Real-time PCR assay targeting IS481 of Bordetella pertussis and molecular basis for detecting Bordetella holmesii

Real-time PCR in clinical microbiology: applications for routine laboratory testing

Real-time PCR has revolutionized the way clinical microbiology laboratories diagnose many human microbial infections. This testing method combines PCR chemistry with fluorescent probe detection of amplified product in the same reaction vessel. In general, both PCR and amplified product detection are completed in an hour or less, which is considerably faster than conventional PCR detection methods. Real-time PCR assays provide sensitivity and specificity equivalent to that of conventional PCR combined with Southern blot analysis, and since amplification and detection steps are performed in the same closed vessel, the risk of releasing amplified nucleic acids into the environment is negligible. The combination of excellent sensitivity and specificity, low contamination risk, and speed has made real-time PCR technology an appealing alternative to culture- or immunoassay-based testing methods for diagnosing many infectious diseases. This review focuses on the application of real-time PCR in the clinical microbiology laboratory.

A multiplex PCR for detection of Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila, and Bordetella pertussis in clinical specimens

A multiplex PCR was developed that is capable of detecting four of the most important bacterial agents of atypical pneumonia, Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila, and Bordetella pertussis in uncultured patient specimens. These organisms cause similar symptomologies and are often not diagnosed because they are difficult to identify with classical methods such as culture and serology. Given this, the overall impact of these pathogens on public health may be grossly underestimated. The molecular test presented here provides a simple method for identification of four common, yet diagnostically challenging, pathogens.

Molecular pathogenesis, epidemiology, and clinical manifestations of respiratory infections due to Bordetella pertussis and other Bordetella subspecies

Bordetella respiratory infections are common in people (B. pertussis) and in animals (B. bronchiseptica). During the last two decades, much has been learned about the virulence determinants, pathogenesis, and immunity of Bordetella. Clinically, the full spectrum of disease due to B. pertussis infection is now understood, and infections in adolescents and adults are recognized as the reservoir for cyclic outbreaks of disease. DTaP vaccines, which are less reactogenic than DTP vaccines, are now in general use in many developed countries, and it is expected that the expansion of their use to adolescents and adults will have a significant impact on reducing pertussis and perhaps decrease the circulation of B. pertussis. Future studies should seek to determine the cause of the unique cough which is associated with Bordetella respiratory infections. It is also hoped that data gathered from molecular Bordetella research will lead to a new generation of DTaP vaccines which provide greater efficacy than is provided by today's vaccines.

External quality assessment for molecular detection of Bordetella pertussis in European laboratories

Although the PCR for the detection of Bordetella pertussis is routinely performed in diagnostic laboratories, no quality assessment program has so far been described. We report on the results obtained with two external quality assessment proficiency panels sent to European laboratories. The first proficiency panel contained a series of dilutions of three previously characterized B. pertussis clinical isolates and two negative controls. No false-positive results were reported by six laboratories providing seven data sets. The reported limits of detection of the three B. pertussis strains varied between 4 and 4,000, 9 and 9,000, and 3 and 30,000 CFU/ml, respectively. The second proficiency panel, composed of a series of dilutions of reference strains of B. pertussis, B. holmesii, B. hinzii, and B. bronchiseptica, as well as negative controls, was sent to nine laboratories. One laboratory reported a negative result for a sample and reported a B. parapertussis-positive sample to be positive for B. pertussis. By using the B. pertussis-specific target gene pertactin, one laboratory detected B. pertussis with 100% specificity. All other laboratories, which used IS481-based assays, reported positive results for the samples containing B. holmesii and B. bronchiseptica, species that have occasionally been recovered from human respiratory samples. These data show that the choice of the target gene is particularly critical for the species specificity of B. pertussis PCR assays.

Molecular genetic methods in the diagnosis of lower respiratory tract infections

Molecular diagnostic techniques, such as PCR, have become useful tools for the rapid etiological diagnosis of lower respiratory tract infections. Nucleic acid amplification tests (NAATs) have been evaluated for detecting most respiratory pathogens, and commercial assays are available for some pathogens. However, standardized protocols are needed before these assays are introduced into routine diagnostic use. For pneumonia, NAATs offer advantages over conventional tests for the detection of Mycoplasma pneumoniae, Legionella spp. and Chlamydia pneumoniae. For pneumococcal pneumonia in adults, PCR adds little to existing diagnostic tests, and is unable to distinguish pneumococcal colonization from infection when testing respiratory samples. Although less sensitive than culture-based methods, several commercial molecular diagnostic assays have been developed for tuberculosis and are useful rapid tests for selected patients. PCR can now be considered the rapid diagnostic test of choice for pertussis and some respiratory virus infections. Further work is required to better characterize the role of molecular diagnostic tests for diagnosing lower respiratory tract infections, and to develop standard assays that can be readily adopted by routine diagnostic laboratories.

