 99.5% (n = 199) on repeat testing, while the Phadebact Monoclonal GC test (Kara Bio Diagnostics AB, Huddinge, Sweden) identified 95.5% (n = 191) of the gonococci on both initial and repeat testing; 8 of the Phadebact-negative isolates were all of the same rare serovar (serovar 1B-17). The mean PLU for the gonococcal isolates was 9,014 (range 2,264 to 10,845) compared with a mean of 51 (range, 8 to 109) for the 69 nongonococcal isolates. We conclude that the AccuProbe culture confirmation test provides a rapid and accurate objective means of identifying cultured N. gonorrhoeae isolates.

Comparison of monoclonal antibody methods and a ribosomal ribonucleic acid probe test for Neisseria gonorrhoeae culture confirmation

Recently, a chemiluminescent nucleic acid probe test that specifically detects the ribosomal ribonucleic acid of Neisseria gonorrhoeae has been released for clinical laboratory use (AccuProbe Neisseria gonorrhoeae). In this study, three coagglutination tests (GonoGen I, Meritec GC, and GC Omni), the GonoGen II immunofiltration method and the Micro Trak Neisseria gonorrhoeae fluorescent monoclonal antibody test were compared with AccuProbe for identification of gonococci. Strains tested (n = 376) included 194 Neisseria gonorrhoeae, 82 Neisseria meningitidis, 32 Neisseria lactamica, 32 Neisseria species, 32 Moraxella catarrhalis, 2 Moraxella spp. and 2 Kingella denitrificans. The GonoGen I, Meritec GC and GC Omni coagglutination tests produced clearly positive results for 93.8%, 92.3% and 95.9% of the gonococci, respectively. The GonoGen II unequivocally identified 91.8% and the MicroTrak fluorescent antibody test identified 90.7% with 2+ or greater fluorescence. AccuProbe identified 100% of the gonococci tested. GonoGen I and GonoGen II were 98% specific, Meritec GC was 99% specific and the specificity of the GC Omni, MicroTrak fluorescent antibody and AccuProbe tests was 100%. While antibody-based tests were reliable when results were clearly interpretable, the AccuProbe was the only confirmatory test that was 100% accurate. Serotyping studies indicate that an array of beta-lactamase positive and negative gonococcal serotypes fail to react with the monoclonal antibody-based tests in general and with the fluorescent antibody test in particular.

Evaluation of a urease-based confirmatory enzyme-linked immunosorbent assay for diagnosis of Neisseria gonorrhoeae

A new urease-based enzyme-linked immunosorbent assay utilizing novel monoclonal antibodies was evaluated for the culture confirmation of Neisseria gonorrhoeae, with 270 isolates of N. gonorrhoeae, 56 isolates of diverse Neisseria spp., and 29 Moraxella isolates. The test was highly specific (100.00%) and sensitive (97.83%). No cross-reactions were observed with any of the Neisseria or Moraxella isolates tested. Fifty percent (3 of 6) of the false-negative results were obtained with isolates of serovar IA-4, a serovar rarely encountered in North America.

A DNA sequence for the discrimination of Neisseria gonorrhoeae from other Neisseria species

A 350 base pair Neisseria gonorrhoeae DNA restriction fragment was cloned after subtractive hybridization to Neisseria meningitidis DNA. This restriction fragment hybridized to 105 out of 106 N. gonorrhoeae strains tested. While three N. meningitidis strains did not hybridize to this probe, Neisseria mucosa DNA exhibited cross-hybridization. This particular clone was used to screen a N. gonorrhoeae genomic DNA library. A positive 2.4 kilobase pair clone was shown by DNA sequencing to contain two long open reading frames. One open reading frame did not hybridize to N. mucosa and other Neisseria species, while it retained specificity for the original 105 N gonorrhoeae strains. This open reading frame also showed significant homology to cytosine DNA methyltransferases.

Laboratory methods in genitourinary medicine. Methods of diagnosing gonorrhoea

Gonorrhoea is normally diagnosed presumptively by the presence of intracellular Gram-negative cocci on a Gram stain and confirmed by culture of the causative organism, Neisseria gonorrhoeae. Alternative methods have been evaluated extensively for the detection of gonococci in clinical specimens including immunological techniques such as ELISA and immunofluorescence, DNA probes, genetic transformation and the limulus lysate assay. Some of these tests have proved as sensitive and specific for the detection of gonorrhoea in symptomatic men as the Gram stain but offer no advantage in time or cost. In women, no test has been found that shows a sensitivity and specificity sufficiently adequate for clinical use. Culture in men and women remains the method of choice for diagnosis. In addition the need to obtain the infecting organism for antibiotic susceptibility testing has not been overcome. In contrast, the rapid identification of N gonorrhoeae can be achieved within four hours using either monoclonal antibodies or by the detection of preformed enzymes. New methods for both the detection and identification of N gonorrhoeae should be carefully evaluated particularly for use in cases of child and sexual abuse where medico-legal problems may arise.