Unsafe Ice Cream Goes Undetected

An ice cream manufacturing plant in New York City in the early 1970s, in compliance with the required testing procedures to ensure the microbiological safety of its food product, routinely sent samples of the ice cream to a local quality control microbiology laboratory. The laboratory performed viable plate counts to detect coliform bacteria. The presence of co­nform bacteria indicates contamination with human fecal matter, making the ice cream unsafe for con­sumption. The plates were overgrown with coliform bacteria, and the technician at the testing laboratory recorded TNTC, the standard notation for too nu­merous to count. Records were compiled indicating

unsafe levels of contamination but no action was taken. This was because the Board of Health inspec­tor who examined the records did not recognize the abbreviation TNTC and was looking for a number greater than 10 per 100 mL to signal a contamination problem. The inspector did not inquire as to the meaning of TNTC, and it was not until another in­spector visited the facility and viewed the records that the problem was detected. The underlying prob­lem with the ice cream was in the plumbing of the building, which had connected the effluent from the restrooms directly to the influent for water used in the manufacture of the ice cream.

count relies on the reproduction of individual bacter­ial cells to form visible colonies, which are counted to enumerate numbers of bacteria in a sample. Another problem and source of possible error associated with this technique is in the enumeration of bacteria that grow in chains or clumps that are hard to disperse. For example, a chain containing ten attached cells will grow into one colony instead of ten. Therefore using the viable plate count method to measure numbers of bacteria that tend to remain attached to one another can lead to erroneously low values.

The viable plate count procedure is selective be­cause no one combination of incubation conditions and media allows all types of bacteria to grow.

Direct Count Procedures

Bacteria can also be enumerated by direct countinj procedures. In this procedure, counting is done with out the need to first grow the cells in culture. In om direct count procedure, dilutions of samples are ob served under a microscope, and the numbers of bac terial cells in a given volume of sample are counted These numbers are used to calculate the concentra tion of bacteria in the original sample (FIG. 10-9) Special counting chambers, such as a hemocytometa or Petroff-Hausser chamber, are sometimes em­ployed to determine the number of bacteria. These chambers are ruled with squares of a known area and are so constructed that a film of liquid of known

FIG. 10-9 The direct counting procedure using a Petroff-Hauser counting chamber. The sample is added to a counting chamber of known volume. The slide is viewed and the number of cells determined in an area delimited by a grid. In the counting chamber shown, the entire grid has 25 large squares for a total area of 1 mm² and a total volume of 0.02 mm³, formed by the spacing of an overlying coverslip. There are 12 cells within the single large grid (composed of 16 smaller boxes) in this example. Assuming the number of cells in this single grid is representative of all the grids, the number of cells within the total area under the grid is 12 cells. The concentration of cells is therefore 300/0.02 mm³.

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