Why a faster method matters in Indicator testing

Microbiological analysis of food and a well thought out environmental monitoring plan forms the backbone of an effective safety and quality system for the food industry. The presence of undesirable organisms including both quality indicators and pathogenic organisms has necessitated routine microbiological testing with a goal to ensure food safety, quality and regulatory compliance. Routine testing for quality indicators primarily involves the enumeration of aerobic bacteria (AC), E. coli/coliforms (EC/CC) and yeasts and molds (YM).

Limitations of traditional agar-based methods

Traditional agar-based methods come with a number of challenges.

  • Extra work: Traditional agar-based enumeration methodologies are labor -intensive and involve the careful preparation of analyte-specific media.
  • More time: Sterilization of small batches of media can take an hour or more while a much longer sterilization time is needed for large volume media preparation processes.
  • Increased effort: Certain media contain heat labile components that require boiling for specified time periods under constant supervision.

Adding to the challenges listed above, problems can occur during agar media production. Batch-to-batch variability can be introduced by overheating, which can result in:

  • pH drift
  • darkening
  • poor gel strength and
  • reduced bacterial recovery.

Reduced bacterial recovery due to the toxic by-products of the Maillard reaction if reducing sugars are present in the medium, is especially concerning.

Adding to the complexity of media preparation processes are the prolonged incubation times of conventional microbiological methods which have resulted in the evolution of analytical methods with shortened incubation times.

Designed to perform comparably to the relevant ISO and FDA/BAM reference methods, the 3M™ Petrifilm™ Rapid Plates solutions meet the microbiology testing needs for meaningful and actionable results in a shorter time-period. Combined with the value of lot-to-lot consistency, increased productivity and convenience of the sample ready 3M™ Petrifilm™ solutions, the innovative 3M Rapid Petrifilm portfolio is specifically designed to meet the need for expedited microbiological results along with enhancements to ease of interpretation. The most commonly tested indicator organisms and the associated 3M™ Petrifilm™ Rapid plates are outlined below.

Yeasts and Molds

Mycological evaluation of food, food ingredients and environmental samples is an important component of food quality assessment, particularly spoilage and prediction of shelf life. The ubiquitous presence and highly varied nature of yeast and mold mycoflora in the environment has required the development of a large variety of traditional agar-based media (Dichloran Rose Bengal Chloramphenicol, acidified Potato Dextrose Agar, DG-18, Malt Extract agar, etc.,) with incubation times ranging from 5-14 days. As a convenient, unitary replacement solution for a variety of agar-based media, the 3M™ Petrifilm™ Rapid Yeast and Mold Count plate enables quantitative enumeration of yeasts and molds within 48-72 hours from a wide variety of high and low water activity food matrices.

Figure 1: Smoked Salmon (left panel) and Yogurt (right panel) on 3M™ Petrifilm™ Rapid Yeast and Mold plates after 60 hours of incubation at 25C


Aerobic bacteria

Like Yeast and Molds, aerobic bacteria are also ubiquitously present and indicate the overall microbiological quality of foods. Measured as the standard plate count (SPC) or the aerobic plate count (APC), the total quantitative enumeration of bacteria is one of the most frequently performed tests to assess the quality of foods. The group of organisms that belong to this category can be defined as mesophilic organisms that can grow in aerobic conditions at moderate temperatures (20-45C). The enumeration of these bacteria can be a useful tool to monitor adherence to GMPs by serving as an important indicator of trends and process drifts in routine operations.

Conventional agar-based methods that utilize Standard Methods Agar /Plate Count Agar (SMA/ PCA), or Skim Milk Plate Count Agar (SMPCA) can be time-consuming with counts available after incubation of 48-72 hours at 30-35C. The proprietary dual sensing technology of 3M™ Petrifilm™ Rapid Aerobic Count Plate leverages the oxidative and fermentative metabolic pathways of bacteria to provide end users with quantitative results within 18-24 hours for a wide variety of matrices with significantly enhanced interpretation of spreader colonies. Given the impact of high levels of aerobic bacteria on spoilage and shelf-life of foods, specifically those that are perishable in nature, the faster time to results afforded by the 3M™ Petrifilm™ Rapid Plates provides the user with valuable and actionable results in an earlier time frame.

Figure 2: Raw milk isolate on 3M™ Petrifilm™ Rapid Aerobic Count Plate (right panel) or standard methods agar (left panel) after 24 hour incubation at 32C


Escherichia coli
and coliform bacteria 

The E. coli/coliform bacteria are perhaps the oldest class of bacteria to serve as quality and hygiene indicators with coliform bacteria having served as an indicator for hygiene in water and foods since the late 1800’s. Defined as aerobic or facultatively anerobic, gram negative non-spore forming rods, coliforms are characterized by their lactose fermenting ability with acid and gas at 35 C within 48 hours. The pervasive presence of E. coli in feces and subsequent detection in water established this organism, a subset within the coliform group as a hygiene indicator whose presence was presumed to ‘indicate’ the increased likelihood of pathogen presence.

In addition to fermenting lactose, a majority of E. coli (‘typical’) produce the enzyme glucuronidase which can be detected via chromogenic or fluorogenic means. Conventional agar-based methods for the detection of E. coli and coliform are time consuming and labor-intensive with confirmation steps for detecting gas production that can take anywhere from 48 hours to several days. Coliforms are susceptible to most heat processing conditions (e.g., pasteurization of milk); therefore, their presence in heat treated foods generally indicates an inadequate heat-kill process or potential post-processing contamination.

The unique construction of the 3M™ Petrifilm™ plate enables a single step detection via a gas bubble of the lactose fermenting ability of this important class of organisms. With the 3M™ Petrifilm™ Rapid Coliform Count plate it is possible to get an early warning of coliform contamination within 6-14 hours of incubation with a confirmed positive result in 24 hours.

Additionally, the recently introduced 3M™ Petrifilm™ Rapid E. coli/coliform count plate enables the simultaneous detection via a dual readout of lactose fermenting coliforms along with beta-glucuronidase producing E. coli within 18-24 hours further streamlining the complicated series of steps needed by the conventional methodologies for the detection of these organisms.

Figure 3: Chicken Salad on 3M™ Petrifilm™ Rapid E. coli/coliform count plate after an incubation of 18 hours at 35C


The 3M™ Petrifilm™ Rapid Plates portfolio of solutions described above enhance testing efficiencies, by providing accurate and rapid solutions that serve to improve food safety programs and help create a safer food supply. 3M™ Petrifilm™ Rapid Plates can help you increase efficiency and maintain accuracy by producing results up to two times faster than traditional methods, so you can confidently make time-sensitive decisions.



Need more information on the total 3M Food Safety testing solution? Simply reach out and we’ll be happy to help you with any questions you have. Contact a rep here.

Dr. Sailaja Chandrapati

Sailaja Chandrapati, BS, MSc, MS, PhD, is a Senior Scientist and Lead Product Development Specialist in the Food Safety Department, 3M Company St. Paul, Minn. She also holds an Adjunct Associate Professor position in the Department of Food Science and Nutrition at the University of Minnesota, Twin Cities, MN. She is a technical expert in the areas of food microbiology and spore science along with sterilization and disinfection monitoring. Since joining 3M in 2000, she has 26 issued patents, 34 filed patent applications multiple poster presentations and is an author of 6 papers and 1 book chapter. Dr. Chandrapati is a member of ASM and IAFP and a past reviewer for the American Journal of Infection Control.