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840 Part 8: Food Safety and Food Allergensother food types including vegetables (Al Saif and Brazier 1996),
ready-to-eat salads (Bakri et al. 2009), vacuum-packaged beef
(Bouttier et al. 2010), retail ground meats (Jobstl et al. 2009), red
meats, ground products, pork and chicken products (Rodriguez-
Placios et al. 2007, 2009, Norman et al. 2009, Von Abercron
et al. 2009, Weese et al. 2009b, Weese et al. 2010), and in swine
operations with the highest prevalence levels observed in young
animals (Norman et al. 2009). Speculation as to the sources
ofC. difficileon meat have included animal hides, processing
equipment, the environment and employees, and noting that the
spores are probably able to survive processing, cleaning, and
disinfection (Weese 2009a).
Rupnik (2007) strongly suggests that the current data provide
preliminary evidence or support speculation that animal reser-
voirs and transmission via foods may be a source of community-
associated infections. Regardless, there are currently consider-
able knowledge gaps in our understanding ofC. difficileand its
links to food-borne disease; to date, there is no direct evidence
of illness and an identified source, which suggests that a greater
understanding of the epidemiology ofC. difficileis warranted.Antimicrobial-Resistant PathogensAlthough mentioned in regards to some of the pathogens dis-
cussed above, the emergence of antimicrobial resistance in food-
borne pathogens warrants further review in this chapter. Doyle
and Erickson (2006) consider that antimicrobial resistance in
pathogens is a concern and will continue to be an issue for the
future. Over the last 50 years or more, the use of antimicrobials
has been a mainstay for the treatment of infectious disease in
humans and animals. However, over the same period, the use
of antimicrobials has led to the emergence of antimicrobial re-
sistance in pathogens. Newell et al. (2010) considers that any
kind of antimicrobial can select for emergence of resistance and
promote the dissemination of resistant bacteria and resistance
genes. As a consequence, resistance can be selected in the agri-
cultural sector and can contribute to the public health burden.
Woteki and Kineman (2003) and Teale (2002) suggest that the
overuse of antimicrobials in agriculture and healthcare have been
identified as contributing factors to drug-resistant strains. Food,
therefore, can be both a source of antimicrobial-resistant bacteria
and resistance genes; therefore, foods harboring antimicrobial-
resistant bacteria can be a significant threat to human illness.
Studies have highlighted the presence of resistantSalmonella,
Campylobacter,E. coli, and other such pathogens in the food
chain potentially contributing to disease.
Indirectly, these antimicrobial-resistant organisms can also be
an indirect source of bacterial resistance by transferring resis-
tance genes to other organisms or commensals. Studies have
highlighted the role of mobile genetic elements in the transfer of
resistance (Barlow 2009). Kruse and Sorum (1994) highlighted
the transfer of resistance using plasmids. Regardless of how re-
sistance occurs, and its complexities, antimicrobial resistance
appears to have emerged in food-borne pathogens, thus posing a
new challenge in understanding its epidemiology and the means
to control it.Some of the important antimicrobial-resistant pathogens in-
clude:Salmonella, there are well-documented studies of food-
borne illness associated with drug-resistantSalmonella.The
pathogen has been associated with a variety of foods includ-
ing meats, eggs, produce, and so on. Studies have highlighted
the movement of drug-resistant strains from food to humans
(Endtz et al. 1991, Engberg et al. 2001). Other studies have
linked drug use at the farm with the emergence of drug-resistant
strains (Logue et al. 2003, Lin et al. 2007, Logue et al. 2010,
Newell et al. 2010), thus leading to potential transfer to humans
via foods or direct contact with animals.
InSalmonella, some species appear to have the ability to
resist multiple antimicrobial agents. Multidrug-resistantS. ty-
phimuriumDT104 is an example of a strain possessing resistance
to five different antimicrobials (Threlfall 2000). Newly emerg-
ing resistance to extended spectrumβ-lactam antimicrobials has
also become an issue (Patterson 2006, Carattoli 2008).
InCampylobacter, a similar situation has also been observed
with poultry being a primary source of the pathogen and the
intensive nature of poultry rearing has led to the emergence of
drug-resistantCampylobacter. In recent years, the emergence of
fluoroquinolone-resistantCampylobacterhas been observed and
in some countries, the emergence of resistance has been linked
to the use of fluoroquinolones in poultry production (Aarestrup
and Engberg 2001, Humphrey et al. 2005) and a consequent in-
crease in fluoroquinolone-resistant strains occurring in humans
(Endtz et al. 1991, Engberg et al. 2001). In light of this ev-
idence and the potential risks for food-borne disease, the use
of fluoroquinolones for use in poultry production was banned
in 2005. Regardless of the ban, however, it would appear that
fluoroquinolone-resistantCampylobacterare still present in pro-
duction birds and meat (Price et al. 2007, Han et al. 2009a). In
addition, the emergence of macrolide-resistantCampylobacter
is also of concern. Lin et al. (2007), Lunagtongkum et al. (2009),
and Logue et al. (2010) highlighted the effect of dosing to select
resistant strains, however, in the absence of selective pressure,
levels of resistant strains appear to drop in favor of nonresistant
strains, suggesting that the fitness cost associated with resistance
maintenance may be too high to promote persistence (Han et al.
2009b, Hao et al. 2009, Logue et al. 2010).
InE. coli,the emergence of multidrug-resistant strains has also
been evident. CommensalE. colimay also be a source of resis-
tance to otherE. coliand pathogens. Studies by our colleagues
have highlighted resistance carried on plasmids that form islands
of resistance (Johnson et al. 2004, Johnson et al. 2010) and ap-
pear to be co-selected with virulent strain types, suggesting that
virulence and resistance are complementary (Johnson and Nolan
2009). Travers and Barza (2002) noted that drug-resistant strains
appear to be more virulent than drug susceptible strains. In addi-
tion, transfer of resistance genes fromE. colito other pathogens
has been demonstrated and has been shown in both the animal
and human gut (Gast and Stephens 1986, Su et al. 2003, Poppe
et al. 2005,Yan et al. 2005) and in foods (Kruse and Sorum 1994,
Walsh et al. 2008).
In gram-positive organisms, methicillin-resistantS. aureus
(MRSA) has emerged as a significant problem; once thought
of as a nosocomial infection, community acquired MRSA have