Yersinia enterocolitica, a notable foodborne pathogen, is typically transmitted through contaminated food and untreated water. However, human-to-human, animal-to-human, and blood transfusion-associated transmissions have also been documented. Among its six biotypes, biotypes 1B and 2-5 exhibits significant pathogenicity attributed to a conserved 70-kb virulence plasmid, pYV/pCD, and specific chromosomal genes. Notably, some biotype 1A strains, often lacking the pYV plasmid and traditional chromosomal virulence genes, frequently cause gastrointestinal diseases in humans. This organism's pathogenesis relies on its capabilities to invade the intestinal wall, regulated by plasmid genes, and to produce heat-stable enterotoxins, controlled by chromosomal genes.
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Y. enterocolitica was discovered over 60 years ago but rose to prominence as a human and veterinary pathogen in the late 1960s when it was increasingly linked to foodborne gastrointestinal infections. This bacterium is part of the genus Yersinia, which includes multiple facultatively anaerobic bacteria belonging to the family Enterobacteriaceae.
Y. enterocolitica is responsible for a range of clinical and immunological manifestations, from intestinal diseases like enterocolitis in infants to rare extraintestinal infections including urinary tract infections, osteoarticular infections, and erythema nodosum. Its geographical distribution is varied, with more than 50 distinct serotypes exhibiting geographical specificity. For instance, serotype O:3 prevails in Australia, Europe, and Canada, while O:8 is predominant in Japan.
Epidemiology and Serotypes
Y. Enterocolitica exhibits a wide range of serotypes, with serotypes O:3, O:9, and O:5,27 commonly associated with human infections. The bacterium thrives in cooler environments, contributing to its persistence in food and water sources. Geographical differences in serotype distribution highlight regional epidemiological patterns, impacting disease surveillance and outbreak management strategies.
Modes of Transmission
Transmission of Y. enterocolitica occurs through various modes, each playing a critical role in its epidemiology.
Fig. 1 Mode of transmission of Y. enterocolitica (Sabina Y., et al. 2011).
Foodborne Transmission
Y. Enterocolitica is a significant foodborne pathogen causing sporadic illnesses and occasional outbreaks. The bacterium has been isolated from various foods, including pork, beef, raw milk, and even tofu and powdered milk. The organism's ability to multiply under adverse conditions like commercial refrigeration aids its survival in food products. In developed countries, most infections are sporadic, although large outbreaks have occurred. In the United States, children are often infected indirectly during the cleaning and preparing of raw or undercooked pork chitterlings. Similar outbreaks have been reported from untreated water sources, contaminated milk, bean sprouts, and tofu.
Human-to-Human Transmission
Person-to-person transmission is rare but has been documented. For instance, a familial outbreak in Japan was attributed to an infected carrier with diarrhea, and a nosocomial outbreak indicated human-to-human transmission of the bacterium. These instances highlight the possibility of transmission through contaminated food handled by infected individuals or via direct contact in healthcare settings.
Animal-to-Human and Waterborne Transmission
Y. enterocolitica can also spread through direct or indirect contact with infected animals, often found in the intestinal tracts of pigs, rodents, cattle, and pets like cats and dogs. The pig is considered the main reservoir. The bacterium's adaptability to extreme environmental conditions, including survival in cool temperate zones and aquatic environments, facilitates its transmission through water contaminated by these animals.
Direct Transmission
In rare cases, Y. enterocolitica causes extraintestinal disease, which can arise from direct environmental exposure. For instance, a construction worker developed an axillary abscess after a finger injury, suggesting direct inoculation from an environmental source. Similar direct transmissions have been proposed for butchers who sustain frequent hand injuries.
Blood Transfusion-Associated Transmission
Y. enterocolitica can be transmitted through contaminated blood, being one of the first recognized causes of posttransfusion sepsis. Since the first reported case in 1975, over 60 additional cases have been documented. The bacterium can cause septicemia in blood transfusion recipients when donor blood, sometimes from healthy individuals or those with a history of diarrhea, is contaminated.
Molecular Insights in Virulence
The pathogenicity of Y. enterocolitica varies widely among its six biotypes due to differences in their genetic makeup, particularly the presence of the pYV plasmid.
Virulence Factors of pYV-Bearing Strains
The virulence plasmid pYV encodes several critical factors, such as Yersinia adhesin A (YadA), Ysc-Yop type III secretion system (TTSS), invasin (Inv), and the heat-stable enterotoxin (Yst). These factors enhance the bacterium's ability to invade host tissues, resist phagocytosis, and survive under adverse conditions, collectively contributing to its pathogenicity.
The Lack of Virulence of Biotype 1A Strains
Biotype 1A strains typically lack the pYV plasmid and the chromosomal virulence genes found in more pathogenic biotypes. Their ubiquity in the environment and asymptomatic presence in animals suggest an avirulent nature. However, they are still frequently isolated from human infections, raising questions about their true pathogenic potential.
Pathogenicity in Some Biotype 1A Strains
Despite lacking traditional virulence genes, some biotype 1A strains cause gastrointestinal diseases similar to those caused by pathogenic biotypes. Recent studies suggest that ystB-encoding Yersinia heat-stable enterotoxin may play a significant role in the pathogenicity of these strains, indicating the need for further research into their virulence mechanisms.
Pathogenesis
The pathogenesis of Y. enterocolitica involves several complex processes, including adaptation, adhesion, invasion, and systemic dissemination.
Adaptation
Upon entering the host through contaminated food, Y. enterocolitica must adapt to the intestinal environment. The pYV plasmid plays a crucial role in this process by encoding genes that allow the bacterium to thrive at human body temperature and resist host defenses.
Adhesion
Adherence to host cells is facilitated by various factors, including YadA and other fimbrial adhesins. While pYV-bearing strains exhibit stronger adherence properties, biotype 1A strains also produce certain fimbriae that may contribute to their pathogenicity.
Invasiveness
Invasion of nonphagocytic epithelial cells is a hallmark of Y. enterocolitica infection. The invasion (Inv) protein facilitates this process by binding to β1 integrins on the host cell surface. While biotype 1A strains invade less efficiently, some possess the ail gene, which may enhance their invasive capabilities.
Local and Systemic Dissemination
Following the invasion of the intestinal epithelium, Y. enterocolitica can disseminate to local and systemic sites. The bacterium's ability to resist phagocytosis and complement-mediated lysis, largely due to the pYV plasmid, allows it to proliferate extracellularly and establish infections in various tissues.
Conclusion
Y. enterocolitica is a versatile pathogen with multiple transmission routes and complex virulence mechanisms. While the presence of the pYV plasmid is a significant determinant of virulence, some biotype 1A strains can also cause disease despite lacking traditional virulence factors. Continued research into the molecular and genetic basis of Y. enterocolitica virulence will enhance our understanding of this pathogen and improve strategies for preventing and controlling infections.
Understanding the diverse transmission pathways and virulence factors of Y. enterocolitica is crucial for developing effective public health interventions and ensuring food safety. Through concerted efforts in surveillance, research, and education, we can mitigate the impact of this pathogen on global health.
Reference
- Sabina Y., et al. Yersinia enterocolitica: mode of transmission, molecular insights of virulence, and pathogenesis of infection. Journal of Pathogens. 2011, 2011(1): 429069.
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