Survival of Listeria monocytogenes in experimentally infected mice

Why Are Some Listeria monocytogenes Genotypes More Likely To Cause Invasive (Brain, Placental) Infection?

Although all isolates of the foodborne pathogen Listeria monocytogenes are considered to be pathogenic, epidemiological evidence indicates that certain serovar 4b lineages are more likely to cause severe invasive (neuromeningeal, maternal-fetal) listeriosis. Recently described as L. monocytogenes “hypervirulent” clones, no distinctive bacterial trait has been identified so far that could account for the differential pathogenicity of these strains.

Although all isolates of the foodborne pathogen Listeria monocytogenes are considered to be pathogenic, epidemiological evidence indicates that certain serovar 4b lineages are more likely to cause severe invasive (neuromeningeal, maternal-fetal) listeriosis. Recently described as L. monocytogenes “hypervirulent” clones, no distinctive bacterial trait has been identified so far that could account for the differential pathogenicity of these strains. Here, we discuss some preliminary observations in experimentally infected mice suggesting that serovar 4b hypervirulent strains may have a hitherto unrecognized capacity for prolonged in vivo survival. We propose the hypothesis that protracted survivability in primary infection foci in liver and spleen—the first target organs after intestinal translocation—may cause L. monocytogenes serovar 4b hypervirulent clones to have a higher probability of secondary dissemination to brain and placenta.

Animal models of listeriosis: a comparative review of the current state of the art and lessons learned

Listeriosis is a leading cause of hospitalization and death due to foodborne illness in the industrialized world. Animal models have played fundamental roles in elucidating the pathophysiology and immunology of listeriosis, and will almost certainly continue to be integral components of the research on listeriosis. Data derived from animal studies helped for example characterize the importance of cell-mediated immunity in controlling infection, allowed evaluation of chemotherapeutic treatments for listeriosis, and contributed to quantitative assessments of the public health risk associated with L. monocytogenes contaminated food commodities. Nonetheless, a number of pivotal questions remain unresolved, including dose-response relationships, which represent essential components of risk assessments. Newly emerging data about species-specific differences have recently raised concern about the validity of most traditional animal models of listeriosis. However, considerable uncertainty about the best choice of animal model remains. Here we review the available data on traditional and potential new animal models to summarize currently recognized strengths and limitations of each model. This knowledge is instrumental for devising future studies and for interpreting current data. We deliberately chose a historical, comparative and cross-disciplinary approach, striving to reveal clues that may help predict the ultimate value of each animal model in spite of incomplete data.

Susceptibility of HRS/J mice to listeriosis: dynamics of infection

Congenitally hairless HRS/J homozygous (hr/hr) mice as well as phenotypically normal littermates (hr/+) were found to exhibit unusual susceptibility to infection with Listeria monocytogenes with 50% of the animals dying within a 10-day period (LD50) at an infecting inoculum approaching 200 microorganisms. In marked contrast to the outbred CD-1 strain as well as other Listeria-susceptible mice, HRS/J hosts are virtually incapable of limiting infection with virulent Listeria. The dynamics of infection reveal early uncontrolled bacterial growth within the peritoneal cavity, followed by a sharp increase of bacterial load in the spleen of both HRS/J homozygotes and heterozygous littermates. Spleen indices obtained for mutant mice indicate substantial splenomegaly which parallels the onset of infection in that organ. Assessment of the exudate population within the peritoneal cavity during infection indicates that HRS/J mice produce an early sustained influx of polymorphonuclear leukocytes while exhibiting a diminished macrophage inflammatory response. Additionally, it was shown that the mutant strain expresses significant increases in the total number of recoverable peritoneal leukocytes in response to other phlogistic stimuli.

Rapid method for the isolation of Listeria monocytogenes from experimentally infected mice

Listeria monocytogenes was successfully isolated from experimentally infected mice by placing homogenized tissues in phosphate-buffered saline (PBS), trypsin, peptone, or pepsin followed by incubation at 37 C for 24 hr. A larger number of Listeria isolates were recovered from the trypsin or PBS splenic homogenate suspensions incubated at 37 C for 2 hr than from the other diluent suspensions. Holding infected tissues at 4 C for at least 3 months did not increase the efficiency of Listeria isolation. Listeria L-forms were not isolated from mice injected with the bacterial form. The in vitro viability of Listeria L-forms suspended in PBS or PBS splenic homogenate was greatly reduced when held at 4 C.