4 Gaps in Your Environmental Testing Program: #3 - The Great Plate Count Anomaly

In this series, product manager Stefan Widmann investigates four things you might miss when testing your food production environment.

What is the great plate count anomaly?

Only 1% of bacteria can be cultivated with the knowledge and techniques currently at our disposal. The “great plate count anomaly” is the term we use to describe the observation that microscopic cell counts are significantly higher than corresponding counts of “colony forming units” on agar plates. A couple of examples can illustrate this phenomenon best: while 50% of the microorganisms of the oral flora can be cultured with agar plates, most of the gastrointestinal flora cannot be cultured at all. The reasons for this are numerous, but the organism community surrounding the bacterium in question, including other bacteria as well as plants and animals, may play an important role. 

Aerobic plate count methods rely on very general supplements, which exclude most bacteria groups. Technically, this is not really part of the great plate count anomaly, as some bacteria are able to grow on special agar plates under special conditions (such as anaerobic or microaerophilic conditions).

Why should food producers care about the great plate count anomaly?

The great plate count anomaly does not pose significant problems in day-to-day testing runs, as aerobic plate counts are specific to a given production environment and, as such, are always relative to an established baseline determined for that production environment.

However, plate methods are very time-consuming, requiring an incubation period of up to three days, depending on the protocol in effect. There are direct methods that do not require a cultivation step to count bacteria: microscopes provide a comprehensive view of bacteria but are also very time-consuming, while flow cytometers currently on the market are expensive and require intense training to master. While such direct methods are common in water treatment facilities, they are not very common in the food industry. There, instant counting technologies must be correlated with established agar-based methods, which are still the accepted standard for environmental hygiene management. 

How can food producers detect what agar plates can’t?

Thanks to advances in technology, formerly bulky and unwieldy devices are becoming portable, more user-friendly and more affordable. This is also true of direct counting methods that can detect all intact bacteria in a sample. Broadening the scope of bacteria detected and – crucially – reducing or eliminating time otherwise spent waiting for incubation can help shift the focus of hygiene monitoring from post-production to pre-production. In this way, food producers can reduce the waste associated with products with a shorter shelf life and of greater concern regarding hygiene, such as fresh meat and dairy products.

This article originally appeared in International Food & Meat Topics 31 (3).