Listeria monocytogenes poses unique challenges to food industry

An outbreak of Listeria monocytogenes (L. monocytogenes) in the fresh produce industry has made headlines once again, this time with suspected contamination of bagged salad.

While the food industry has controls in place to combat L. monocytogenes just as they do for other foodborne pathogens, outbreaks occur. A recent article looks into the disease transmission factors associated with L. monocytogenes and points out some unique and difficult problems confounding current food safety controls and the best practices to avoid L. monocytogenes contamination in the food processing environment.

First of all, the article explains that Listeria is so troublesome because it adapts well to a wide variety of environments and can readily colonize on most surfaces. In fact, Listeria can be found in about 7% of produce growing areas and is widely distributed in nature too, with a detection rate of about 3% of soils sampled in undisturbed areas.

In addition, the organism is very hardy and survives well on a variety of surfaces, especially cold, wet environments, including within refrigeration units, on drains in floors, and on any damp surfaces if not cleaned effectively. It can also be transient in an operation through soil, on shoes and in dust.

Furthermore, the vulnerability of cut, nutrient rich fruits and vegetables support survival and multiplication of the organisms during storage and shipping. While the Listeria phenomenon is a relatively new public health problem in produce, it is related to supply chain factors such as centralized processing of fruits and vegetables and extended refrigeration times which provide ample conditions for the bacteria to multiply. This, along with the rising demand for convenience items such as bagged salads and fresh cut fruits and vegetables, has contributed to increased instances of L. monocytogenes in the world’s food supply and the human infections associated with the bacteria, known as listeriosis.

To make matters more difficult, symptoms of listeriosis of may not present for up to three weeks after exposure to L. monocytogenes. This long incubation period hamper efforts to identify suspect products and increases the attack-rate in outbreaks, the article explains. Symptoms in those who do become ill, however, can progress rapidly to life-threatening sepsis and eventually to shock and death faster than most other foodborne pathogens.

Due to these dangers, food manufacturers must adhere to strict sanitation and testing programs to ensure their products are safe and effectively reduce the likelihood that L. monocytogenes will reach food-contact surfaces or products that enter the food chain.

Keys to preventing of L. monocytogenes in food production environments include vigilance and management oversight first and foremost. Upper management involvement in sanitation problems is critical, along with management support for needed capital improvements in facilities that have issues. Moreover, sufficient resources must be applied to the frequent and ongoing monitoring of the environment and the verification of sanitation.

To do this, the first line of defense is the design and maintenance of equipment and the environment. All surfaces and processing equipment that food comes in contact with, even the hard-to-clean surfaces and hidden areas, must be rigorous cleaned and sanitized. The effectiveness of this cleaning also needs to be analyzed and verified. There are several popular testing methods including ATP bioluminescence that are common in the industry today.

In addition, microbial testing is perhaps one of the most effective prevention strategy, yet it is largely underutilized, the article states. Both surfaces and products (raw, in-process, and finished product) should be under a microbial monitoring program. Diligence in executing the sampling program must be maintained; with sampling schedules developed based on risk assessments. Importantly, management needs to plan for an immediate and effective response if unsatisfactory tests results are received.

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