Listeria Is A Bacterium With Many Subspecies Biology Essay


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Martin Fischer once said "Don't forget that the flavours of wine and cheese depend upon the types of infecting microorganisms." Flavours as well as some micro organism present in food could affect health, i.e. food could become one of the sources of many fatal diseases. One of those infecting micro-organism is Listeria monocytogenes .WHO estimates that foodborne and waterborne diarrhoeal diseases taken together kill about 2.2 million people annually, 1.9 million of them children (WHO 2010). Governments all over the world are increasing their effort to improve food safety, and carrying out range of work to ensure that our food is safe to eat, and hence they are implementing food safety management system such as HACCP in every food manufacturing company.

What are Food safety management systems?

Food safety managements systems are set of programs or guidelines which should be applied by food companies or any organization related to food supply or food consumption. One of the important objectives of food safety management system is to ensure that consumer is safe from any foodborne illness. Recently many serious outbreaks of foodborne disease have been reported from every continent, which describe the significant importance of these diseases in both the public health and social sector. "Foodborne illness occurs when a person consumes food contaminated with pathogenic bacteria, viruses or parasites. This condition is often called "food poisoning" (Anon 2009). The most common bacteria which cause food poisoning are Campylobacter, Salmonella, E-coli O157, Listeria, Clostridium, Staphylococcus, Bacillus Cerus, etc. Listeria is more likely to cause death than any other bacteria; hence detail study on Listeria is required to control the outbreak by taking some precautionary measures which can help to save many lives.


Listeria is a gram-positive rod shaped bacterium. If we observe it under a microscope, Listeria species sometimes appear as gram positive rods arranged in short chains. In smears they can be coccoid, hence they are usually mistaken for Streptococci, and longer cells resembles as Corneybacteria. There are many species of Listeria but only Listeria monocytogenes is the major cause of Listeriosis. To distinguish Listeria monocytogenes from different species of Listeria, Haemolytic activity on blood agar can be used but still this method is not an absolute method of differentiation; further biochemical assays may be necessary to distinguish between different Listeria species. (Todar 2008)

Classification of Listeria Monocytogenes:















Listeria monocytogenes is a facultative anaerobic, motile (20-250C) short gram positive rod of regular appearance, cells are 0.5 - 2.0 µm in length and 0.4 - 0.5µm in diameter. Optimum temperature of growth is 35-370 C but strains can exhibit wide growth temperature ranges from 1- 450 C. Majority of Listeria monocytogenes can survive in cold temperatures hence they are usually grouped in "psychrotropic spoilage bacteria." It has also been seen that Listeria can survive minimum pasteurization heat treatment.

Glass and Doyle (1989) studied survival and growth of Listeria in range of foods during storage at 40C for up to 12 weeks. Growth of L monocytogenes was best on sliced chicken and turkey. Microbial count ranging from 103 to 105 cfu/g was noticed in 4 weeks when compared with other food items like Ham, which showed a count of 104 cfu/g in 6 weeks. Based on this observation, the author concluded that pH was the more crucial factor in determining growth patterns. L monocytogenes either grow poorly or not at all on products that have pH less than 5. However, George et al (1988) using lab media rather than foodstuff observed that L monocytogenes were capable of flourishing between pH ranging from 4.4 (200 C) to 4.6 (100 C) and could not be sustained below a pH of 5.2 at 40 C. This suggests that the storage temperature is an important factor in determining growth control with respect to the pH value (Forsythe et al 1998).


Listeriosis is a serious infection which is mainly cause by eating food contaminated with the bacterium Listeria monocytogenes. The high risk groups which are normally affected by Listeriosis include elderly people, pregnant women, fetus of pregnant women, and people with weakened immune system. Listeriosis is a rare disease with a very high fatality rate, causing about 43 percent of the food poisoning deaths in the United States. L monocytogenes can also cause flu like symptoms in healthy individuals if the infected food contains large number of Listeria colonies. In advance stages of the disease the subject might suffer from meningitis, frequently accompanied by blood infection. In pregnant women, usually symptoms of mild influenza like illness may occur but infection can cause serious effect on the fetus which normally leads to abortion, still birth, delivery of acutely immune compromised infant (CDC 2009). The infective dose for Listeria is very low. Even 100 to 1000 cells of Listeria can cause infection. According to Centre for Disease Control and Prevention (CDC): "The annual incidence of Listeriosis decreased by 34% between 1989 and 1993; an analysis of the incidence trend from 1996 to 2006 revealed a 36% decline. However, outbreaks continue to occur. In 2002, an outbreak that resulted in 54 illnesses, 8 deaths, and 3 fatal deaths in 9 states was traced due to consumption of contaminated turkey meat." According to WHO Listeriosis is considered the emerging food borne disease because the role of food in its transmission has only been recently recognized. Outbreaks of Listeriosis have been reported from many countries, including UK, Australia, Switzerland, France, Canada and the United States.