Rapid detection of Bordetella pertussis by real-time PCR using SYBR green I and a LightCycler instrument

A polymerase chain reaction (PCR) assay in real-time for detection of B. pertussis using SYBR green I as the reporter fluorophore and LightCycler instrument (a thermocycler coupled to a fluorescence detection device) was established and evaluated. The amplified amplicon using series diluted control prototype strain (ATCC strain #9797) of B. pertussis was analyzed for the fluorescent melting profile, and melting temperature (Tm) was determined. When examined, amplicons using a representative set of clinical isolates of B. pertussis were found to have the same Tm value (86 +/- 0.5 degrees C, the specificity parameter of detection) as the control prototype strain as expected. Amplified product was also analyzed and detected by agarose gel electrophoresis. The detection limit by fluorescent profile and Tm analysis was 10-fold better than that detected by agarose gel analysis.

Differential detection of B. pertussis from B. parapertussis using a polymerase chain reaction (PCR) in presence of SYBR green1 and amplicon melting analysis

Nucleic acid (DNA) from control stock strains of B. pertussis and B. parapertussis (B. pertussis strain # 9797 and B. parapertussis strain # 15234 from ATCC) was amplified by polymerase chain reaction (PCR) targeting pertussis toxin (PT) promotor region, in presence of SYBR green1 a dye that fluoresces on binding specifically to double stranded DNA; and fluorescent melting profile of amplicon (amplified DNA) was studied. Amplicon of B. pertussis and B. parapertussis generated distinctly different melting bands with melting temperature (Tm) at 89.8 and 91.7 degrees C, respectively. Melting profile and Tm of each randomly selected isolates of B. pertussis and B. parapertussis was identical to that of respective control strains. Distinct difference in Tm between B. pertussis and B. parapertussis specific amplicons allowed differential detection of the two Bordetella species based on a single PCR product. The amplified product of serial diluted control stock of bacteria was analyzed by both agarose gel electrophoresis and melting profile analysis. The analytical sensitivity of detection (1-10 CFU equivalent in the tested volume) by melting profile and Tm analysis was in agreement with that obtained by agarose gel analysis.

Comparison of culture and PCR for detection of Bordetella pertussis and Bordetella parapertussis under routine laboratory conditions

A PCR assay for the detection of Bordetella pertussis and Bordetella parapertussis was compared with the conventional culture method under routine laboratory conditions. Detection of B. pertussis was based on the amplification of a section of the IS481 insertion sequence and confirmation of positive results was based on a sequence of the pertussis toxin promoter region. Detection of B. parapertussis was based on the amplification of a section of the IS1001 insertion sequence. An internal control was included. Data were available for the period 28 November 2000 to 9 July 2003. In this period, 3096 patients were examined for infection with B. pertussis and B. parapertussis by culture and PCR on the same day. B. pertussis was found in 496 (16 %) patients; 208 (42 %) were diagnosed by PCR alone whereas 17 (3 %) were diagnosed by culture alone. B. parapertussis was found in 64 (2 %) patients. The sensitivity of the PCR was 97 % and of culture 58 %. The specificity of PCR was 93 % when regarding culture as 100 % sensitive. There was a significant relationship between laboratory method and age, as the superiority of PCR was most marked in the age group 0.5-3 years. The PCR assay proved highly sensitive for the diagnosis of pertussis. The specificity estimate of the PCR assay suffers from the influence of a gold-standard method with a low sensitivity. The PCR assay is considered highly specific due to the amplification of two different sequences in two separate assays.

