Quantitative Duplex PCR of Clostridium botulinum Types A and B Neurotoxin Genes

Evaluation and Optimization of Microdrop Digital PCR for Detection of Serotype A and B Clostridium botulinum

Clostridium botulinum is the causative pathogen of botulism. Laboratory detection of C. botulinum is essential for clinical therapy treatment of botulism due to the difficulty in diagnosis, especially in infant botulism. The extreme toxicity of botulinum neurotoxin (BoNT) requires a sensitive detection method. Due to the detection limit of real-time quantitative PCR (q-PCR), a more sensitive detection method, micro-drop digital PCR (ddPCR) was applied in C. botulinum main serotypes A and B. The following performance criteria were evaluated by ddPCR: analytical sensitivity; repeatability; and diagnostic specificity. The limit of detection (LOD) was 0.84 and 0.88 copies/μl for BoNT A and B genes, respectively, by ddPCR with high specificity, compared to 5.04×10² and 6.91×10² copies/μl by q-PCR. It was increased 10 times compared with q-PCR in spiked stool samples. This improvement in sensitivity was especially important in clinical samples as more positive samples were detected by digital PCR compared with q-PCR. Meanwhile, enrichment time for low bacteria content samples was shortened by four hours both in serotypes A and B C. botulinum by ddPCR compared with q-PCR, which are important for laboratory diagnosis and epidemiology work.

A quadruplex real-time PCR assay for rapid detection and differentiation of the Clostridium botulinum toxin genes A, B, E and F

Clostridium botulinum is the aetiological agent of botulism, a disease marked by flaccid paralysis that can progress to asphyxiation and death. This species is defined by the production of one of the botulinum neurotoxins (BoNTs), which are the most potent toxins known. Because of their potency, these toxins have the potential to be used as biological weapons, and therefore C. botulinum has been classified as a category A select agent. There are four related but antigenically distinct BoNT types that cause disease in humans, A, B, E and F. The mouse bioassay is the current gold standard by which BoNTs are confirmed. However, this method is expensive, slow and labour-intensive. Although PCR-based assays have been used extensively for the detection of BoNT-producing bacteria in food, animals and faecal samples, and recently to help diagnose disease in humans, no real-time quantitative PCR (qPCR) assay has yet been developed that can identify and differentiate all four BoNTs that cause disease in humans. This report describes the development of a qPCR single-tube assay that uniquely identifies these four BoNTs responsible for human disease. A total of 79 C. botulinum isolates with varying toxin types was evaluated in this study, as well as numerous near-neighbours and other bacterial species. The results showed that this quadruplex assay was capable of detecting any of the four toxin genes in a given sample at a sensitivity of about 130–840 fg genomic DNA and could detect the presence of up to all four BoNT genes simultaneously in a given sample. The assay was also functional in the presence of extraneous organic matter commonly found in various environmental samples.

Change of thermal inactivation of Clostridium botulinum spores during rice cooking

Cooked and packed rice (CaPR), a popular rice product in Japan, is distributed with oxygen-absorbing agents and without refrigeration. When the final product was inoculated with spores of several strains of proteolytic Clostridium botulinum at a dose of 10(3) spores per g (2 x 10(5) spores per package) and incubated at 30 degrees C, the bacteria grew and produced neurotoxins in 40 days. To simulate more realistic cases of contamination, the same dose of spores was inoculated before the cooking process. When cooked at 100 degrees C for 30 min, a small number of spores survived and the toxins were detected in some of the samples after incubation for 180 days. However, when cooked at 100 degrees C for 15 min immediately followed by 105 degrees C for 15 min, neither survivors nor the toxins were detected during incubation for 270 days after cooking. Even when inoculated with 10(5) spores per g of one of the most heat-resistant strains, 213B, viable spores were not detected after cooking. The inactivation by these heating conditions in different media indicated that the spores were inactivated >1,000-fold more in rice suspension than in cooked meat medium or phosphate buffer. It was therefore suggested that rice contains component(s) that facilitates thermal inactivation of C. botulinum.

Rapid and simple detection of Clostridium botulinum types A and B by loop-mediated isothermal amplification

AIMS

To develop a convenient and rapid detection method for toxigenic Clostridium botulinum types A and B using a loop-mediated isothermal amplification (LAMP) method.

METHODS AND RESULTS

The LAMP primer sets for the type A or B botulinum neurotoxin gene, BoNT/A or BoNT/B, were designed. To determine the specificity of the LAMP assay, a total of 14 C. botulinum strains and 17 other Clostridium strains were tested. The assays for the BoNT/A or BoNT/B gene detected only type A or B C. botulinum strains, respectively, but not other types of C. botulinum or strains of other Clostridium species. Using purified chromosomal DNA, the sensitivity of LAMP for the BoNT/A or BoNT/B gene was 1 pg or 10 pg of DNA per assay, respectively. The assay times needed to detect 1 ng of DNA were only 23 and 22 min for types A and B, respectively. In food samples, the detection limit per reaction was one cell for type A and 10 cells for type B.

CONCLUSIONS

The LAMP is a sensitive, specific and rapid detection method for C. botulinum types A and B.

SIGNIFICANCE AND IMPACT OF THE STUDY

The LAMP assay would be useful for detection of C. botulinum in environmental samples.