C. Difficile Toxins A and B Support Development of Diagnostics
February 14, 2017
By: List Labs
By: Mary N. Wessling, Ph.D. ELS
List Biological Laboratories, Inc.’s products are being used to confront one of the most pressing problems in health care today: stopping the worldwide spread of illness caused by Clostridium difficile (CD, C Difficile Toxin), a gram-positive, spore-forming anaerobic bacillus. The spores are highly resistant to adverse environmental conditions and are frequently among the contaminants in food products, where they germinate . The CD pathogen causes severe diarrhea and pseudomembranous colitis, among other dangerous gastrointestinal ailments. At present, the estimate is 500,000 cases in hospitals and long-term care facilities, with an annual mortality rate of 15,000 to 30,000 in the US and worldwide . The estimated annual cost of treating CD infections ranges from $436 million to $3.2 billion per year in the US alone .
C Difficile Toxin in Hospitals
Historically, CD was mainly a problem for hospitals and long-term care facilities, but infections may spread rapidly into the community, especially among persons who have required antibiotic treatments that kill off competing strains in the intestines and allow CD to multiply. CD is a rapidly evolving bacterium, with hypervirulent strains contributing to the increase in mortality. At present, 234 unique genomes have been identified that cause most of the hospital outbreaks in the US and Europe. The current testing of stool samples to confirm diagnosis requires up to 3 days to differentiate between dangerous CD infection (CDI) and less harmful causes of diarrhea. A delay in diagnosis of CD presents a difficulty in treatment, making the development of more rapid diagnostic techniques a high priority.
C Difficile Toxin Types A & B
Clostridium difficile produces two major toxins, C Difficile Toxin A aka TcdA (Product #152) and C Difficile Toxin B aka TcdB (Product #155), the latter being the more virulent. These toxins inactivate the Rho-GTPase through glycosylation, and the structural bases for their activities have been clarified by X-ray diffraction, biochemical assays, and molecular dynamics . The List Labs Toxin A and B products are playing an important role in the search for a more rapid and accurate methods to diagnose CDI. To meet the demands of those more sensitive and exacting methods, List’s difficile toxins are of higher purity than previously available products. The purity of List Labs’ toxins enables diverse creative and fascinating scientific approaches that adapt and modify known analytical techniques to CDI testing. What follows (in chronological order of publication) are recent studies that relied on List Labs’ products for their results.
C Difficile Infection Studies
Molecular diagnostic techniques are increasingly being used to quantify the seriousness of infection and to distinguish CDI from other causes. The study by Moura et al  used List Labs’ products (purified TcdA and TcdB) for a proteomic analysis that identified and quantified the protein factors involved in CD toxin production through an enhanced mass spectrometric (MS) method. This method provided a basis for development of improved MS methods that demand only small samples and contribute to a better understanding of toxin-mediated illnesses, their prevention and therapy.
In the same year, Lei and Bochner  used List Labs’ Toxins A and B in phenotype microarrays (PMs) under different culture conditions. Noting that with the evolution of the CD genome, multiple lineages evolved independently; they examined how the Toxin B cytopathic effect caused cell rounding and used that to measure the virulence of CD under different conditions. Using the PMs, they developed a 1-day test that compared the pure List Labs’ toxins and unpurified toxins to see parallels, which could be measured by colorimetry. This provided a more rapid test than the 3-day diagnostic tests currently in use.
In 2014, Huang et al  used CD toxin B to approach the problem of distinguishing between infection, colonization, and live and dead CD organisms. Their real-time microelectronic sensor-based analysis had high sensitivity and relied on reading the impedance of cells applied to microelectrodes to detect specific cellular processes through quantification over time. The method is rapid: the authors reported that 80% positive results were obtained within 24 hours. Concentration of the CD in stool measured by their method correlated with clinical severity, providing a method to monitor the progress of CDI in patients.