Laboratory diagnosis of pertussis infections: the role of PCR and serology

This study reports on practical laboratory aspects of pertussis diagnosis. PCR assays were applied to respiratory specimens obtained during a large study of infants (less than 5 months old) admitted to paediatric intensive care units (n = 122), children (less than 15 years old) admitted to paediatric wards (n = 16) and their household contacts (n = 320). Estimation of antibodies to pertussis toxin and culture for Bordetella pertussis were attempted on specimens from the same patients, where available, and the overall utility of the diagnostic PCR assays was assessed by comparison to these results. A PCR assay for the human mitochondrial cytochrome oxidase (HMCO) gene was used for quality control of the extracted samples and an internal process control (IPC) was included in each sample to test for PCR inhibition. Four of 458 samples were considered unsuitable (three HMCO negative, one IPC negative) and excluded from further analyses. Positive PCR results were considered valid if they were either (i) positive for both of two B. pertussis gene targets (pertussis toxin S1 promoter and the insertion element IS481), i.e. consensus PCR positive, or (ii) repeatably positive in only one assay. Using these criteria, 52 of 454 (11.5 %) samples were considered as PCR positive for B. pertussis. Six of 356 samples were culture-positive for B. pertussis, 1/88 infants, 3/14 children and 2/254 contacts, giving an overall isolation rate of 1.7 %. Using these data, PCR gave an almost fivefold increase in diagnostic yield compared with culture (McNemar's test; P < 0.0001). Sera from 9/111 infants, 5/10 children and 14/210 contacts were positive. Serology and PCR results showed a high level of agreement (113/121) for infants and children. PCR demonstrated a significant improvement in diagnostic yield over culture. Serological testing also resulted in a significant increase in diagnostic yield compared to culture alone. PCR is a useful technique, but validity of results must be assured by careful control. Rapid diagnosis of B. pertussis infection particularly in infants by PCR, together with serological assays, can enhance surveillance systems for pertussis in all age groups.

Coqueluche : diagnostics biologiques

Whooping cough is a respiratory infection particularly severe for infants. The agents of the disease are the bacteria Bordetella pertussis and B. parapertussis. Before vaccination, the disease affected majority of the children. Due to the high circulation of the bacteria, adolescents and adults were regularly in contact with the bacteria and had natural boosters. Forty years after generalized vaccination in some countries, a change in the transmission of the disease is observed. Children between 2 and 5-10 years of age are not infected (depending of the vaccine used and the vaccine calendar) because they were vaccinated in their infancy. However, adolescents and adults are becoming susceptible because they do not receive any vaccinal or natural boosters. They can be infected and contaminated infants are too young to be vaccinated. Clinical symptoms in adolescents and adults, previously vaccinated or infected, are very variable and for this reason biological diagnosis are now necessary to confirm the infection. These diagnostics are culture, PCR and serology.

Evaluation of real-time PCR for detection of and discrimination between Bordetella pertussis, Bordetella parapertussis, and Bordetella holmesii for clinical diagnosis

PCR is increasingly being used as a diagnostic test for the detection of Bordetella pertussis and Bordetella parapertussis DNA, as it has improved sensitivity and specificity in comparison to conventional techniques. The assay described here uses the two insertion sequences IS481 and IS1001 for B. pertussis and B. parapertussis, respectively, with detection by molecular beacons. The real-time PCR for IS481 detects both B. pertussis and Bordetella holmesii, and the real-time PCR for IS1001 detects both B. parapertussis and B. holmesii. By performing both assays discrimination between B. pertussis and B. parapertussis can be obtained. The sensitivity was 1 to 10 CFU/ml for B. pertussis, 10 CFU/ml for B. parapertussis, and 10 CFU/ml for B. holmesii in both assays. The clinical sensitivity of the B. pertussis assay was not affected by duplexing with an internal control PCR. Real-time PCR, conventional PCR, and culture were performed on 57 clinical samples. Eight of the 57 (14%) were found positive by culture, 19 of 57 (33%) were found positive by conventional PCR, and 22 of 57 (39%) were found positive by real-time PCR. One sample was inhibitory. When the B. pertussis assay was compared with a clinical standard for B. pertussis infection, sensitivity was 38, 83, and 100% and specificity was 100, 97, and 97% for culture, conventional PCR, and real-time PCR, respectively. The real-time PCR designed for B. pertussis and B. parapertussis provides sensitive and specific diagnosis of B. pertussis and B. parapertussis infections and is therefore suitable for implementation in the diagnostic laboratory.

