Predictive Microorganisms as an Indication of Pathogen Contamination

The original use of “indicator” organisms to signal the possible presence of pathogenic bacteria dates back more than a hundred years. Since then, the use of these organisms as predictors of pathogens has been extended from water to a variety of raw and processed food products. When used to predict the possible presence of pathogens, the term “index organism” is now preferred; “indicator organisms”, in comparison, are used as a means of assessing process integrity. Various index organisms have been used, including: coliforms, “fecal” or “thermo-tolerant” coliforms, E. coli, Enterobacteriaceae, enterococci, and “aerobic plate count”. Despite the enteric connotations associated with the names of some of these organisms, they are not confined to the intestinal tract and often represent bacteria of widely divergent origin. Some coliforms and Enterobacteriaceae can even grow on refrigerated muscle foods. Numerous studies have failed to find solid relationships between coliforms, Enterobacteriaceae, or E. coli, and pathogens such as Salmonella, Listeria monocytogenes, and Campylobacter on a variety of meat and poultry products. The few studies which have actually analyzed the predictive value of various index organisms have drawn similar conclusions. Numerous international and national advisory committees, including the FAO/ WHO and the National Research Council’s Subcommittee on Microbiological Criteria, have concluded that it is invalid to attempt to predict the safety of a meat or poultry product based on the levels of APC, coliforms, “fecal” coliforms, Enterobacteriaceae, E. coli, or enterococci found on the product. Index and Indicator Organisms: History and Terminology For more than a century, microbiologists have used indirect tests as a means of detecting possible pathogen contamination of water and foods. These indirect assays have typically involved either aerobic plate counts (APCs) or groups of organisms (coliforms, enterococci, and Enterobacteriaceae) which are thought to co-exist with enteric pathogens in the intestinal tracts of humans and animals. In the 1890’s, Schardinger (1892) began testing water for Escherichia coli. The presence of E. coli was used as an indication of fecal contamination, and, possibly, of the presence of Salmonella typhi. Over time, such testing was extended first to dairy products, then to food products, and food processors began to use indicator assays as a means of determining the adequacy of a process. The reader is referred to Mossel (1967) for a historical review of the role of indicator organisms in the determination of food safety. In view of the dual functions which these indirect assays served, researchers attempted to clarify the situation (Mossel, 1978). The terms “index”, “marker”, “simulator”, and “surrogate” were suggested for reference to organisms whose presence at certain levels indicates the possible occurrence of pathogens (Brodsky, 1995). In contrast, “indicator organisms” or “hygiene marker organisms” are those whose detection is indicative of a failure in GMP or integrated system control which results in a food product of unacceptable microbiological quality (Brodsky, 1995). Testing for index organisms, then, serves a predictive function while testing for indicator organisms is a means of assessing process integrity (Mossel, 1978). Although the distinction between the functions of index and indicator organisms is an important one, workers in this field have often used the terms interchangeably, leading to considerable confusion. This situation is further confounded by studies in which a particular group of organisms functions as both index organisms and as indicators. This review will use the terms “index organism” and “indicator organism” as defined above; “index/indicator organism” will be used when referring to information common to both index and indicator usages.