environmental sampling S.aureus

[SKA]

What are the differences in NA and BP medium plates in regard to environmental sampling.
How come it will only settle on BP medium plates? what does agglutination have to do with it?

[theprof]

OK, I'll run through this for you but you seem to be unaware of some of the features of BP medium, so forgive me if I go back to the beginning.

S. aureus selective media utilise a number of different
toxic chemicals to achieve selectivity. Some of the ingredients used include sodium chloride, tellurite, lithium chloride and various antibiotics. In Baird-Parker they are lithium chloride and potassium tellurite.

Most selective media are suitable for the enumeration
of normal or unstressed S. aureus. However, due to processing, preservation or other adverse conditions,
sub-lethal stress may occur, resulting in the increased sensitivity of S. aureus to the selective agents. Because injured cells exhibit an increased sensitivity to selective agents, S. aureus may go undetected in conventional selective enumeration procedures. This is precisely what occurs when NA media is used.

(continued)

[theprof]

It has been demonstrated that the recovery of heated or dried cells of S. aureus may be lost or its activity reduced by heating or drying and that blood, which contains catalase, or the addition of pyruvate, helped in the enumeration by destroying hydrogen peroxide produced by recovering cells. Baird-Parker agar is most satisfactory in enumerating injured cells when compared with other staphylococcal selective media. Some authors have suggested that most staphylococcal species of clinical significance can be identified on the basis of a few key characteristics. These include colony morphology, coagulase production, oxygen requirements, haemolysis, novobiocin resistance, acetylmethylcarbinol (acetoin) production, aerobic utilization of selected carbohydrates and certain enzyme activities.

On a non-selective agar such as tryptic soy agar or nutrient agar, most staphylococcal species grow abundantly in 18-24 hours when incubated at 35"C, with colony diameter generally 1-3 mm. Colony morphology may be an aid to species identification, and colony pigmentation is of importance.

[theprof]

The value of monitoring live S. aureus is to ascertain
the hygienic quality of the food and the potential of the live
organisms to grow and produce the enterotoxins in foods.

Detection of staphylococcal enterotoxins has been a subject of much research in the past 25 years. Animals have been used to detect toxins in the past but are not
practical for routine testing.

Latex agglutination tests, gel diffusion and methods such as ELISA are the now preferred Immunological methods employed to detect the toxin in foods.

Commercial kits can detect enterotoxin A, B, C, D, and/or E
either singularly or in combination.