736 Modern Food Microbiology
Bacteroides fragilis
This obligately anaerobic, Gram-negative bacterium is of potential significance as a foodborne
pathogen since it produces an ileal loop-positive enterotoxin and is often associated with human
diarrhea, as are A. hydrophila and P. shigelloides. The enterotoxin was first demonstrated in 1984, and
enterotoxic strains of B. fragilis were first associated with human diarrhea in 1987.
B. fragilis is estimated to constitute between 1% and 2% of the human intestinal biota. As a non-pore
former, it is more sensitive to aerated environments than the clostridia and yet it has been recovered
from municipal sewage. This species differs from most other Bacteroides in being catalase positive,
and like most others it can grow in the presence of 20% bile.
The B. fragilis enterotoxin is produced as a single chain with a molecular weight of about 20,000 Da.
It differs from the classic bacterial enterotoxins in belonging to a class of zinc-binding metalloprotease,
designated metzincins. The enterotoxin has a wide range of protein substrates, and it undergoes
autodigestion. The intestinal damage that it causes is believed to be due, at least in part, to its proteolytic
action. It elicits a positive response in ileal loops of lambs and other animals.
Since the etiological agent is identified in only around 50% of foodborne outbreaks in the United
States, it is clear that previously unrecognized agents need to be included. B. fragilis along with
Klebsiella pneumoniae
60 and Enterobacter cloacae
61 may warrant more attention. The latter two
organisms produce heat-stable enterotoxins that are similar to the heat-stable enterotoxin (ST) of
E. coli, and their potential significance in foods has been noted.
109
Erysipelothrix rhusiopathiae
This bacterium (E·ry·si·pe’·lo·thrix rhu·si·o·pa’·thi·ae) is phylogenetically closely related to Listeria
(see Chapter 25), and like L. monocytogenes, it causes disease in animals and humans. It is the cause of
erysipelas in swine and erysipeloid in humans. Because of these similarities, it seems to be a “logical”
candidate for a foodborne pathogen although such cases do not seem to have been reported. In general,
erysipeloid is a localized disease of the hands and arms of handlers of fresh meat and fish, but systemic
involvements are not unknown. Erysipeloid in pigs is characterized by diamond-skin lesions.
The organism is a facultative anaerobe, catalase negative (in contrast to the listeriae), oxidase
negative, and generally produces H 2 S. At least 23 serovars are known. The only other species is
E. tonsillarum, which was separated from E. rhusiopathiae based on its primary habitat of porcine
tongues, and because of serovar differences.
100
One of the first studies of the incidence of this organism in foods is that of Ternstr¨ om and Molin,
104
who in 1982 undertook a study of foodborne pathogens in meats in Sweden. They examined 135
samples consisting of equal numbers of chicken, beef, and pork, and found E. rhusiopathiae in 36%
and 13%, respectively, of pork and chicken, but none in beef. In one plant, 54% of pork loins were
positive, and many of the isolates possessed mouse virulence. Of 112 retail pork samples examined
in Japan, 34% contained this bacterium, and the 38 isolates represented 14 serovars.
98 In a study of
meat samples from 93 wild boar and 36 deer in Japan, 44% of the wild boar and 50% of the deer
samples contained E. rhusiopathiae, representing 13 serovars.
54 In a study of 750 chickens in Japan,
Erysipelothrix spp. were recovered from 15.7% of skin samples, and from 59.2% of 179 feather
samples.
76 E. rhusiopathiae represented 273 of 297 isolates and the remainder were E. tonsillarum.
In another study of 153 chicken samples in Japan, 30% contained Erysipelothrix spp. with 65 of 67
being E. rhusiopathiae.
77
Bacteroides fragilis
This obligately anaerobic, Gram-negative bacterium is of potential significance as a foodborne
pathogen since it produces an ileal loop-positive enterotoxin and is often associated with human
diarrhea, as are A. hydrophila and P. shigelloides. The enterotoxin was first demonstrated in 1984, and
enterotoxic strains of B. fragilis were first associated with human diarrhea in 1987.
B. fragilis is estimated to constitute between 1% and 2% of the human intestinal biota. As a non-pore
former, it is more sensitive to aerated environments than the clostridia and yet it has been recovered
from municipal sewage. This species differs from most other Bacteroides in being catalase positive,
and like most others it can grow in the presence of 20% bile.
The B. fragilis enterotoxin is produced as a single chain with a molecular weight of about 20,000 Da.
It differs from the classic bacterial enterotoxins in belonging to a class of zinc-binding metalloprotease,
designated metzincins. The enterotoxin has a wide range of protein substrates, and it undergoes
autodigestion. The intestinal damage that it causes is believed to be due, at least in part, to its proteolytic
action. It elicits a positive response in ileal loops of lambs and other animals.
Since the etiological agent is identified in only around 50% of foodborne outbreaks in the United
States, it is clear that previously unrecognized agents need to be included. B. fragilis along with
Klebsiella pneumoniae
60 and Enterobacter cloacae
61 may warrant more attention. The latter two
organisms produce heat-stable enterotoxins that are similar to the heat-stable enterotoxin (ST) of
E. coli, and their potential significance in foods has been noted.
109
Erysipelothrix rhusiopathiae
This bacterium (E·ry·si·pe’·lo·thrix rhu·si·o·pa’·thi·ae) is phylogenetically closely related to Listeria
(see Chapter 25), and like L. monocytogenes, it causes disease in animals and humans. It is the cause of
erysipelas in swine and erysipeloid in humans. Because of these similarities, it seems to be a “logical”
candidate for a foodborne pathogen although such cases do not seem to have been reported. In general,
erysipeloid is a localized disease of the hands and arms of handlers of fresh meat and fish, but systemic
involvements are not unknown. Erysipeloid in pigs is characterized by diamond-skin lesions.
The organism is a facultative anaerobe, catalase negative (in contrast to the listeriae), oxidase
negative, and generally produces H 2 S. At least 23 serovars are known. The only other species is
E. tonsillarum, which was separated from E. rhusiopathiae based on its primary habitat of porcine
tongues, and because of serovar differences.
100
One of the first studies of the incidence of this organism in foods is that of Ternstr¨ om and Molin,
104
who in 1982 undertook a study of foodborne pathogens in meats in Sweden. They examined 135
samples consisting of equal numbers of chicken, beef, and pork, and found E. rhusiopathiae in 36%
and 13%, respectively, of pork and chicken, but none in beef. In one plant, 54% of pork loins were
positive, and many of the isolates possessed mouse virulence. Of 112 retail pork samples examined
in Japan, 34% contained this bacterium, and the 38 isolates represented 14 serovars.
98 In a study of
meat samples from 93 wild boar and 36 deer in Japan, 44% of the wild boar and 50% of the deer
samples contained E. rhusiopathiae, representing 13 serovars.
54 In a study of 750 chickens in Japan,
Erysipelothrix spp. were recovered from 15.7% of skin samples, and from 59.2% of 179 feather
samples.
76 E. rhusiopathiae represented 273 of 297 isolates and the remainder were E. tonsillarum.
In another study of 153 chicken samples in Japan, 30% contained Erysipelothrix spp. with 65 of 67
being E. rhusiopathiae.
77