Food Microbiology: How to Become a Quality Control Freak

There are two steps to the enrichment process. The first step is the pre-enrichment, which allows for resuscitation of the target microorganism in a non-selective or only slightly selective environment. The second step is the selective enrichment, which uses a more selective environment to suppress growth of non-target microorganisms while promoting the growth of the target microorganism.

Enumeration testing: Enumeration testing refers to the determination of the total number of viable cell counts present in a sample. There are two basic types of enumeration methods, direct and indirect. The most common direct methods include the aerobic plate count, also known as heterotrophic plate count, total plate count, or standard plate count. These methods provide a direct measure of the microbial count. The most probable number (MPN) technique is a widely used indirect enumeration method. An MPN is estimated from the positive or negative results obtained in one or more decimal dilutions of the sample.

It is important to remember that microbiological analyses and laboratory processes are susceptible to variation. Therefore, it is critical to monitor variation on a regular basis. Commercially manufactured microorganism preparations are commonly used as daily process controls to monitor methods, equipment, techniques, and environmental conditions. Quantitative microorganism preparations are manufactured to deliver a pre-determined value of colony-forming units (CFUs), so labs are able to consistently recover specified levels of target microorganisms. Inability to recover specified levels of target microorganisms indicates that something is wrong with the process, and the problem must be investigated to maintain quality standards.

Automated technologies such as real-time polymerase chain reaction (PCR) and ribotyping instruments are growing in popularity because of the level of speed and accuracy with which they can detect, identify, and/or enumerate foodborne pathogens. Control organisms are needed for validation and verification of these instruments. The quality control strains used for validation and verification should be selected based upon the manufacturer’s recommendations.

New batches of media: Testing new batches of culture media that will be used for the isolation of organisms or enrichment of a sample is critical because you must verify the ability of the media to adequately support microbial growth. This process will also reduce the risk of obtaining false-negative results due to the use of defective media. Testing new media will also help in detecting batches of media that may be slightly more inhibitory than normal. Media with selective or differential properties should be tested to ensure the selectivity of the media.

Media testing can be complicated by the fact that a fresh culture may be more likely to grow on a slightly more inhibitory medium than a microorganism contaminant that was somehow weakened by a component of the test sample. This problem can be partially overcome by challenging the medium with a low CFU count. The use of positive and negative control cultures is essential to confirm that the process, media, or reagents in question will be able to detect the target organism. Ready-to-use quantitative microorganism preparations that are designed to deliver a small number of CFUs can be very beneficial.