This biofilm-producing terrorist is the bane of all industrial microbiologists. Industry can be humming along quite happily and then up pops Listeria and its panic stations. The micro response team rushes to the site armed with gauzes, swabs, sampling and HACCP (Hazard Analysis and Control of Critical Points) plans to do combat.
As we all know the genus Listeria is a gram positive rod, psychrotrophic, and displays a peculiar tumbling motility caused by a low number of peritrichous flagella which beat in a clockwise motion due to a defective CheY gene (Dons et al, 2004). This organism is ubiquitous and is found primarily in soil (Sutherland et al, 2003). The only species that is truly a human opportunistic infector is Listeria monocytogenes, public enemy number one. Its sibling Listeria ivanovii is attempting to cause confusion in the ranks of those over-worked industrial microbiologists. L. ivanovii has shown similar pathogenicity as seen by L. monocytogenes, in mice and other animals, but is rarely seen in humans (FDA/CFSAM, 2003). Are these two species protected or masked by Listeria innocula the harmless one? With the perceived threat of Listeriosis, the government bodies are debating the move towards zero tolerance for the genus. The federal government food body FSANZ standard only states that L. monocytogenes absence is required in ready-to-eat products and the FAO/WHO risk assessment concluded that levels of L. monocytogenes <100 cells per gram has the same risk as zero cells per gram (FAO/WHO, 2001). To complicate matters, Dussarget (2004) stated that of the 13 known serovars of L. monocytogenes, only 1/2a, 1/2b and 4b are responsible for 98% of reported human Listeriosis cases. The serovar 4b is associated with the majority of food borne outbreaks and sporadic cases. This single genus has been responsible for more product recalls and media hype than any other micro-organism. We all have heard of Conroy’s and the two deaths from ham in Adelaide in the last few months. Industries that produce ready-to-eat products all have great concern for this ubiquitous terrorist.
Industry has spent millions on the combat, control and the eradication of this organism. As with all terrorist organizations, the sleeper cells are very hard to find and the fact that Listeria produce a fatty acid biofilm on solid surfaces makes it very difficult to treat with standard chloride based surface sanitizers. This biofilm aids the survival of Listeria due to its lipid composition which is hydrophobic and thus prevents the entrance of water-based sanitizers; it also acts as a food reserve and selects for the survival of other symbiotic organisms that aid in the survival and proliferation of Listeria (Sutherland et al, 2003) (Somers & Wong, 2004). The destruction of one biofilm may lead to the establishment of others from that original source and to product contamination. Biofilms are living entities and thus, when critical mass is achieved, cells detach and contaminate the product. This is known to the industrial microbiologist as ‘spitting’. There is reported resistance developing in the standard chemicals used in the eradication of biofilms (Chavant, 2004). The only effective way to clean down contaminated areas is by high-pressure (area needs to be sealed) acid washes as well as physical scrubbing followed by contact sanitization (quats, chlorine, acid and peroxide sanitizer) - the chemical equivalent of hunting down terrorist cells with thermonuclear warheads. Listeria has also displayed an ability to survive and thrive in some of the most extreme environments found in industry such as saturated brine. Listeria has been associated with many of our most loved and highly consumed foods. These include: ice cream, soft cheeses, smoked salmon, pate, fermented meats, cooked further processed chicken meats and fresh leaf produce (Sutherland et al, 2003). This cowardly bacterium attack the elderly, infirmed and the defenseless fetus with relatively low infective doses, 2 to 3 log less than is required to infect healthy adults (CFSAN, 2003). To complicate matters further, this organism presents to the treating clinician as flu -like systems and initial diagnosis may be difficult.
The total number of victims recorded in Australia is 3 cases in 1,000,000 and is steadily decreasing as the industrial microbiologist is slowly eradicating all known niches. The consumers demand for ‘fresh’ products with minimal preservatives and additives results in additional pressures on the industrial microbiologist to discover strategies to meet the consumer demand without endangering the public. This has resulted in the steady development of non-thermal treatments such as microwave and radio frequency, ohmic and inductive heating, high pressure processing, pulsed electrical field and pulsed light, just to name a few that are in development or have been used in commercial food manufacturing (FDA/CFSAN, 2000). These intervention strategies amplify nature’s only controls in controlling these terrorists. For example, high pressure processing uses water pressures to burst the cell. There is a plethora of methods available for the industrial microbiologist to screen and identify this organism. The selection of methods is primarily based on quality and turnaround time. The longer a company’s product takes to reach the market the more it costs the company. Therefore, there is always pressure to find faster methods to screen out negatives. Some of the most common rapid methods are based either on ELISA type tests (BioMerieux VIDAS, TECRA Unique) or PCR (Oxoid’s BAX and Roche’s real-time PCR protocol). These methods are all automated and have the required regulatory approvals. The covert battle between the industrial microbiologist and Listeria is ongoing with no definite exit time. As long as the consumer enjoys the convenience of ready to eat food, Listeria will be waiting to strike; however, the industrial microbiologist will be there to contain, prevent and eliminate any danger to the public.
