Listeria

Listeria is a type of microorganism that is represented by 6 types of gram-positive rod-shaped bacteria. The microorganism is named after the famous English surgeon, the founder of aseptic and antiseptic methods in medicine, Joseph Lister.

Listeria is quite common on the planet: it is especially well-known in veterinary medicine, as it often affects domestic animals. But in some cases, the microorganism can be dangerous for humans.

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Listeria bacteria

Listeria bacteria are gram-positive, non-spore-forming, rod-shaped microorganisms. Among the 6 common species of these organisms, Listeria monocytogenes is especially important for medicine, as it can cause disease in both animals and humans. Other species of Listeria bacteria, such as Listeria ivanovii, affect only wild and domestic animals, while Listeria innocua, Listeria seeligeri, Listeria grayi, and Listeria welshimeri are generally recognized as harmless microorganisms.

Most experts attribute the few reports of human infections with these microorganisms to the fact that there are not many laboratories in our country that can detect listeria and antibodies to it. According to statistics, the number of human infections with listeria is relatively small, but the number of fatalities in patients with listeriosis is higher than average, and in infants – up to 75%.

Infection during pregnancy is especially dangerous, as it can cause difficulties with bearing a child, stillbirth, premature birth and early mortality of newborns. In addition to women and infants, listeria often causes illness in elderly people and old people who suffer from septic and meningoencephalic forms of listeriosis. First of all, people who are weakened, with poor immune protection, with concomitant oncology or autoimmune pathologies get sick. Over the past few years, the number of cases of listeriosis in people taking drugs has increased.

Listeria monocytogenes

Listeria monocytogenes, the causative agent of listeriosis, is a small, mobile rod that does not form spores and is perfectly stained by the Gram method (Gram+). These organisms belong to the corynebacteria, the most famous representative of which is the diphtheria bacillus. For this reason, microbiologists often characterize this microorganism as a variety similar to diphtheria, and only after careful analysis do they distinguish the microorganism's belonging to the Listeria bacteria.

This organism has the form of a short straight rod with dimensions of approximately 0.4-0.5 by 0.5-2 microns, with smoothed tips. Microorganisms can be located one by one, or in small links of 4-5 bacteria, very rarely - in long chains. They are not capsule-forming.

When placed on a nutrient medium, this microorganism grows in the form of small (up to 2 mm), smooth-convex, oblong colonies, gray-blue or greenish in color, semi-transparent. If the microorganisms are cultivated on a liquid medium, then uniform turbidity is detected with the subsequent appearance of sediment. If the nutrient medium is semi-liquid, then the colonies germinate to a greater extent on the surface. The grown material has a curd or whey odor, which indicates the accumulation of residual substances of carbohydrate metabolism in the culture medium.

Listeria structure

Listeria bacteria are mobile and can have from 1 to 4 flagella, thanks to which they move and perform peculiar "somersaults". They are most active at 20-25°C, and growing at 37°C sharply reduces their mobility, to the point of complete cessation.

Listeria bacteria are facultative aerobes (that is, they require the presence of oxygen for their development) and germinate well in glucose-serum media.

The organisms are very stable in the outdoor environment, can successfully grow in a wide range of temperatures (from +1°C to +45°C) and pH (4-10). Listeria bacteria feel good at low temperatures, can reproduce already at t° +4-6°C in soil, in water bodies, on the surface of plants, in corpse meat, in food products.

The life cycle of the Listeria bacteria also proceeds without problems in the refrigerator: microorganisms are able not only to maintain their vital activity, but also to actively multiply and accumulate on products, while other microorganisms slow down and stop their development. For this reason, placing contaminated food in the refrigerator will not protect a person from infection. Salting products has the same effect on the Listeria bacteria: bacteria can withstand up to 20% salt solution.

However, listeria bacteria die quite quickly when boiled, although they die only within half an hour when pasteurized (at t ° +60-70 °). But listeria can also withstand high temperatures if they are located in the middle of cellular and tissue structures. For example, there are known cases of listeriosis developing after consuming pasteurized dairy products: the bacteria did not die, being in single leukocyte and epithelial cells, which were subsequently found in the sediment.

In the open air, the Listeria bacteria dies when treated with disinfectants (chloramine, formalin), as well as under the influence of ultraviolet rays.

In the external environment, Listeria bacteria retain the ability to live and develop for 90-120 days, in the soil - up to 600 days, on food products - up to 30-90 days.

Listeria and listeriosis

Most often, a person can become infected with the Listeria bacteria during contact with sick domestic animals, or after eating food products contaminated with the bacteria: meat, milk, fish, vegetables, etc. Soft cheeses, meat products, salads are especially susceptible to infection. Meat products can contain Listeria bacteria in 35-45% of cases.

