The objective of this study was to detect Listeria monocytogenes in frozen fish in Lagos, Nigeria. A total of 55 frozen fish from two markets (Iyana-Ipaja and Mushin) were analyzed for the presence of L. monocytogenes using Oxoid Listeria Precis method. Out of the 55 samples analyzed, only 15 were positive for L. monocytogenes. The study showed that L. monocytogenes is a contaminant of frozen fish and may pose serious health implications on consumption of the contaminated fish

Amusan, E.E.

Aquatic Commons

307 ,).
INTRODUCTION
Listeria monocytogeucs is a gram positive bacterium and is the causative agent of a serious illness called listeriosis.
The history of listeriosis dates back approximately 60 years. It was first known as a cause of epidemics and sporadic
cases in some 50 species of animals, the disease appears now with increased frequency among human populations at
risk (Seeliger, 1988). Listeriosis was first described in humans in 1936. when it was isolated in three children wid.
bacteremia and/or meningitis. Later. the genus name was changed 10 Listerella followed by Listeria, in bonor of
Lord Lister. Although monocytosis is not a typical finding in human infections. species monocytogenes remained,
and the microorganism is now known as Listeria monocytogenes (Nancy, 2(02). The recognition of L.
monocytogenes as a significant food borne pathogen occurred only in the early 1980's with the outbreaks of food
borne listeriosis (Schlecht et al., 1983). According to DNA homology values as well as to 16S rRNA cataloguing
results, the genus Listeria presently contains two groups of closely related species: one is constituted by L. gravi and
L. murrayi, the other by L. monocytogenes. L. ivanovii, L. innocua, L welshimeri and L. seeligeri. Among these
seven species, only two arc pathogenic for humans and animals: U ..teria monocytogenes and Listeria ivanovii
(Rocourt and Mollarcl, 1988). L. monocytogenes is widely distributed in the environment and occurs in many food
raw materials. It is an intracellular pathogen and once ingested, can penetrate the intestinal endothelial barrier, the
placental or the hematoencephalic barrier (Goldfine and Shen, 2(07). The groups at bigher risk of contracting the
disease are young people, the old, (over 65), pregnant women and immune compromised people, the YOPI, an
acronym coined by De Cesare and Manfreda (2002). Listeriosis is one of the most severe food borne infections,
with low morbidity but high mortality 30% (Rocourt et al.; 200 I). The yearly medical costs and pl"oructivity losses
from the acute illness from food bome Listeria in the USA are estimated to be one, two and three times the costs
caused by Salmonella spp., Canipylobacter spp. and Escherichia coli Ol57:H7, respectively, despite the prevalence
of these diseases being over 500, 700 and 25 times the number of listeriosis cases, respectively (Anonymous, 2000).
Fish are the cheapest source of animal protein in Africa (Claucas and Ward. 1996). fishes have bigh water content
and freeze between temperatures of 0 and 3:C with an average of about 2C (Desrosier, 1978). Freezing kills some
bacteria, but the ones not killed will grow upon thawing (Frazier and Westhoff, 1988). FAO (1989) report indicated
that the method of catching fish contributes to the bacterial load of frozen fish and observed that trawling offish net
along the bottom sediments of water for a long time could result in exposing the fish to a high bacterial
contamination. Consequent upon this, the aim of this work was to detect the presence of L. monocytogenes in frozen
fish in Lagos, Nigeria,
MATERIALS AND METHODS
Sample Collection: Fifty-five frozen fish samples used in this study included four species: Micromesistus pousasou
(Panla), Scomher scombrus (Titus), Pseudotholithus spp. (Croaker) and Sardinella spp. (Shawa). These samples
were bought from lyana- Jpaja and Mushin markets. They were transported in iceboxes to the Microbiology
laboratory at the Nigerian Institute for Oceanography and Marine Research and analyzed immediately upon arrival.
Isolation and ldentification of Listeria monocytogenes
The isolation and identification of L. monocytogenes was carried out using the Oxoid Listeria Precis method. It is a
rapid culture method for the enrichment, isolation and identification of Listeria monocytogenes. For flexibility.
confirmation was validated using the Oxoid Biochemical Identification System (O.8.I.S.) mono tests. For each
sample, 25g was added to 225ml of one-broth Listeria and stomached for 30 seconds. It was then incubated at 3(fC
for 24holiTS.After 24 h enrichment. the broth cultures were agitated and a loopful was inoculated onto Brilliance
Listeria agar. This was incubated appropriately (37°C for 24-48h). The plates were examined for blue-green
colonies with or without opaque white halos; at least 3 suspected colonies were subcultured on Tryptone Soya Agar
(Oxoid) and incubated at 37°C for 24h. Colonies from UteTSA plates wen: preserved on TSA slants for further tests.
ABSTRACT
The objective of this study was to detect Listeria monocytogenes in frozen fish in Lagos, Nigeria, A total of 55
frozen fish from two markets (Iyana-Ipaja and Mushin) were analyzed for the presence of L. monocytogencs using
