Listeria as a Hygiene Indicator
By Justin Ford, EMLab P&K Analyst
Listeria monocytogenes is a gram-positive bacilli bacterium, about 0.5 x 0.5-2.0 µm in size, and recognized as a human pathogen. The organism may be isolated from environmental sources such as the feces of mammals, birds, fish, insects, and other animals, though the primary source is believed to be soil and decaying vegetable matter. One to five percent of healthy people are thought to carry the organism in their intestinal tracts as a portion of their normal flora. Many infections occur annually, however many minor infections are likely to go unreported. It is of particular interest to the food production industry because occurrence of any contaminated products would be expected to be recalled. In one instance in 1999, 30 million pounds of contaminated meat were recalled.3 Listeriosis (bacterial infection caused by eating food contaminated with Listeria monocytogenes) has been associated with consumption of contaminated milk, soft cheeses, undercooked meat, and unwashed raw vegetables. Listeria monocytogenes is particularly dangerous to immuno-compromised individuals. The mortality rate of symptomatic Listeria infections is about 20-30%; much higher than most food borne infections.
Listeria monocytogenes possesses many characteristics that make it both an effective pathogen and difficult to eliminate from an environment. It is a facultative, intracellular (inside a cell) parasite. This allows the organism to avoid antibody-mediated clearance by hiding within the hosts own cells. It is facultatively anaerobic (can live both with and without oxygen) and capable of reproduction at temperatures just above freezing. The organism is motile at room temperature via end-over-end tumbling in broth and is weakly β-hemolytic on sheep blood agar. As well as being collectively important to the organisms' pathogenicity, these characteristics are useful for identification of L. monocytogenes.
Beyond time consuming phenotypic and genetic testing methods, several rapid identification techniques are available. These include chromogenic (color changing) media culture, immunoassay, and PCR testing. A relatively new chromogenic method for Listeria identification is available in the form of RAPID' Listeria spp. agar. This new culture medium, developed by Bio-Rad, can produce results in a relatively short, 48-hour period. Buffered swabs are the tool of choice when checking for L. monocytogenes contamination.
Because detecting the source of environmental contamination can be tricky and time consuming, a number of easily identifiable bacteria associated with contaminating events are regularly tested for in order to determine the presence, severity, and scope of a contaminating event. Many of these bacteria are dangerous in their own right and would be a cause for concern if isolated from a hospital or kitchen counter top. Others are more benign. For example, E. coli is often used as an indicator of sewage contamination in waterways either via runoff from farmland or from human sewage systems. It is not abnormal to find E. coli in small amounts throughout the environment, so the quantity of bacterial cells may also be of importance. Similarly, L. monocytogenes is found ubiquitously in natural environments. Isolating it from a man made environment should not result in surprise, but it may be a concern when considering the pathogenicity of the organism. Supplemental to its dangerous nature, particularly in medical and food preparation facilities, Listeria exhibits traits that allow it to survive and grow in environments not habitable by other pathogens. It is this fact that makes L. monocytogenes a potential indicator of negligent QA standards, inadequate cleaning procedures, or poor employee hygiene.
Testing for Listeria may be wise even when consumption of the organism is not a concern. For example, in food preparation facilities, the food product may be tested directly, but so should other areas along a production line such as puddles of condensation on the floor, drainage grates, and hard to reach areas beneath machinery. This is particularly important as the organism is so easily tracked in from the outside environment, and can also form bio-films that are resistant to normal cleaning procedures. In these instances, Listeria is serving as an environmental indicator of a more general hygiene issue, one that may not be solved by addressing the bacteria alone.
The wide range of temperatures at which Listeria may grow adds an extra level of complication for the storage of food. While certainly not the only organism to be worried about in these environments, L. monocytogenes may be a useful tool in determining the quality of QA/QC (quality assurance/quality control) and cleaning procedures. As an example, E. coli and L. monocytogenes both prefer moist environments and will readily contaminate food sources. Both are easily tracked into a location by people. However, testing for E. coli contamination in a refrigerated environment may yield deceiving results; while E. coli lies dormant, Listeria could be growing rampantly. Specific testing for Listeria can act as a quick and effective indicator of environmental hygiene and, in some fields, may be required by law.
1. P. Murray, K. Rosenthal, M. Pfaller. (2005). Medical Microbiology (Fifth Ed.). pp 273-276. Elsevier Mosby, Philadelphia, Pennsylvania.
2. T. Jemmi & R. Stephan (2006). Listeria monocytogenes: Food-Borne Pathogen and Hygiene Indicator. Rev. Sci. Tech. pp 571-580.
3. CDC: Listeriosis. 2009. Accessed on March 20, 2010.
4. T. Moretro and S. Langsrud (2004). Listeria monocytogenes: biofilm formation and persistence in food-processing environments. Biofilms, 1, pp 107-121.
This article was originally published on May 2010.