Community-associated infections are a rising concern in the epidemiology of C. difficile
. This study identified a potential food animal source of such infections. Parallel to the findings of this study, recent investigations reported that as many as 47.6% of litters and 90% of herds are infected with C. difficile
). In the Netherlands, patients infected with ribotype 078 were younger (67.4 versus 73.5 years) and had community-associated disease more frequently (17.5% versus 6.7%) than patients infected with ribotype 027 (27
). Similarly to the findings reported here, previous researchers have found genetic relatedness among strains isolated from humans and pigs, suggesting a possible epidemiological relationship (4
There is little published data available on the antimicrobial susceptibility of C. difficile
isolated from pigs and other food animals (20
), and there is even less on antimicrobial resistance genes. Based on this study, the majority of C. difficile
isolates (80.5%) have a MIC of >32 μg/ml for ciprofloxacin. This finding is in agreement with previous reports, which have noted that the older fluoroquinolones, such as ciprofloxacin, have moderate to poor activity against C. difficile
). One recent study found all isolates of swine origin to be sensitive to metronidazole and vancomycin, 98% were sensitive to chloramphenicol, and 90% were sensitive to tetracycline (20
). Previous studies have reported that the majority of C. difficile
strains with tetracycline resistance have the tetM
gene, which is carried on a conjugative transposon (19
). The tetW
gene has the second largest host range of the tetracycline resistance genes, second only to tetM
. In this study, the tetM
gene was found in 97% of resistant isolates and the tetW
gene was found in 32% of tetracycline-resistant isolates of swine origin. There did not appear to be a combined benefit to carrying both genes, as isolates that carried both tetM
genes had a wide range of MICs within the resistant zone.
gene has been found to encode macrolide lincosamide and streptogramin resistance and has been shown to result in high levels of resistance (15
). This gene was found in 91% of erythromycin-resistant isolates in this study. Of those isolates found to carry this gene, 97% were found to have a MIC of >256 μg/ml, which is consistent with previous reports. This gene appears to play a significant role in high-level erythromycin resistance in C. difficile
isolated from swine. A large portion of the isolates that were resistant to both erythromycin and tetracycline were found to carry both the ermB
and the tetM
genes. A link between these two resistance genes has previously been described in C. difficile
isolates of human origin, where they are associated with a Tn916
-like element (31
). Further studies to characterize these elements in swine and to determine if they are linked would be noteworthy.
Toxin gene results for these swine are similar to results previously published, and all combinations of genes have been previously reported for food animals (20
). This study is the first known report of heterogeneity in C. difficile
strains carried within individual pigs, although this has been found previously in humans at a rate of approximately 7% (5
). The findings clearly imply that the characterization of a single isolate from a subject (i.e., pig) could be misleading, as pigs may carry multiple strains at the same time. Such carriage of multiple strains by one pig could promote the horizontal transfer of resistance genes and toxin genes between strain types.
The major toxinotype, toxinotype V, which was found commonly in the present study, has also been reported previously (1
). Toxinotype V-like has been reported recently (8
). However, to the best of our knowledge, this is the first study indicating its occurrence among isolates of swine origin. Toxinotypes V and O have also been reported in humans (8
). The common occurrence of toxinotype V in the present study underscores a potential concern, as this type has been associated with community-associated disease and increasing prevalence in CDI in humans (6
). Further investigation is necessary, as some recent reports have found toxinotype V in asymptomatic people (21
), and others found that there was no association between virulence attributes and clinical outcomes (28
Previous studies have determined that PFGE has a high index of discrimination for C. difficile
). PFGE results showed many samples clustering within toxinotypes as well as within farms. For example, a large cluster of 100% similar nontoxigenic isolates were mostly from one Ohio farm, although one isolate within that cluster was from a second Ohio farm. All clusters noted were found to carry only one toxinotype, although one toxinotype was found to split into separate clusters within the dendrogram, showing diversity within the toxinotype. This finding was expected, as toxinotyping only describes one small part of the genome and PFGE is descriptive of the total genomic DNA.
In comparisons of swine strains to control CDC isolates, clusters included isolates of swine and human origin. Although this is concerning for public health reasons, further information is needed to determine the epidemiological relationship of these isolates. As mentioned by previous studies which have found highly similar isolates in human and swine, it is possible that strains humans and strains in pigs are of a common source, but these findings do not show a direct causal or epidemiologic relationship among the isolates (4
). Further research is needed to determine the potential transmission of swine isolates to humans, either through contaminated food, environmental contamination, or other exposure routes.