We have received several claims that the VMRD Clostridium septicum direct FA conjugate does not bind certain cultures of Clostridium thought to be C. septicum. To investigate these claims, we obtained one of the putative C. septicum isolates. The stench it produced in anaerobic culture supported its identity as a Clostridium species, and the swarming behavior on agar reported by our customer suggested C. septicum. A Gram stain showed Gram-positive rods with some spore formation. This culture did not react with our C. septicum direct FA conjugate. It also did not react with our C. chauvoei, C. novyi, or C. sordellii direct FA conjugates. All that could be seen by FA microscopy were faint outlines of long rods. We were prepared to develop a new C. septicum conjugate that would react with this ornery culture but first we had to be certain of its identity. DNA analysis by PCR amplified a segment of the 16S ribosomal RNA gene. The closest sequence match—homology was 100%—in GenBank was C. fallax. An earlier clue suggesting that the sample was not C. septicum was that chopped meat broth turned partially black instead of pink after 48 hours of growth1. The species name, fallax, meaning “deceptive” in Latin2, was most appropriate for this sample, which misled even the best bacteriologists to believe that it was C. septicum.
1. Sneath, Peter H. A.. “Bergey’s Manual of Systematic Bacteriology, Volume 2.” Maryland: Williams & Wilkin, 1986. Page 1188
2. Ibid. Page 1167