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1.
Distribution and hazards
Listeria monocytogenes (Listeria monocytogenes) is widely present in nature.
It exists in soil, surface water, sewage, wastewater, plants, green feed, and rotten vegetables, so it is easy for animals to eat it.
Bacteria, and spread through the oral-fecal route
.
Listeria monocytogenes is a pathogen of zoonotic diseases and the most common food-borne pathogen.
2.
Inspection method
According to the first method in GB4789.
30-2016, the inspection procedure is shown in Figure 4-8
.
Figure 4-8 Listeria monocytogenes test procedure
Three, matters needing attention
(1) Plate separation
The typical colony morphology of Listeria monocytogenes on CHROMagar Listeria chromogenic medium is a regular blue colony with a transparent halo (diameter <3mm), see color picture 14
.
The transparent halo around the colony is an important feature that distinguishes Listeria monocytogenes from other Listeria species.
It should be noted that although the culture time specified in the standard is 24 to 48 hours, some strains cannot be cultured for 24 hours.
The colony of Listeria monocytogenes on the PALCAM plate is a small gray-green colony with a brown-black hydrolysis circle around it, as shown in color picture 15
.
This medium cannot distinguish Listeria monocytogenes from other species of Listeria, and the colony morphology of all Listeria bacteria on this medium is the same
(2) Dyeing and morphological observation
The bacterial age cannot exceed 48h during Gram staining, because the bacterium is Gram-positive when it is young, but it can easily become Gram-negative when it exceeds 48h
.
Listeria is a small, spherical bacillus, slightly curved in some media, with blunt ends, single, short chain, cells connected to each other in a V shape, or clusters of cells are arranged in parallel along the long axis.
(3) Power test
The national standard method does not give the culture temperature for the power test.
Related Links: Food Microbiology Inspection Salmonella Inspection