2008 Canadian Listeriosis outbreak

In July 2008 there was a major outbreak of Listeriosis which was directly linked to deli meat supplied by "Maple Leaf foods" a major food company in Canada. According to "The Public Health Agency of Canada" the total numbers of deaths reported were 23, where Listeriosis was underlying or contributing cause and 57 confirmed cases of Listeriosis were reported from different states of Canada. Ontario lab results have confirmed that Listeriosis was an underlying or contributing cause of death during the outbreak. It was reported that about 220 products of Maple Leaf food were contaminated; each stamped with "97B" near the best before date and most of them were Deli meat. The following table given by The Public Health Agency of Canada (PHAC) reflects the number of confirmed cases and deaths where Listeriosis was the underlying or contributing cause. (CDC 2009)


Confirmed Cases

Deaths where Listeriosis was the underlying or contributing cause



















New Brunswick






Deli meat are also called as ready to eat meat (RTE) which are in form that is edible to eat without any additional processing to achieve food safety and may require additional preparation for palatability, aesthetic and culinary purposes


The deli meat production involves two major Processes Upstream process and Downstream process. If the conventional definition of critical control point is considered, then the preparatory steps where animal is slaughter cannot be consider as CCP's. But Gill et al (1997) have suggested that overall hygienic quality of cooked meat can be improved only if we improve the hygienic quality of raw meat. To minimise the risk of contamination all upstream processes should employ CCP's, with special emphasis on steps at which Listeria could enter from animal onto meat. From handling of raw meat to distribution of cooked meat there are various process at which Listeria can cause contamination, hence extra care should be taken to avoid Listeria contamination at those steps. NACMCF (1993) has proposed seven Critical Control Points which should be implemented during upstream processes for controlling and minimising the risk of contamination. These steps include skinning, post-skinning, evisceration, final washing, chilling, storage, and labelling. The elements of hygiene will differ according to the type of processing. There will be significant difference between hygiene standard required in plant manufacturing cooked meat and hygienic standard required in a place where animals are slaughter .Before implementing HACCP in deli meat industry there are certain PRP's which are very important and should be strictly followed .To identify Critical Control Points at each process steps Codex Decision tree can be useful, which consist of some questionnaire by which we can decide whether to apply CCP or PRP at that particular step.

The first step before implementing HACCP plan is to develop a flow sheet diagram for the production of deli meat. Flow sheet diagram will provide clear, simple outline of the processes involved in production of deli meat.

Flow sheet diagram for the Production of Deli meat

Pre-Slaughter condition

Arrival of animal & holding

Upstream processing

Washing/Chemical rinsing

Stunning & Exsanguinations


Steam pasteurization

Carcass fabrication/Grinding




Downstream processing

Vacuum packaging

Aseptic Packaging



Proper Labelling

The HACCP team should perform onsite inspection of all the processes to verify the precision and completeness of flow diagram. Once those initial tasks are completed, then HACCP team can implement HACCP plan in deli meat production which should give certain critical control points at steps where control actions are necessary to avoid Listeria contamination. As the production consist of two major processes, HACCP analysis is done separately for both the processes.



Risk Assessment

Process step



Hazard elimination




Overall concern



Listeria monocytogenes

Present on skin of animal

Use of chemicals


Likely to be present




Post-skinning wash

Listeria monocytogenes

Pathogen left after skinning

Temp up to 75°C






Evisceration /removal of unwanted body parts

Listeria monocytogenes

Conatami nation by person or object

Sterilization /sanitation programme






Final Washing

Of Carcass

Listeria monocytogenes

Cross contamination

Steam pasteurization




Most important step



Listeria monocytogenes

Listeria growth

Maintain the temp.below 4°C






Process step


CCP description

Control actions

Critical limits

Target tolerance




Listeria monocytogenes

Chemical dehairing

Use of chemicals to remove pathogens

Dehairing with minimum microbial load

Minimum microbial load

Validate by microbiological tests

Regular test should be conducted. All the skin should be removed

Final Washing

Of Carcass

Listeria monocytogenes

Steam pasteurization

Hot water spraying

Temperature of hot water should be more than 75°C


Validated by Calibrated thermometer

Proper observation of temperature log


Listeria monocytogenes


Maintain below 4°C& less storage time

Temperature should remain 4°C

Absence in 25 g

Validated by calibrated thermometer

Proper observation of temperature every week


The HACCP team should appoint monitoring officer at every CCP's in case if any deviation occurs at certain step than that officer could inform to staff that is trained in HACCP and responsible for that process. As the production of meat starts with collection of animal the plant operator should purchase the animals use for meat processing from certified supplier who is applying certain hygienic rules in terms of facilities, herd management and transport of animal. In addition it is important to have some records from the supplier which states the nature and origin of animal feed used, veterinary medicinal drugs or other treatment administered to animal, occurrence of any disease that may affect safety of product. This information should be available and checked by competent authorities when animal enters the slaughterhouse. From above table it can be observed that there are certain pre-requisite programmes that should be implemented to minimise the risk of Listeria contamination. Before implementing control actions at various critical control points plant operators should follow certain PRP's which could include good hygienic practices. For good sanitation programme it is always better to divide the sanitation programme into separate programs.

