Avian botulism, a paralytic disease of birds, often occurs on a yearly cycle and 
is increasingly becoming more common in the Great Lakes. Outbreaks are caused by 
bird ingestion of neurotoxins produced by Clostridium botulinum, a 
spore-forming, gram-positive, anaerobe. The nuisance, macrophytic, green alga 
Cladophora (Chlorophyta; mostly Cladophora glomerata L.) is a potential habitat 
for the growth of C. botulinum. A high incidence of botulism in shoreline birds 
at Sleeping Bear Dunes National Lakeshore (SLBE) in Lake Michigan coincides with 
increasingly massive accumulations of Cladophora in nearshore waters. In this 
study, free-floating algal mats were collected from SLBE and other shorelines of 
the Great Lakes between June and October 2011. The abundance of C. botulinum in 
algal mats was quantified and the type of botulism neurotoxin (bont) genes 
associated with this organism were determined by using most-probable-number PCR 
(MPN-PCR) and five distinct bont gene-specific primers (A, B, C, E, and F). The 
MPN-PCR results showed that 16 of 22 (73%) algal mats from the SLBE and 23 of 
31(74%) algal mats from other shorelines of the Great Lakes contained the bont 
type E (bont/E) gene. C. botulinum was present up to 15000 MPN per gram dried 
algae based on gene copies of bont/E. In addition, genes for bont/A and bont/B, 
which are commonly associated with human diseases, were detected in a few algal 
samples. Moreover, C. botulinum was present as vegetative cells rather than as 
dormant spores in Cladophora mats. Mouse toxin assays done using supernatants 
from enrichment of Cladophora containing high densities of C. botulinum (>1000 
MPN/g dried algae) showed that Cladophora-borne C. botulinum were 
toxin-producing species (BoNT/E). Our results indicate that Cladophora provides 
a habitat for C. botulinum, warranting additional studies to better understand 
the relationship between this bacterium and the alga, and how this interaction 
potentially contributes to botulism outbreaks in birds.