There is a wide spread belief that grapefruit seed extract (GSE) has nearly
'universal' antimicrobial activity. Even more aggravating is the belief that
this antimicrobial efficacy is a 'natural' property of GSE. Both popularized
claims are likely false.

There are two studies I know of that tested the antimicrobial efficacy of
GSE (Takeoka et al., 2001 and von Woedtke et al., 1999).  Both studies
confirmed that most commercial preparations of GSE contain preservatives.
The preservatives appear to be responsible for the antimicrobial activity.
There is nothing in pure grapeseed extract with antimicrobial activities.

The preservatives are not naturally found in grapefruit seeds nor are they a
consequence of GSE manufacture. Preservatives are likely added to the GSE.
The preservative benzethonium chloride exists in some commercial
preparations of GSE at levels as high as 8%. I don't think there is much
hope for pure GSE inhibiting Paenibacillus larvae infections.

All antimicrobial tests were based on microbe growth inhibition using direct
microbe-GSE challenges. No bees involved. I haven't found any studies of GSE
and AFB under 'hive' conditions. Nor have I found anyone using AFB infected
larvae as a bioassay. Why do it at all? I am very sure pure GSE will not be
useful.

Richard Goetze

See:

Takeoka G; Dao L; Wong RY; Lundin R; Mahoney N; (2001). Journal of
agricultural and food chemistry. (49), 3316. Identification of benzethonium
chloride in commercial grapefruit seed extracts. (Author Affiliation:
Western Regional Research Center, Agricultural Research Service, U.S.
Department of Agriculture, 800 Buchanan Street, Albany, California 94710,
USA. [log in to unmask]).

von Woedtke T; Schluter B; Pflegel P; Lindequist U; Julich WD; (1999).
Aspects of the antimicrobial efficacy of grapefruit seed extract and its
relation to preservative substances contained.  Die Pharmazie (54), 452.