Personal hygiene: All employees should be trained for good hygienic practices. All employees and other persons who enter and work in manufacturing plant must follow strict sanitation programme and regular medical examination of all the employees should be carried out every month.

Pre-operational sanitation: Prior to start of daily operation it should be ensure that all the equipments used for cutting or processing of raw meat should be sanitized properly.

Operational sanitation: During processing all sanitary practices should be followed strictly. High risk area should be separated from low risk area and proper transport of deliveries from low risk to high risk area should be control.

Maintenance: Every month preventive maintenance and proper schedule should be establish and documented. Cleaning of premises every week

Water quality: Water use for various processes should be free from any contamination i.e. sterilized water should be used.

Proper disposal of waste.

Control actions that can be implemented at various CCP's

Slaughter lines must be designed such that it can allow continuous or constant progress of slaughter process and which can decrease the rate of cross contamination in slaughter line.

There should be separate rooms for emptying and cleaning of stomachs and intestine of animals.

It should be ascertained that there is separation of time and space in between stunning, bleeding, evisceration and further skinning process.

Washing of final carcass is an important step since steam pasteurization could kill most of the microbial load present onto meat. Temperature of hot water should be maintained.

Storage of raw meat is considered to be one of the most critical steps.

Storage temperature should be maintain around 4°C and it should be ensured that raw meat is sent for further processing and it should not be stored for longer time.

Downstream processing

In downstream processing HACCP plays a vital role in decreasing the microbial load onto the meat and ensuring that the processed meat is safe to eat. Downstream processing includes steps which reduce or limit the growth of Listeria if these steps are analysed in HACCP we can easily identify the Critical control points at those steps and hazard elimination or control actions will ensure reduce number of Listeria in meat or even eliminating the risk of contamination by Listeria.

Process step



Aseptic Packaging




Design of plant should be such that raw meat handling area should be at a distance from cooked meat handling area which can reduce the risk of cross contamination

To minimise the risk, all preparation steps should employ GMP level of hygiene with special emphasis on steps at which Listeria can enter onto meat.

Curing of meat with certain herbs which can prevent the growth of Listeria and increase the shelf life of cook meat.

If the water activity of meat is between 0.92-0.95 then pH should be reduced to 5.1-5.2.

Ensure meat, particularly in case of chicken that it is heated to minimum temperature of 75°C.

Aseptic packaging or vacuum packaging to avoid cross contamination. Packaging section should be considered as high care unit.

Labelling is also one of the important steps in production of processed meat. Consumer should be given proper instruction related to storage and handling of processed meat. Proper labelling should also include date of manufacturing date till which product could be used once its open and list of ingredients.

Proper distribution in well refrigerated transportation for maintaining the given temperature to various retail shops.

Proper documentation keeping all the records including lab analysis sheets for the audits.

If there is any deviation then corrective action may includes destroying product, holding product and testing it, and recalling product if it's already in commerce.

HACCP Verification and Documentation Every month company should carry out audits to ensure that HACCP plan is working efficiently. An audit checklist depends upon the seven principles of HACCP which has been use to draw up HACCP plan. Verification ensures that HACCP plan is adequate and working as intented.Verification includes activities such as review of HACCP plan, CCP records, and review of microbial lab analysis sheets. Every CCP records lab sheets should be well documented and it should be user friendly and easily accessible. Every documents of HACCP should include

Title of the document

Product description.

Ingredients used and equipment.

Critical limit.

Corrective actions.

Responsible personnel.

Signature of employee and supervisor.

DEVIATION: Deviation occurs when one of the critical limit at certain critical control point is not met correctly or deviation also means when effective monitoring is not done properly if deviation occurs then it means that product could be unsafe for consumers hence following corrective activities can be done.

Identifying and eliminating the deviation.

Ensuring CCP is back under control after corrective action is taken.

Ensuring that deviation does not occur again.

Ensure that no unsafe products enter in market.

Proper documentation is important in case of deviation.


Due to globalization and increased world-wide distribution of food products, food safety should require increased attention all over the world. Since "prevention is better than Cure", it is very important that we make sure that the meat we eat is safe from Listeria, as meat is general food product which can be consume by high risk group who are mostly affected by listeria. It should be ensured that there is proper implementation of HACCP . Every Meat processing industry and food processing industry should implement HACCP which will reduce the fatality rate due to Listeria. The tragic event in Canada has taught lessons to the entire food government agency and now they are ensuring that similar events never happen again.

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