In this article we will discuss about:- 1. Introduction to Yersinia Enterocolitica 2. The Organism of Yersinia Enterocolitica and its Characteristics 3. Pathogenesis and Clinical Features 4. Isolation and Identification 5. Association with Foods.
- Introduction to Yersinia Enterocolitica
- The Organism of Yersinia Enterocolitica and its Characteristics
- Pathogenesis and Clinical Features of Yersinia Enterocolitica
- Isolation and Identification of Yersinia Enterocolitica
- Yersinia Enterocolitica’s Association with Foods.
1. Introduction to Yersinia Enterocolitica:
Yersinia enterocolitica is one of three species of the genus Yersinia recognized as human pathogens; Yersinia Enterocolitica causes predominantly a gastroenteritis, while Y. pseudo tuberculosis is associated mainly with mesenteric adenitis.
In terms of their social impact, both pale into insignificance when compared to Yersinia pestis, responsible for the bubonic plague which killed an estimated 25% of the European population in the 14th Century.
The genus Yersinia is named after the French bacteriologist Alexander Yersin who, in 1894, first described the organism responsible for the bubonic plague. It was created to accommodate former members of the genus Pasteurella that were clearly members of the Enterobacteriaceae.
In 1964, comparison of B. enterocoliticum with a number of closely related isolates, identified by other workers as Pasteurella spp., led Frederiksen to propose the creation of the new species Yersinia enterocolitica.
Further definition within the genus has occurred with the creation of seven new species from non-pathogenic strains previously described as ‘Yersinia enterocolitica-like’. Those most commonly isolated from foods, Y. frederiksenii, Y. intermedia, Y. kristensenii, Y. mollaretii, and Y. bercovierii can be readily distinguished from Yersinia Enterocolitica on the basis of a few biochemical tests.
The importance of Yersinia Enterocolitica as a cause of foodborne illness varies between countries. In England and Wales, laboratory reports of Yersinia Enterocolitica infections, mostly sporadic cases, increased from 45 in 1980 to more than 590 in 1989 when it outnumbered cases of both Staph. aureus and Bacillus food poisoning.
Although the increase may simply reflect improved detection and reporting, a much smaller increase was observed over the same period in reports of Y. pseudo tuberculosis. Since this is detected by the same methods, it suggests that the figures indicate a genuine increase in incidence.
Yersiniosis is most common in the cooler climates of northern Europe, particularly in Belgium, and in North America where a number of large outbreaks have been reported. It also displays a different seasonal variation from most other foodborne pathogens with a peak in reported cases occurring in the autumn and winter.
2. The Organism of Yersinia Enterocolitica and its Characteristics:
Yersinia enterocolitica is a member of the Enterobacteriaceae; an asporogenous, short (0.5-1.0 by 1-2 µm) Gram-negative rod which is facultatively anaerobic, catalase- positive and oxidase-negative. It can grow over a wide range of temperature, from — 1 °C to +40 °C, with an optimum around 29 °C and has a number of temperature-dependent phenotypic characteristics.
For example, it is non-motile at 37 °C, but motile with peritrichous fiagella below 30 °C. Like other psychrotrophs, though able to grow at chill temperatures, it does so slowly and at 3 °C has been found to take 4 days to increase by 2-log cycles in broth media. It is heat sensitive but with considerable variation between strains; measured D values in whole milk at 62.8 °C have varied from 0.7-57.6 s.
Optimal growth occurs at a pH 7-8 with a minimum (in broth at 25 °C) varying between 5.1 and 4.1 depending on the acidulant used. As the temperature decreases so the minimum growth pH increases. Growth is possible in broth media containing 5% salt but not 7% salt at 3 °C or 25 °C.
Yersinia Enterocolitica can be isolated from a range of environmental sources including soil, fresh water and the intestinal tract of many animals. Surveys have found the organism in numerous foods including milk and dairy products, meats, particularly pork, poultry, fish and shellfish, fruits and vegetables.
Most food isolates are however non-pathogenic and are known as environmental strains. The species can be subdivided by bio-typing, serotyping and phage typing and pathogenicity appears to be associated only with certain types, each with a particular geographical distribution (Table 7.10).
In Europe, Canada, Japan and South Africa human yersiniosis is most frequently caused by biotype 4, serotype O3 (4/O3) and, to a lesser extent in Europe and Japan, bio-serotype 2/O9.
Strains of bio-serotype 4/O3 from Europe, Canada and South Africa can be distinguished by phage typing. In the United States bio-serotype 1/O8 most commonly causes human yersiniosis, although a wider range of serotypes is encountered, e.g. O13a, O13b, O5,27.
A number of techniques other than bio typing and serotyping have been described which claim to distinguish pathogenic from environmental strains of Yersinia Enterocolitica relatively simply and are therefore more within the capacities of routine laboratories.
These include the ability of pathogenic strains to auto-agglutinate at 37 °C, their dependency on calcium for growth at 37 °C, and their uptake of congo red dye, and they are usually associated with the presence of the 40-48 MDa virulence plasmid (see below).
These tests are not completely reliable due to a number of problems such as the expression of plasmid-encoded phenotype in culture, occurrence of atypical strains and the possibility of plasmid loss during isolation. A test for pyrazinamidase activity which is not plasmid mediated may offer some advantages in this respect.
3. Pathogenesis and Clinical Features of Yersinia Enterocolitica :
Illness caused by Yersinia Enterocolitica occurs most commonly in children under seven years old. It is a self-limiting enterocolitis with an incubation period of 1-11 days and lasting for between 5 and 14 days, although in some cases it may persist for considerably longer. Symptoms are predominantly abdominal pain and diarrhoea accompanied by a mild fever; vomiting is rare.
