Comparison of an automated Most Probable Number (MPN) technique to traditional plating methods for estimating populations of total aerobes, coliforms and E. coli associated with freshly processed broiler chickens

Authors: Line, John; Stern, Norman; Seal, Bruce; Oakley, Brian

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2011
Publication Date: 9/1/2011
Citation: Line, J.E., Stern, N.J., Seal, B.S., Oakley, B. 2011. Comparison of an automated Most Probable Number (MPN) technique to traditional plating methods for estimating populations of total aerobes, coliforms and E. coli associated with freshly processed broiler chickens. Journal of Food Protection. 74(9):1558-1563.

Interpretive Summary: Traditional microbiological techniques for estimating populations of viable bacteria associated with a given sample type primarily fall into three main catagories and include direct plating techniques, membrane filtration, and Most Probable Number (MPN) calculations (Swanson et al., 2001). These methods can be laborious and time consuming (Line et al., 2001). The MPN technique is especially tedious as multiple series of tubes must be inoculated at several different dilutions. There are numerous MPN methods available in the literature, but all require the time-consuming process of setting up multiple tubes or wells at multiple dilutions which forms the statistical basis for the test (add MPN REFS). Recently, an instrument (TEMPOTM) has been developed to automate the MPN technique and reduce the effort required to estimate some bacterial populations. The TEMPOTM instrument employs a unique micro-channeled card into which the sample is introduced under vacuum automatically creating the dilution series necessary for MPN estimation. Samples are added to vials of prepared dehydrated media and introduced into the cards in an automated vacuum chamber. The cards are then removed from the vacuum chamber and incubated as appropriate. Following incubation, the cards are placed in a reading chamber equipped with a camera capable of detecting fluorescence associated with growth of organisms in specific channels on the cards. Computer software is utilized to interpret the results from the card and an MPN estimation is automatically calculated. The primary purpose of our study was to compare the TEMPOTM automated MPN system to traditional direct plating on agar or PetrifilmTM for estimating populations of total aerobic microorganisms, total coliforms, and E. coli from poultry associated samples, specifically, freshly processed broiler chicken carcasses (post-chill whole carcass rinse [WCR] samples) and drip-line samples from a commercial processing facility. In addition we wanted to determine if sampling of the cumulative post-chill drip collected over time from an entire flock of broilers would give a more representative estimation of bacterial contamination associated with the flock as a whole as compared to the WCR technique frequently employed to estimate bacterial populations associated with post-chill carcasses.

Technical Abstract: Recently, an instrument (TEMPOTM) has been developed to automate the Most Probable Number (MPN) technique and reduce the effort required to estimate some bacterial populations. We compared the automated MPN technique to traditional microbiological plating methods or PetrifilmTM for estimating the total viable count of aerobic microorganisms (TVC), total coliforms (CC), and E. coli (EC) populations on freshly processed broiler chicken carcasses (post-chill whole carcass rinse [WCR] samples) and cumulative drip-line samples from a commercial broiler processing facility. Overall, 120 broiler carcasses, 36 pre-chill drip line samples and 40 post-chill drip line samples were collected over 5 separate days (representing 5 individual flocks) and analyzed by the automated MPN and direct agar plating or PetrifilmTM methods. The TVC correlation coefficient between the automated MPN and traditional methods was found to be very high (0.972) for the pre-chill drip samples which had mean Log values of 3.09 and 3.02, respectively. The TVC correlation coefficient was slightly lower (0.710) for the post-chill WCR samples which had lower mean Log values of 1.53 and 1.31, respectively. Correlations were high between the methods for the pre-chill CC and EC samples with correlation coefficients of 0.812 and 0.880, respectively. The estimated number of total aerobes was generally greater than the total number of coliforms or E. coli recovered for all sample types. Significantly more bacteria were recovered from the pre-chill samples than the post-chill WCR or cumulative drip samples. The automated MPN instrument was easy to utilize and allowed a single operator to perform the three analytical tests in less time than it took 4 trained laboratory technicians to conduct the tests by traditional methods.