Oxoid Listeria Precis method. Out of the 55 samples analyzed. only 15 were positive for L. monocytogenes. The
study showed that L. monocytogenes is a contarrnnant of frozen tish and may pose serious health implications.
Keywords: Listeria monocytogeues, frozen fish, Oxoid Listeria Precis.
AMUSAN, E.E.
Fish Technology Department ..Nigerian Institute for Oceanography and Marine Research
nkamboesthC'I-:tIJl·ahoo.coJII
DETECTlO~ Of USTERlA M01\OCYTOGENES IN FROZEN FrSH rN LAGOS, NIGF.RrA
75
308
l_: __PauLL ,",au. 0"""'" _ To"" _
Figure I: Occurrence of L. monocytogenes at Iyana-Ipaja and Mushin Markets
6 • IYANA-IP ;\.l_-\
8
--------
ACKNOWLEOG£MEXrS
The author is grateful to the Director of Fish Technology Department, ;-\igerian Institute: for Oceanography and
Marine Research, for supporting this work.
CONCLUSION
In conclusion. the presence of this pathogen reveals that hygiene and safety of frozen fish is compromised. The
isolation of L. monocytogenes from frozen fish mdicates that contamination may take place at several stages in the
production chain between harvesting and production for consumption. Frozen fish undergo various processing steps
that inactivate this bacterium but cross-contamination nf products could occur and this poses a major problem.
All isolates were subjected to O.8.I.S. mono test. The O.B.I.S. mono is a rapid colourimetric test for the
determination of D-alanyl aminopeptidase (OALAase). It is designed for the differentiation of presumptive Listeria
species that have been isolated from selective media and plated on to a secondary medium for further biochemical
testing. It provides presumptive identification of Listerin monocytogenes but does not replace full biochemical
testing.
RESULTS ANU D1SCt.:SSION
Listeria monocytogenes was detected in 150ut of 55 samples. Iyana-Ipaja and Panla fish (Microll1esislus poutasou )
had the highest occurrence of this organism. The occurrence of L. monocytogenes in frozen fish in the study was
27'Yo.· L. monocytogenes was 1I0tdetected in Croaker fish in both markets. (I was found in Titus fish at lyana-Ipaja
but did not occur in Mushin markets. The results arc shown below in the Table I and Figlirel.Li~terin
monocytogenes has become an Important food associated pathogen. lis ability to persist In food processing
environment and multiply under a wide range of temperature makes it a significant threat to public health. The
results show that the frozen fish in the two markets in Lagos are contaminated with L. monocytogenes, The presence
of L. monocytogenes on frozen fish may have been from the handlers. tables. knives and the environment (Cruz et
al .• 200R). Aquatic creatures are potential bactei iUII1sources of L. monocytogenes. This bacterium has been isolated
regularly from a wide variety of seafood products. including fresh, frozen, fermented. smoked and salted fish
derived from aquaculture as ....ell as captive fisheries (Dabrowski ('f ClI •• 2000: FAO, 1989; Ikeh et (1/., 2010; Salihu
et al. 2008; Weagant et ul., 1988). The occurrence of L. mouocytogencs in seafood is reported (() range from 0% to
75% (Ben L.:mbarek. 1994). however, it was 27% in this study, Tna study conducted in 'sukka, Nigeria, Ikch et al.,
(20 10) reported an incidence of 40% in frozen fish. The number and type of bacteria found on frozen fish is
dependent on many factors, of which, source of the fish contributes the major factor (Eze et (II., 20 II). Also, the
kind and number of microorganisms found on frozen fish is dependent on the source of the fish, additional
contamination introduced in the fishing boat, freezing temperature during storage. severity of freezing process with
respect to lethality to microorganisms and contamination by handlers and market sellers. Karnat and Nair. (1994)
reported the absence of I.. monocytogenes from tropical fish and attributed this to environmental factors. L.
monocytogenes was not detected in Croaker fish 10 both markets. It was not found in Titus fish in Mushin market.
L. monocytogencs is a contaminant of frozen fish: notwithstanding. In this study the detected quantities arc low. In
Nigeria, there have not been reports of any epidemic outbreaks or sporadic cases of listeriosis due to the
consumption offrozen fish but the findings in this research work show)' that human listeriosis could be a problem in
Lagos.
30<)
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Anonymous. (2000). Economics of foodbornc disease: feature. hnp./'w\\ w.crx.usda. Gov /briefing/
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Scomber scombf'lls (Titus)
5310
sPP o5o5Pseudot hotiIII us
(Croaker)
510Sardiuella spp. (Shuwa)
5710
Mtcrontcsi stu»
(Pallia)
poutusou
o
monorvtogrne
.)
Positive
Samples tor
L
Number of
Analyzed
Samples
Number I Positive
uf Samples for L.
\nalY.le mr}llnr.ltogenes
d
~ilmple
r----------+------------4 s
Mushin Markerfi~h Specie .s-Common i\ames Iyana-Ipaja vlarket
Sites/Occurrence
Table I: Occurrence of Li.'/c:na 11l001Or,1'lflgt>II(!S III I rozen FIsh
Samples
310
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Detection of Listeria monocytogenes in frozen fish in Lagos, Nigeria

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