Comparison of real-time PCR and conventional hemi-nested PCR for the detection of Bordetella pertussis in nasopharyngeal samples

We directly compared a conventional hemi-nested PCR assay with a real-time (LightCycler) PCR assay for the detection of Bordetella pertussis in nasopharyngeal samples. Of the 152 samples tested, 49 (32%) were positive by first-round conventional PCR, 56 (37%) were positive by the hemi-nested PCR assay, and 39 (26%) were positive by the real-time assay. All samples testing positive with the real-time assay were also positive by the hemi-nested assay (both first- and second-round PCR), and all culture-positive samples tested positive by both PCR assays. These results suggest that the hemi-nested assay is more sensitive than the real-time assay for detecting B. pertussis.

Description of a multiplex Bordetella pertussis and Bordetella parapertussis LightCycler PCR assay with inhibition control

While culture for Bordetella species is highly specific, sensitivity is extremely variable due to patient age, immunization status, antibiotic treatment, and specimen transport conditions. We evaluated a real-time multiplex PCR assay as an alternative to culture for the detection and differentiation of Bordetella pertussis and Bordetella parapertussis. The PCR conditions allowed the simultaneous detection of one B. pertussis organism and five B. parapertussis organisms per reaction. An inhibition control was incorporated into the assay. Of 163 total samples evaluated, 37 of 38 samples positive by either culture or direct fluorescent antibody testing (DFA) were also positive by PCR (97% sensitivity). Of 125 culture- or DFA-negative samples, 101 were also negative by PCR (81% specificity). The described multiplex assay is a rapid, sensitive, contamination-limiting, real-time PCR assay that controls for inhibition. The assay performs well using liquid or swab samples and from dried material on slides.

Detection and discrimination of B pertussis and B holmesii by real-time PCR targeting IS481 using a beacon probe and probe-target melting analysis

A beacon probe was designed to detect one of the two documented single nucleotide changes in IS481 target allele of Bordetella holmesii genome as compared to Bordetella pertussis. PCR amplified product targeting a region of IS481 in presence of the probe was subjected to a post-PCR hybridization and melting cycle. Hybrid of the probe with B. pertussis specific target had a different thermal stability than that with allele having the single nucleotide change in B. holmesii. The melting of B. pertussis-probe hybrid occurred in a single phase; while that of B. holmesii-probe hybrid was biphasic-one for allele identical to that in B. pertussis and the other for that with a single nucleotide change in B. holmesii genome, with a difference in melting temperature (T(m)) of 6.5 degrees C. The characteristic melting profile and T(m) analysis was the basis for discriminatory detection of B. pertussis from B. holmesii. The method was applied in a representative set of clinical isolates of B. pertussis and B. holmesii and the result was in agreement with conventional culture method.

Bordetella pertussis PCR: simultaneous targeting of signature sequences

The absence of analytical controls for polymerase chain reaction (PCR)-based diagnostic tests for Bordetella pertussis limits their clinical utility. In this study, multiplex PCR simultaneously targeted two specific Bordetella pertussis sequences, the chromosomal repeated insertion sequence IS481 (IS) and the pertussis toxin promoter region (PT). A multi-target hybridization-EIA (Hyb-EIA) method in a 96-well microtiter-plate format was used to detect amplicons. Forty-seven (15%) of the 318 nasopharygeal specimens tested positive for at least one DNA target of B. pertussis by PCR, including the 10 known positive samples by culture and/or direct fluorescent antibody (DFA). Forty-six of the 47 PCR positive samples were considered positive for B. pertussis using the consensus interpretation criteria. Simultaneous detection of multiple chromosomal regions may identify false-positive and -negative results due to analytical variations or potential sequence polymorphism, and uncover a wider range of pathogenic strains.

