Listeria species are ubiquitous and can be found in soil, water, vegetation, and the gastrointestinal tracts of animals. The genus includes 17 currently identified species, with Listeria monocytogenes and Listeria ivanovii being pathogenic to humans. L. monocytogenes causes the disease listeriosis, which has high fatality rates of 20-30% despite its low incidence. Virulence is multifactorial, with key factors including internalins, listeriolysin O, and the ability to grow intracellularly. Control relies on proper food handling and temperatures above 70°C, as L. monocytogenes can survive and grow at refrigeration temperatures.

1. Listeria species
Johnson Mwove
Jomo Kenyatta University of Agriculture and
Technology
Advances in Food Microbiology and Safety
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2. Occurrence
■ Members of the genus Listeria are ubiquitous.
■ They can be found on
– Decaying vegetation and in soils
– Animal feces
– Sewage
– Silage
– Dairy products - raw milk; soft cheeses
– Meat - fresh and frozen meat
– Vegetables
– Poultry,
– Seafood products Etc 2

3. Species identified
■ The genus Listeria is currently comprised of 17
species (Molla, Yilma, & Alemayehu, 2004; Orsi &
Wiedmann, 2016).
■ Based on their relatedness to Listeria
monocytogenes, the Listeria species are
classified into
– Listeria sensu strictu group
– Listeria sensu lato group.
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4. Species identified
Listeria sensu strictu
■ Listeria monocytogenes,
■ Listeria seeligeri,
■ Listeria ivanovii,
■ Listeria welshimeri,
■ Listeria marthii,
■ Listeria innocua,
Listeria sensu lato
■ Listeria grayi,
■ Listeria fleischmannii,
■ Listeria floridensis,
■ Listeria aquatica,
■ Listeria newyorkensis,
■ Listeria cornellensis,
■ Listeria rocourtiae,
■ Listeria weihenstephanensis,
■ Listeria grandensis,
■ Listeria riparia, and
■ Listeria booriae
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5. Serotypes
■ In addition to the 17 species, various
serovers have been identified
■ In Listeria sensu strictu group, Listeria
are characterized by the possession of
antigens that give rise to 17 serovars.
■ Listeria monocytogenes , is represented
by 13 serovars, (Jay, 2000).
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6. Characteristics of Listeria sensu strictu
■ Gram-positive bacterium
■ Facultative anaerobic
■ Non-spore-forming,
■ Rod-shaped.
■ Flagellar motility - flagellin
■ Positive Voges-Proskauer reaction – can
produce acetoin from fermentation of glucose
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7. Characteristics of Listeria sensu strictu
■ Ability to grow at temperatures as low as 4 °C,
■ Positive catalase reaction – Catalase enzyme
■ Inability to reduce nitrate to nitrite – no nitrate
reductases
■ All species are capable of fermenting D-
arabitol, α-methyl D-glucoside, cellobiose, D-
fructose, D-mannose, N-acetylglucosamine,
maltose, and lactose
■ None of the species can ferment inositol, L-
arabinose, and D-mannitol (Orsi & Wiedmann,
2016)
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8. Characteristics of Listeria sensu lato group,
■ Gram-positive bacterium
■ Non-motile (except for Listeria grayi),
■ Able to reduce nitrate (except for Listeria
floridensis),
■ Are negative for the Voges-Proskauer test
(except for Listeria Grayi and one of the
Listeria Aquatica strains)
■ Can grow at temperatures below 7 °C except,
species in the proposed new genus
MesoListeria (Orsi & Wiedmann, 2016).
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9. Growth characteristics of Listeria monocytogenes
■ Tolerant to salt - 13–14% sodium chloride
■ Can grow under both aerobic and anaerobic conditions (Yousef & Lou,
1999).
Characteristics
Temperature
1 - 45°C
-1.5 - 45°C
(Optimum 30–37°C)
pH 4.1 - 9.6
(Optimum – 6-8)
4.0–9.6
(Optimum 6.0–8.0)
Water activity
0.90 - 0.93 0.90; Optimal 0.97
(Jay, 2000). (Yousef & Lou, 1999)
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10. Pathogenicity
Pathogenic species
■ Two species are considered pathogenic to
humans (Orsi & Wiedmann, 2016):
– Listeria monocytogenes
– Listeria Ivanovii,
■ Listeria monocytogenes causes listeriosis
■ Although the incidence of listerosis is low, what
is significant is that very high fatalities ranging
from 20 to 30%.
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11. listeriosis
Symptoms
■ Typical symptoms (Orsi & Wiedmann, 2016; Ramaswamy et al.,
2007) include:
■ Septicemia,
■ Abortions, and
■ Encephalitis
■ Gastrointestinal illnesses without systemic infections
Groups at risk include
■ Pregnant women,
■ Neonates,
■ Immunocompromised patients (as in HIV/aids infection),
■ The elderly.
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12. Pathogenicity
Virulence properties
■ Virulence is multifactorial - Many factors affecting the
pathogenicity of Listeria monocytogenes
– its capacity for intracellular access and growth – can
cause parasite-directed endocytosis – Internalin (A
&B)
– iron compounds – Increased growth
– superoxide dismutase - offers a defense against
phagocytes
– bacterial surface protein
– hemolysin
■ The most significant virulence factor associated with
Listeria monocytogenes is listeriolysin O (Jay, 2000;
Yousef & Lou, 1999)
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13. Pathogenicity
■ Ingestion - may survive the stomach environment and penetrates
the intestinal epithelial cells.
■ The organism is then taken up by macrophages and non-
phagocytic cells - surface protein internalin is required, as it binds
to the receptors on the host cells to instigate adhesion and
internalization.
■ Following internalisation into host cells, the bacteria releases
listeriolysin O which breaks down the vacuole wall
■ They then spread from cell to cell by a process involving actin
polymerisation.
■ In infected hosts, the bacteria cross the intestinal wall at Peyer's
patches to invade the mesenteric lymph nodes and the blood.
■ The main target organ is the liver, where the bacteria multiply
inside hepatocytes.
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14. Infectious dose
■ The approximate infective dose of L. monocytogenes is
estimated to be 10 to 100 million colony forming units
(CFU) in healthy hosts, and only 0.1 to 10 million CFU in
individuals at high risk of infection (Farber, Ross, &
Harwig, 1996).
■ The overall mortality is still high (25 to 30%)
Control
■ Overall, standard pasteurization protocols (Jay, 2000).
■ Temperatures above 50°C are lethal to Listeria
monocytogenes .
■ Freezing can also lead to a reduction in Listeria
monocytogenes numbers (Yousef & Lou, 1999).
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15. Standards on Listeria monocytogenes
■ The EU has established microbiological food safety
criteria for Listeria monocytogenes in RTE foods in
Regulation (EC) No 2073/20053 of 15 November 2005
on microbiological criteria for foodstuffs to ensure
compliance.
■ EAS 451-1:2007, Microbiology of food and animal
feeding stuffs — Horizontal method for the detection
and enumeration of Listeria monocytogenes (East
African Community Secretariat, 2007)
■ KS CAC/GL 61:2007: Guidelines on the application of
general principles of food hygiene to the control of
Listeria monocytogenes in foods / Food Hygiene
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17. Conclusion
■ Members of the genus Listeria are ubiquitous.
■ Genus has 17 currently identified species
■ Several serovars have also been identified,
Listeria monocytogenes , is represented by 13
serovars
■ Two species are considered pathogenic to
humans (Orsi & Wiedmann, 2016):
– Listeria monocytogenes
– Listeria Ivanovii,
■ Virulence of these bacteria is multifactorial.
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18. THANKS
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