Characterization of the interaction between Yersinia enterocolitica biotype 1A and phagocytes and epithelial cells in vitro

Abstract

Yersinia enterocolitica strains of biotype 1A are increasingly being recognized as etiological agents of gastroenteritis. However, the mechanisms by which these bacteria cause disease differ from those of highly invasive, virulence plasmid-bearing Y. enterocolitica strains and are poorly understood. We have investigated several biotype 1A strains of diverse origin for their ability to resist killing by professional phagocytes. All strains were rapidly killed by polymorphonuclear leukocytes but persisted within macrophages (activated with gamma interferon) to a significantly greater extent (survival = 40.5% +/- 17.4%) than did Escherichia coli HB101 (9.3% +/- 0.7%; P = 0.0001). Strains isolated from symptomatic patients were significantly more resistant to killing by macrophages (survival = 48.9% +/- 19.5%) than were strains obtained from food or the environment (survival = 32.1% +/- 10.3%; P = 0.04). Some strains which had been ingested by macrophages or HEp-2 epithelial cells showed a tendency to reemerge into the tissue culture medium over a period lasting several hours. This phenomenon, which we termed "escape," was observed in 14 of 15 strains of clinical origin but in only 3 of 12 nonclinical isolates (P = 0.001). The capacity of bacteria to escape from cells was not directly related to their invasive ability. To determine if escape was due to host cell lysis, we used a variety of techniques, including lactate dehydrogenase release, trypan blue exclusion, and examination of infected cells by light and electron microscopy, to measure cell viability and lysis. These studies established that biotype 1A Y. enterocolitica strains were able to escape from macrophages or epithelial cells without causing detectable cytolysis, suggesting that escape was achieved by a process resembling exocytosis. The observations that biotype 1A Y. enterocolitica strains of clinical origin are significantly more resistant to killing by macrophages and significantly more likely to escape from host cells than are strains of nonclinical origin suggest that these properties may account for the virulence of these bacteria.

FIG. 1

Time course of escape of Y. enterocolitica W22703c (biotype 2), A3064 (clinical, biotype 1A), T83 (clinical, biotype 1A), 61525 (clinical, biotype 1A), 937 (clinical, biotype 1A), AM5 (nonclinical, biotype 1A), IP2222 (nonclinical, biotype 1A), Eco88 (nonclinical, biotype 1A), and AM8 (nonclinical, biotype 1A) from HEp-2 epithelial cells. Bacteria were incubated with HEp-2 cells for 3 h and then exposed to 100 μg of gentamicin per ml for periods ranging from 1.5 to 16.5 h. At the end of this period, gentamicin-containing medium was replaced with fresh medium without gentamicin. After a further 3-h incubation, the medium was removed for enumeration of viable bacteria. Data are the mean of three independent experiments.

FIG. 2

Transmission electron micrographs of macrophages incubated for 1 h with Y. enterocolitica 937 (clinical, biotype 1A) (A), for 19 h with Y. enterocolitica 937 (B), and for 19 h with Y. enterocolitica AM5 (nonclinical, biotype 1A) (C). Findings similar to those shown in the figure were observed with three other strains of Y. enterocolitica biotypes 1A (two clinical and one nonclinical). At 1 and 19 h, strains of clinical origin appeared to induce phagosome fusion (arrowheads), and after 19 h of incubation, bacteria were observed within spacious vacuoles (s) or tight vacuoles (large arrow). In contrast, nonclinical isolates were identified in small numbers and often appeared degraded. Small arrows indicate bacteria. n, nucleus. Bar, 2 μm.