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Infection and Immunity, July 2001, p. 4657-4660, Vol. 69, No. 7
Department of Pathology, University of
Cambridge,1 and Public Health
Laboratory, Addenbrooke's Hospital,2 Cambridge,
England
Received 3 January 2001/Returned for modification 29 January
2001/Accepted 29 March 2001
The infectivities of 66 Listeria monocytogenes isolates
were assessed by intragastric inoculation of mice. Eight were poorly infective. Serovars 4b and 1/2 were more infective than serovars 3 and
4nonb. A noninfective isolate was cleared more rapidly from the cecum
than were infective isolates, suggesting that survival in the gut may
relate to infectivity.
Although Listeria
monocytogenes is a foodborne pathogen and is relatively common in
foods (7, 16, 30), human infections are rare. Reasons
include host susceptibility and dose, but little is known about how
L. monocytogenes causes disease or whether all isolates are
equally infective via the oral route in humans.
Virulent isolates are hemolytic, with the hemolysin (5)
enabling bacteria to escape from phagolysosomes; however, even hemolytic isolates vary in virulence for mice (11, 29),
and additional virulence genes have been identified (23,
25). Most studies of virulence have used intravenous (i.v.) or
intraperitoneal routes of inoculation of mice or cell culture models to
assess the behavior of wild-type, laboratory, or genetically
manipulated strains (4, 8, 11, 19, 28). Yet the ability of
L. monocytogenes to survive in the gastrointestinal tract
and invade through the mucosa may be limiting factors for natural
infection and require virulence factors different from those necessary
for survival and growth in the reticuloendothelial system or for
invasion and multiplication within cells in vitro.
The behavior of L. monocytogenes in mice following
intragastric (i.g.) inoculation has been studied previously (1,
14, 24), but the number of isolates examined has been small.
This note reports on a study of the infectivity of 66 phenotypically characteristic smooth hemolytic isolates from different sources and
belonging to a range of serovars following i.g. inoculation into
immunocompetent mice. In a preliminary experiment, growth curves were
determined for specially selected isolates to establish that
differences in infectivity could be detected by i.g. inoculation and to
choose the best time to screen for infectivity for a large number of isolates.
Preliminary in vivo growth curves.
Methods for preparation of
mouse-passaged log phase cultures, i.g. inoculation of BALB/c mice, and
viable bacterial counts in liver and spleen have been described
previously (2). Cecal contents were also removed and
weighed, dilutions performed in phosphate-buffered saline (PBS), and
viable counts were performed on PALCAM agar plates (Oxoid,
Basingstoke, United Kingdom), which were incubated for 48 h at
30°C.
0019-9567/01/$04.00+0 DOI: 10.1128/IAI.69.7.4657-4660.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Variation in the Infectivity of Listeria
monocytogenes Isolates following Intragastric Inoculation of
Mice
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FIG. 1.
Viable bacterial counts in spleen (a), liver (b), and
cecal contents (c) of mice inoculated i.g. with 2 × 109 CFU of selected isolates of L. monocytogenes. Each point represents the mean of the
log10s of the viable count per gram of tissue for three
mice on days 1 to 4 and two mice on days 5 and 7.
Screening experiments. The infectivities of 66 L. monocytogenes isolates of different serovars from different sources were then assessed. Each serovar group included both clinical and food isolates except 4nonb, for which no clinical isolates were available. Four groups also contained environmental isolates. Three mice were inoculated i.g. with 2 × 109 CFU, and viable bacterial counts in spleen and liver were determined 3 days later, as described previously (2). The means of the log10s of the viable counts were then calculated. Isolates with a mean of <103 CFU in the spleen and/or liver were arbitrarily defined as being poorly infective. Many experiments were repeated at least once to confirm results.
Isolates demonstrated a wide variation in infectivity (Fig. 2). The majority were of intermediate or high infectivity, with 103 to 106 CFU being recovered from the spleen and liver. Eight isolates (12%) were poorly infective, and one of these, a food isolate of serovar 1/2b, was not recovered from any livers or spleens.
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FOOTNOTES |
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* Corresponding author. Present address: Department of Pathology, Adelaide University, Adelaide, South Australia 5005, Australia. Phone: 61 8 8303 5390. Fax: 61 8 8303 4408. E-mail: angela.barbour{at}adelaide.edu.au.
Present address: Public Health Laboratory, Department of Pathology,
Dorset County Hospital, Dorchester, Dorset DT1 2JY, England.
Present address: Department of Microbiology and Immunology,
Medical School, University of Newcastle, Newcastle upon Tyne NE2 4HH, England.
Editor: D. L. Burns
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