Sometimes the pain resulting from acute terminal ileitis and mesenteric lymphadenitis (inflammation of the mesenteric lymph nodes) is confined to the lower right hand side of the body and prompts a mistaken diagnosis of appendicitis and subsequent surgery.
A problem of post-infection complications such as arthritis and erythema nodosum (a raised, red skin lesion) can occur in adults, the latter particularly in women. This appears to be mainly associated with serotypes O3 and O9 and is therefore more common in Europe.
Ingested cells of pathogenic Yersinia Enterocolitica which survive passage through the stomach acid adhere to the mucosal cells of the Peyer’s patches (gut-associated lymphoid tissue). Adhesion is mediated through bacterial outer membrane proteins that are encoded for on a 40-48 MDa plasmid possessed by all pathogenic Yersinia Enterocolitica.
The plasmid is essential but not the sole prerequisite for virulence since cell invasion is controlled by chromosomal genes. The adhered cell is taken up by the epithelial cell by endocytosis where it survives without significant multiplication and can exert cytotoxic activity. Released into the lamina propria, it invades phagocytic cells and multiplies extracellularly producing a local inflammatory response.
Damage to the absorptive epithelial surface results in malabsorption and a consequent osmotic fluid loss characterized by diarrhoea. Other plasmid-encoded characteristics are thought to contribute to this process, such as the production of outer membrane proteins that confer resistance to phagocytosis, auto-agglutination at 37 °C, and resistance to serum.
A heat-stable enterotoxin (9000-9700 Da) is produced by Yersinia Enterocolitica but its role in pathogenesis, if any, is unclear. It bears some similarity to E. coli ST immunologically and in its ability to induce fluid accumulation in ligated ileal loops and to stimulate guanylate cyclase activity.
Elaboration of enterotoxin in the gut is unlikely since production usually ceases at temperatures above 30 °C. Production in inoculated foods has been shown, but the observed incubation period is inconsistent with a foodborne intoxication.
Finally, the ability to produce the toxin is not confined to pathogenic Yersinia Enterocolitica, but has also been demonstrated in numerous environmental strains and a number of other Yersinia species as well.
4. Isolation and Identification of Yersinia Enterocolitica:
A large number of procedures for the isolation and detection of Yersinia Enterocolitica have been developed. Enrichment procedures usually exploit the psychrotrophic character of the organism by incubating at low temperature, but this has the disadvantage of being slow with the attendant possibility of overgrowth by other psychrotrophs present.
Some workers have included selective agents in their enrichment media but some strains, such as serotype O8, are reported to be sensitive to selective agents. The most commonly used enrichment media are phosphate buffered saline (PBS) or tryptone soya broth (TSB) most usually incubated at 4 °C for 21 days.
Yersinia Enterocolitica and related species are more alkali resistant than many other bacteria so the pH of enrichment media is sometimes adjusted to 8.0-8.3 or cultures subjected to a short post-enrichment alkali treatment.
The best results for the selective isolation of Yersinia Enterocolitica from foods and enrichment broths have been obtained with cefsulodin / irgasan / novobiocin (CIN) agar. In addition to the antibiotics, the medium contains deoxycholate and crystal violet as selective agents and mannitol as a fermentable carbon source.
After incubation at 28 °C for 24 h, typical colonies of Yersinia Enterocolitica appear with a dark- red centre surrounded by a transparent border. Isolates can be confirmed and bio-typed by biochemical tests.
In vitro tests to distinguish between environmental and pathogenic strains of Yersinia Enterocolitica have been referred to above. Techniques using gene probes to detect the virulence-associated plasmid by a colony hybridization test have also been used with some success and offer the possibility of detecting potentially pathogenic strains in foods without the need for lengthy enrichment procedures.
5. Yersinia Enterocolitica‘s Association with Foods:
Pigs are recognized as chronic carriers of those Yersinia Enterocolitica serotypes most commonly involved in human infections (O3, O5,27, O8, O9). The organism can be isolated most frequently from the tongue, tonsils and, in the gut, the caecum of otherwise apparently healthy animals.
Despite this, pork has only occasionally been shown to be the vehicle for yersiniosis, although a case control study in Belgium, which has the highest incidence of yersiniosis, implicated a national prediliction for eating raw pork. In 1988/9 an outbreak of yersiniosis in Atlanta involving 15 victims (14 children) was strongly associated with the household production of pork chitterlings.
A number of outbreaks of yersiniosis have been caused by contaminated milk including the largest hitherto recorded which occurred in 1982 in Tennessee, Arkansas and Mississippi in the United States. In this instance pigs were implicated as the original source of contamination, but not demonstrated to be carriers of the same O13 serotypes causing the infection.
It was presumed that the organism was transferred from pigs, via mud, onto crates used to transport waste milk from the dairy to the pig farm. The crates were returned to the dairy and inadequately washed and sanitized before being used again to transport retail milk.
As a consequence the outside of packs was contaminated with Yersinia Enterocolitica which was transferred to the milk on opening and pouring. It was subsequently demonstrated that the organism involved could survive for at least 21 days on the outside of milk cartons held at 4 °C.
Contaminated water used in the production of beansprouts and in the packaging of tofu (soya bean curd) was responsible for two outbreaks in the United States in 1982. A number of approaches to the control of yersiniosis have been proposed which are generally similar to those proposed for the control of other zoonotic infections such as salmonellosis.
These include pathogen-free breeding and rearing of animals, a goal which may not be achievable in practice, and hygienic transport and slaughter practices.
Work in Denmark on contamination of pork products with Yersinia Enterocolitica has identified evisceration and incisions made during meat inspection as critical control points and has further shown that excision of the tongue and tonsils as a separate operation significantly reduces contamination of other internal organs.