A person has a high risk of contracting listeriosis if his professional activity is related to birds, animals or their products. This is especially true for workers in meat and poultry processing plants, farms, milk processing plants and fish farms.

Listeria bacteria in meat (chilled) slow down their activity somewhat, but the bacteria do not die completely. If the meat was frozen at a temperature of -10-28°C, then even under such conditions the pathogen is detected after 1 year or more, and it is absolutely viable. When meat and offal are treated with sodium chloride, listeria remain viable for 2 months or more. When sausages are heat treated, the listeriosis pathogen can die only after one and a half to two hours. All of the above facts make it clear that if you suspect an animal of having listeriosis, you should absolutely not eat its meat.

Listeria bacteria are found in fish no less often than in meat. Various microorganisms that are present in fish products along with listeria bacteria, to some extent restrain their reproduction. For this reason, the number of listeria bacteria in a chilled fish product may not be critical (up to 100 bacteria per 1 g). But the conditions created during salting and cold smoking inhibit the development of other microorganisms, while listeria at this time begin to actively reproduce. This explains the presence of a greater number of listeria bacteria in cold-smoked fish products, salted fish and preserves.

The listeriosis pathogen can enter fish processing plants with caught fish (especially freshwater fish), with plant additives, dirty water, and with packaging containers.

Workers who perform the initial stage of fish processing (cleaning scales, cutting) are particularly at risk.

And, despite the fact that the entry of the Listeria bacteria into the human body with food is considered the most common, it should be remembered that Listeria can penetrate in other ways - through the respiratory system, the conjunctiva of the eye and through scratches and wounds on the skin. A sick person, or a carrier of parasites, excretes bacteria with feces, urine. Particularly dangerous are carriers of the infection who are unaware of their illness, infecting others. Infection can occur in utero, as well as during labor.

Listeria GOST

The method for detecting and determining listeria bacteria in food products is designated as GOST R 51921-2002. This generally accepted method applies to all food products, including baby, medical and special nutrition: meat products (including poultry), internal organs and less valuable parts of carcasses, fish products, dairy products, vegetables, margarine, mayonnaise, ready-made salads. GOST defines the method for detecting listeria monocytogenes bacteria in the listed products.

The method is based on sowing a certain portion of the product being studied in a liquid selective nutrient medium, with subsequent transfer to diagnostic-selective media and germination of the crops under optimal conditions.

The differentiation of grown cultures is carried out in stages: first, they determine whether the grown bacteria belong to the genus Listeria, after which they proceed to confirm their belonging to the species Listeria monocytogenes.

Antibodies to Listeria

Antibodies to listeria bacteria are an indicator of infection with these microorganisms, which cause the development of listeriosis. Determination of antibodies to listeria is carried out in the following situations:

• if listeriosis is suspected;
• in cases of severe intoxication, high temperature, rash, enlarged liver and spleen;
• when diagnosing an unexplained infection.

The test is also mandatory for women who have previously suffered from this disease when planning a pregnancy.

No preparation is required for the test. A medical worker takes blood from a vein for testing.

Normally, there should be no antibodies to Listeria bacteria. In other situations, the analysis may show the following results:

• 1:50 – the titer is questionable;
• 1:100 – weakly positive titer;
• 1:200 to 1:400 – positive;
• 1:800 and higher – the titer is strongly positive.

A positive result indicates the presence of a current infection in the body.

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Selective additive for the isolation of listeria

The selective supplement for the isolation of listeria can be used for the bacteriological diagnosis of listeriosis or for the detection of listeria bacteria on environmental objects.

Listeriosis is quite widespread in the world, but in our country, fortunately, the percentage of detection of patients and carriers of listeria bacteria is relatively low. Nevertheless, the use of selective media is an important point of sanitary and hygienic requirements and bacteriological control of products.

Selective additives can be purchased separately or as part of a ready-made medium for isolating and culturing listeria bacteria. Most often, the additive is produced in the form of a powder, which is then added to the broth or other media where optimal conditions for culturing listeria will be created. In addition, inhibitory substances are added to the medium, which suppress the simultaneous growth of other bacterial flora present.

The material is examined according to GOST to determine Listeria monocytogenes. Incubated at t° +30°C for 24 hours.

The shelf life of selective additives is about 2 years.

When Listeria enters the body, it spreads quite quickly, affecting those parts of the body and the central nervous system where medications cannot reach in the required volume. For this reason, listeriosis treatment is not always effective, especially if the diagnosis is made quite late. In this regard, early detection of listeria bacteria is the most important moment in the successful treatment of the disease.