The Clostridium perfringens necrotic enteritis toxin B, NetB, was recently proposed as a new key virulence factor for the development of necrotic enteritis (NE) in broilers. The aim of the present study was to investigate the presence of the netB gene and the in vitro production of the NetB toxin in a well characterized collection of 48 C. perfringens Type A isolates, obtained from Danish broiler flocks. The investigation revealed netB gene prevalences of approx. 50% and 60% among isolates from diseased (NE) and healthy flocks, respectively. Only minor nucleotide variations were observed between the isolates in the coding sequence (CDS) of the netB gene, and the promoter region was observed to be completely conserved. However, in vitro NetB production was only observed in 4 out of 14 netB-positive C. perfringens isolates recovered from healthy birds, whereas 12 out of 13 netB-positive isolates from NE birds were shown to produce the NetB toxin. It is therefore proposed that genotype, i.e. presence of the netB gene, in itself is inadequate for predicting virulence of C. perfringens, and future investigations should focus on the bacterial phenotypes; the regulatory mechanisms involved in the expression of NetB, and potentially also other toxins, and its implications for the virulence of individual C. perfringens strains.
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