Yersinia spp. infections

S G FENWICK AND M G COLLETT

GENERAL INTRODUCTION: FACULTATIVELY ANAEROBIC GRAM-NEGATIVE RODS

Introduction

The genus Yersinia is named after Alexandre Yersin, a French bacteriologist, who first isolated the plague bacillus in Hong Kong in 1894 while investigating a catastrophic epidemic of bubonic plague that killed an estimated 60 000 people. He named the bacterium Pasteurella pestis. Taxonomical revision now places this organism in the Yersinia genus, the oldest member of which is Y. pseudotuberculosis, so named because in 1883 Malassez and Vignal described a disease in guinea pigs characterized by nodules in internal organs resembling tuberculosis (cited by Schiemann, 1989).¹⁰⁷ Yersinia enterocolitica has undergone a number of name changes since it was first isolated by McIvor and Pike in 1934, and called Flavobacterium pseudomallei. Since then it has been named Bacterium enterocoliticum, Pasteurella pseudotuberculosis X, Pasteurella pseudotuberculosis atypique, Pasteurella pseudotuberculosistype b and Les germes X. In 1964, Frederiksen demonstrated that these strains constituted a separate species which he named Yersinia enterocolitica, and at the same time proposed that the genus Yersinia be included in the family Enterobacteriaceae (cited by Bottone, 1981).¹¹ The relationship between Y. pestis, Y. pseudotuberculosis and other Yersinia spp. was defined by DNA hybridization studies in 1980.⁷

Yersinia pseudotuberculosis infections occur in a wide range of mammalian and avian species and epidemics in laboratory animals and captive birds are common, particularly in the northern hemisphere.⁸⁹ There is an hypothesis that the emergence of Y. pseudotuberculosis, which has a very close antigenic relationship to Y. pestis, may have limited the spread of plague in the last pandemic in Europe, and that the spread of infection to other parts of the world is due to shipments of livestock and/or bird migration.^{77, 113}

Yersinia spp. pathogenic to livestock include Y. pseudotuberculosis⁷ and Y. enterocolitica. ⁶ Yersinia pseudotuberculosis causes sporadic outbreaks of gastroenterocolitis and/or abortion in cattle, sheep, goats and farmed deer, while Y. enterocolitica, which is generally less virulent than Y. pseudotuberculosis, has been incriminated in outbreaks of enterocolitis in sheep, goats and pigs. Strains of these organisms also cause enteric infections in humans and non-human primates.

The first descriptions of disease in sheep due to Y. pseudotuberculosis were from Australia, by Gilruth in 1909 and Hindmarsh in 1929.⁹⁹ The incrimination of Y. pseudotuberculosis as a possible cause of bovine abortion was first reported by Mair and Harbourne in 1963.⁷⁹ Livestock infections with Y. enterocolitica and Y. pseudotuberculosis have been reported principally from New Zealand and Australia³⁷ but also from many other regions including Canada, the UK, Nigeria, China, Russia, India and Brazil. Infections with these organisms have not been recorded from South Africa. Various other Yersinia spp., considered non-pathogenic, are periodically isolated; they are, however, rarely believed to be significant and are found as commensals in the intestine, soil and water.¹¹⁷

Aetiology

Members of the genus Yersinia are Gram-negative coccobacilli, short rods or ovoid in form, ranging in size from 0,5–1 × 1–3 µm. Ovoid forms tend to be bipolar. The organisms are aerobic and facultatively anaerobic. They do not form endospores or true capsules. The production of flagella depends on the temperature of incubation, being absent at 37 °C but occurring at 22 °C, except in the case of Y. pestis which is non-motile at both temperatures. Using conventional media, Yersinia spp. are infrequently isolated because the colonies they produce after 48 hours of incubation are very small and easily overgrown by normal flora. In addition, rapid production of acid by bacteria such as Escherichia coli causes precipitation of bile salts in enteric media such as MacConkey agar, and this inhibits the growth of Yersinia spp.⁷¹

However, Y. enterocolitica and Y. pseudotuberculosis outgrow competitive microflora at 4 °C,¹¹ and this feature is embodied in a technique known as cold-enrichment.⁹⁵ A two-step enrichment procedure is superior to other methods for the recovery of pathogenic Y. enterocolitica serotypes.⁸⁷ Plating onto a Yersinia selective medium, such as CIN agar (cefsulodin, irgasan, novobiocin), which contains a Yersinia antimicrobic supplement (Difco, USA), is used routinely after enrichment.^{16, 48, 52, 71, 96, 107, 110, 112} Growth of some strains of Y. enterocolitica and Y. pseudotuberculosis have been shown to be inhibited by CIN agar.³⁹ The optimum incubation temperature for Yersinia is 28 to 30 °C. After 40 hours of incubation on selective media, characteristic deep-mauve colonies 1,5 mm in diameter are readily identifiable as Yersinia sp.¹¹⁰

The ability of Y. enterocolitica and Y. pseudotuberculosis, some strains of which are human pathogens, to survive and multiply at 4 °C, the temperature used for the preservation of chilled food, makes them of concern to the human food industry.^{56, 64, 94, 108}

Yersinia spp. are catalase- and oxidase-negative. Acid is produced by fermentation of various carbohydrate substrates. Proteolytic activity is variable, as is production of urease. Nitrates are usually reduced.^{6, 8} The DNA base composition is within the range 46 to 49 per cent GC.¹³