In 2014, an innovative study by Leslie et al  used human intestinal organoids (HOIs) derived from stem cells to model the disruption of barrier functions in the human intestine by CD. The HOIs were generated by directed differentiation of human pluripotent stem cells, which were then further differentiated into intestinal tissue. These HOI’s were subjected to the C. difficile strain, TcdA or TcdB. Increased toxicity under conditions favorable to production of toxins by CD was measured by presence of cell rounding using fluorescent dextran. Injection of TcdA replicates the disruption of the epithelial barrier function and structure observed in HIOs colonized with viable CD.
In 2015, Hong et al  used an extension of a method they had developed previously combined with ELISA, eventually applying single-stranded DNA molecular recognition elements (MRE) to microchips. They used List Labs’ lyophilized toxin B, reconstituted and immobilized on magnetic beads; after incubation with an ssDNA library, 80 randomly selected clones were sequenced and analyzed. The goal was to identify ssDNA MREs that bind to toxin B; eventually, fluorescence was used to examine the structure of one selected MRE bound to toxin B. This very complex process yielded an MRE bound with high specificity with toxin B in human fecal matter; it demonstrated a proof-of-concept diagnostic application.
In quite another approach to preventing the morbidity and mortality attributed to CD, a study by Zilbermintz et al  showed that the antimalarial drug amodiaquine has a protective effect against CD. The drug was one of a group of existing FDA-approved compounds screened to extend their use to as broad-spectrum, host-oriented therapies. Amodiaquine interferes with the functioning of the host protein cathepsin B targeted by CD and other pathogens as well. The aim of their approach is to find therapies that circumvent the effect of pathogen mutations that lead to drug resistance. All toxins used in the study were purchased from List Biological Laboratories. Though not a new diagnostic technique, this discovery promised an approach to CDI using existing pharmacology, which then could reduce the immense cost of treating CD patients .
- Xiao Y et al. Clostridial spore germination versus bacilli: genome mining and current Food Microbiol. 2011 Apr;28(2):266-74. doi: 10.1016/j.fm.2010.03.016. Epub 2010 Apr 1. PMID: 21315983
- Moura H et al. Proteomic analysis and label-free quantification of the large Clostridium difficile toxins. Int J Proteomics. 2013;2013:293782. doi: 10.1155/2013/293782. Epub 2013 Aug 27. PMID: 24066231
- Hong KL et al. In vitro selection of a single-stranded DNA molecular recognition element against Clostridium difficile toxin B and sensitive detection in human fecal matter. J Nucleic Acids. 2015;2015:808495. doi: 10.1155/2015/808495. Epub 2015 Feb 5. PMID: 25734010
- Yin JC et al. Structural insights into substrate recognition by Clostridium difficile sortase. Front Cell Infect Microbiol. 2016 Nov 2a2;6:160. eCollection 2016. PMID: 27921010
- Lei XH , Bochner BR. Using phenotype microarrays to determine culture conditions that induce or repress toxin production by Clostridium difficile and other microorganisms. PLoS One. 2013;8(2):e56545. doi: 10.1371/journal.pone.0056545. Epub 2013 Feb 20. PMID: 23437164
- Huang B et al. Real-time cellular analysis coupled with a specimen enrichment accurately detects and quantifies Clostridium difficile toxins in stool. J Clin Microbiol. 2014 Apr;52(4):1105-11. doi: 10.1128/JCM.02601-13. Epub 2014 Jan 22. PMID: 24452160
- Leslie JL et al. Persistence and toxin production by Clostridium difficile within human intestinal organoids result in disruption of epithelial paracellular barrier function. Infect Immun. 2015 Jan;83(1):138-45. doi: 10.1128/IAI.02561-14. Epub 2014 Oct 13. PMID: 25312952
- Zilbermintz L et al. Identification of agents effective against multiple toxins and viruses by host-oriented cell targeting. Sci Rep. 2015 Aug 27;5:13476. doi: 10.1038/srep13476. PMID: 26310922