Elaine Fraser examines the reasons behind environmental monitoring of both surfaces and air and the possible effects on food.
Listeria species are widespread throughout the environment and can be found in many different food types. Listeria monocytogenes has only been recognised as an important food pathogen since the early 1980s.
There are now known to be six Listeria species. These are L. monocytogenes, L. innocua, L. welshimeri, L. grayi, L. seeligeri and L. ivanovii5. All species are Gram-positive rod-shaped bacteria, which are urease and oxidase negative. Listeria species are catalase positive and produce acid from glucose (without gas)4. Growth occurs over a wide range of temperatures, from 0 to 45°C. Cells exhibit a tumbling motility at 25°C, but are non-motile at 37°C and refrigeration temperatures. Listeria species are facultative anaerobes and grow strongly on a variety of complex media.
Although 26 per cent of healthy individuals carry L.monocytogenes without showing any symptoms of illness, it can cause serious disease in many people. Besides L.monocytogenes (the main pathogen of the Listeria genus), L.seeligeri and L.ivanovii have also been known to cause infection on rare occasions.
Food-borne outbreaks have occurred worldwide and are associated with a number of different food types. Listeria monocytogenes can be found in most foods and is a particular risk in ready-to-eat foods, which are consumed without further cooking.
Certain factors increase the risk of a food becoming contaminated with Listeria. Ready-to-eat foods are prone because Listeria grows at refrigeration temperatures and no further cooking takes place before the food is eaten.
However, this organism is not particularly heat-tolerant. The organism grows fastest between 3037°C, and can grow at pH values between 4.59.02. Because L.monocytogenes grows under the same conditions as other Listeria species, surveys for all species are regarded by some as a useful indicator of the possible presence of L. monocytogenes.
Survival and growth of L. monocytogenes on production equipment is often a problem, especially in hard to clean areas. Floor drains in food processing plants are also known to be sources of Listeria, so it is essential to keep areas dry. The best way to prevent contamination of foods is by implementing an effective hazard analysis critical control point (HACCP) system along with a good manufacturing program (GMP).
The high level of concern regarding L.monocytogenes in foods has led to the development of many media for its isolation. Conventional methods are widely used for detecting and identifying Listeria species, particularly L. monocytogenes. Isolation methods involve enrichment in selective broths followed by isolation of colonies on selective agar media. Food samples can be enriched in Buffered Listeria Enrichment Broth (BLEB, or unbuffered, LEB), Fraser Broth or Half-Fraser Broth. Suitable media for isolating Listeria colonies include PALCAM agar and Oxford agar. The selective agents incorporated into Listeria media commonly include nalidixic acid, acriflavine hydrochloride and polymixin B1.
Oxford agar inhibits all Gram negative bacteria and incorporates aesculin, which L.monocytogenes hydrolyses to produce black zones around the colonies. Other selective agents in the medium, which inhibit the growth of competing flora, include lithium chloride and acriflavine. Ferric iron is incorporated into the medium as an indicator. PALCAM agar contains lithium chloride, ceftazidime, polymixin B and acriflavine hydrochloride as selective agents. A double indicator system (1. aesculin and ferrous iron and 2. mannitol and phenol red) allows easy differentiation of L.monocytogenes. Listeria monocytogenes cannot ferment mannitol, whereas enterococci and staphylococci do ferment mannitol, producing a yellow colour in the agar. Incubation of PALCAM agar under microaerobic conditions also inhibits growth of Bacillus and Pseudomonas species. ALOA (Agar Listeria according to Ottaviani and Agosti) is a chromogenic medium for Listeria isolation. ALOA medium is more selective than PALCAM agar and Oxford agar and differentiates L.monocytogenes from other species. In 1996, ISO published its first standardised method for detecting Listeria species in foods. This is currently under review.
Colonies with the typical appearance of Listeria species on selective agar can be further examined for characteristic cellular morphology, presence of haemolysis, catalase production and other biochemical tests.
Many rapid methods for the detection of Listeria species and L. monocytogenes exist, including PCR, ELISA and novel lateral flow devices such as the Oxoid Listeria Rapid Test (OLRT).
Elaine Fraser is with Oxoid Ltd, Basingstoke, UK.. www.oxoid.com
