Confirmation Testing of Presumptive Positive Assays Using the BioLumix System

In Microbiology, the initial test result using selective or differential media is called Presumptive Test. Most presumptive tests require confirmation. Confirmation can be accomplished using specific reagents and materials. However, due to the critical importance of testing for pathogens and/or objectionable organisms as contaminants; it may be necessary to perform identification of any organisms isolated from samples. Identification measures microorganisms to the species level.

Initial Testing

For testing of any sample for the presence of microorganisms it is critical to perform a measure of total organism counts (viable organisms). BioLumix provides testing for both Total Aerobic Counts (Bacteria) and for Total Yeast and Mold Counts (Fungi). The BioLumix system in this regard mimics testing for both bacteria and fungi using USP or BAM plate methodology. In these initial tests for total counts there isn’t any discrimination of objectionable organisms from common organisms and common flora. Objectionable organisms for Nutraceutical Samples as an example may include E coli, Staphylococcus aureus, Salmonella or Pseudomonas aeruginosa.

Use of Selective Media

For most samples, it will be necessary to test for at least some objectionable organisms. In order to perform tests for each specific objectionable organism it is necessary to use selective media specifically designed to select for the target organisms. For example, for E coli testing, it is necessary to use selective media that contains both inhibitors that prevent the growth of non-E coli organisms and substrates that can utilized by E coli and not by most other microorganisms. BioLumix make use of such a media, referred to as the EC vial.

Confirmation Test

Unique confirmation tests that can be performed directly from the vials are described for the various objectionable organisms.

E. Coli- Indole Test

For samples that grow and detect in the BioLumix system, a series of Confirmation Tests can be utilized to begin the process of understanding whether the organism(s) are genuine E coli or not. For E coli testing a common initial confirmation test is the Indole Test using the Kovacs reagent. The Indole Test measures the presence of any indole in the growth media as a by-product of tryptophan metabolism by E. coli. Figure 1 depicts a negative (yellow ring) and positive Kovacs Reaction (Red ring) at the top of the media in the test vial.

Staphylococcus aureus – Coagulase Test

The BioLumix vial for testing for the presence of Staphylococcus aureus contains inhibitors of non-Staphylococcus organisms and substrates, such as mannitol as the sole carbon source used by S aureus. If growth is found in the BioLumix STA vial, the analyst can begin to confirm the presence or absence of S. aureus, directly from the vial, using the classic coagulase tests. The coagulase test that has been used for decades uses a known antisera specific for S. aureus epitopes. When S aureus is present, the antiserum reacts with the specific epitopes and forms a lattice of antibody-antigen, and the material coagulates within hours. Figure 2 illustrates the coagulase positive (upper tube) and negative (lower tube) reaction.

Pseudomonas aeruginosa- Oxidase Test

The BioLumix vial for testing for the presence or absence of Pseudomonas aeruginosa contains inhibitors such as Centrimide to prevent the growth of non-pseudomonads and substrates for use by P. aeruginosa. If growth is found in the BioLumix PSE vial, the analyst can begin to confirm the presence or absence of P. aeruginosa using the classic oxidase test. When P. aeruginosa is present, the oxidase test strip reacts with the centrifuged precipitate material (bacterium) and yields a rapid dark blue reaction. This reaction is based on the presence of certain cytochrome oxidase that are found intracellularly in the P. aeruginosa.Figure 3 illustrates the positive oxidase color test ( + ) from the negative reaction ( – ).

Salmonella- Immunoassay Strip

The BioLumix vial for testing for the presence of Salmonella contains inhibitors of non-salmonella bacteria and substrates utilized by Salmonella. If growth is found in the BioLumix SAL vial; the analyst can begin to confirm the presence of absence of Salmonella using commercially available test kits that typically make use of Immunological reactive endpoints. One such kit is shown in the cartoon (Figure 4) and depicts immuno-reactive bands on a test strip.

Identification

Any negative confirmation assay indicates that the target organism is absent and the result is negative. However, in the rare occasions that the vial shows growth and the confirmation assay is positive, it does not necessarily mean that an objectionable organism is present. In these situations further identification of the growing organism might be required. The growing organism could be isolated on selective or non-selective medium and identified by any appropriate identification system.

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