Diagnosis of community-acquired pertussis infection: comparison of both culture and fluorescent-antibody assays with PCR detection using electrophoresis or dot blot hybridization

Diagnosis of Bordetella pertussis infection has been difficult due to the low sensitivity of culture. PCR tests have been shown to be more sensitive than culture, but the reported sensitivity of PCR is variable. We evaluated PCR product detection by using either agarose gel electrophoresis (PCR-gel) or dot blot hybridization with (32)P-labeled oligonucleotide probes, and we compared these methods to both culture and direct fluorescent-antibody (DFA) assays with microscopy for the detection of pertussis. This was done with 225 nasopharyngeal swab specimens collected in community clinic settings. The multiplexed PCR amplified the multiply repeated IS481 B. pertussis sequence and a sequence from the human globin gene as a positive control for specimen adequacy. Of 225 specimens, 179 were judged to be adequate for PCR analysis. Among the adequate specimens, 9, 4, and 10 were culture, DFA, and PCR-gel positive, respectively. The sensitivity of PCR-gel versus culture was 89% while the sensitivity of culture versus PCR-gel was 80%. DFA had the lowest sensitivity. Thirty specimens were positive by PCR with dot blot hybridization; no negative control specimens showed a signal above the background. Among the 79 (44%) adequate specimens with clinical data available, the rates of reported cough or persistent cough were similar for persons who were pertussis positive by each assay. The IS481 PCR, with either electrophoresis or dot blot hybridization, is a sensitive assay; however, at this time it cannot completely replace culture without an overall loss in sensitivity for the detection of pertussis. Further study is required to understand the clinical significance of B. pertussis PCR products detected by dot blot hybridization alone.

Real-time LightCycler PCR for detection and discrimination of Bordetella pertussis and Bordetella parapertussis

Real-time PCR assays based on the LightCycler technology were developed for individual (simplex PCR) and simultaneous (duplex PCR) detection and discrimination of Bordetella pertussis and Bordetella parapertussis in clinical samples. The assays were evaluated with 113 specimens from patients with and without symptoms of pertussis. Results were compared to those from conventional culture and TaqMan real-time PCR. The analytical sensitivity ranged from 0.1 to 10 CFU for B. pertussis and B. parapertussis, and intra- and interassay variations were less than 7%. Results were available within 2 h. With the simplex format, 21 of 100 samples from patients with clinical symptoms of pertussis were positive for B. pertussis and/or B. parapertussis. With the duplex format, 18 of 100 samples were positive. LightCycler PCR increased the diagnostic sensitivity over that of culture by 2.0-fold (duplex PCR) (P = 0.08) to 2.3-fold (simplex PCR) (P = 0.02). Our data suggest that duplex PCR in this format showed good analytical sensitivity but lost some sensitivity on clinical samples compared with the simplex format.

The use of TaqMan PCR assay for detection of Bordetella pertussis infection from clinical specimens

OBJECTIVE

The routine clinical laboratory detection of Bordetella pertussis is through culture, which can require 5 to 7 days for the bacteria to grow. Using a polymerase chain reaction (PCR) assay can shorten this detection time while increasing the sensitivity of detection with similar specificity. This study compared culture with TaqMan PCR for detection of B pertussis in clinical specimens and the turnaround time for each assay during the pertussis season.

MATERIALS AND METHODS

Nasopharyngeal swabs in Regan-Lowe transport media were collected from 1556 persons who had symptoms of whooping cough or who had had contact with infected persons; the swabs were submitted for B pertussis detection during the pertussis season. A single nasopharyngeal swab from each patient was submitted for both culture and TaqMan PCR detection. Upon receipt of the specimens, the swabs were inoculated onto Regan-Lowe agar for culture and incubated for up to 7 days. The same swab was processed for PCR detection using TaqMan PCR assay. A second nested PCR was used on positive specimens for resolution purposes. The TaqMan PCR assay was performed 3 to 5 days a week, whereas the culture was performed 6 days a week. All specimens were processed on the same day or earliest possible working day for TaqMan or culture, and specimens queued for resolution by nested PCR were batched.

RESULTS

There were a total of 275 PCR positives and 28 culture positives. After resolution with the second nested PCR, the sensitivity, specificity, positive predictive value, and negative predictive value were 100%, 97.4%, 87.6%, and 100% for TaqMan PCR and 11.6%, 100%, 100%, and 85.7% for culture. The average turnaround time for positive culture was 5.1 days, and the average turnaround time for PCR was 2.3 days.

CONCLUSION

The TaqMan PCR assay has superior sensitivity and shorter turnaround time over culture because it can be finished within one working day. Furthermore, the same swab can be used for culture of the bacteria for antibiotic susceptibility testing. The early detection of pertussis using TaqMan PCR assay allows early intervention on the spread of the disease and the ability to culture the bacteria from the same swab, thereby eliminating the need for a second swab and allowing for detection of antibiotic resistance.