Comparison of methods to determine the microbiological contamination of surfaces of beef carcasses by hydrophobic grid membrane filters, standard pour plates or flow cytometry

A routine hydrophobic grid membrane filter (HGMF) method previously used to estimate the hygienic adequacy of the process of beef carcass production was compared with two variations of this method, the standard pour plate (SPP) method of enumerating aerobic cells, and the flow cytometry method of enumerating autofluorescent particles for 150 excision samples of surfaces of beef carcass. The application of 1.0% 2,3,5-triphenyltetrazolium chloride (TTC) sprayed as an aerosol after incubation of HGMF on trypticase soy agar for 24 or 42 h yielded significantly higher counts (P < 0.001) than HGMF incubated on trypticase soy agar with TTC for the same length of time. TTC applied in aerosol and 24 h incubation provided higher counts than TTC in media and 42 h incubation. The linear regression equations obtained in this study could serve to estimate the conversion of log(10) most probable numbers growth units per millilitre obtained from HGMF on plates with TTC in media or applied in aerosol to SPP log(10) colony forming units/per millilitre. It was assumed that bacteria taken from carcasses from 15 farm sources were representative of bacteria on beef carcasses at other Alberta abattoirs. This facilitated conversion of data obtained by the automated HGMF method to SPP data in verification of the workings of hazard analysis and critical control point systems for beef slaughter processes. Under these experimental conditions no linear relationship was found between fluorescent particles in 80 of the above surface samples and colony forming units produced by the SPP method.