Express Evaluation of Staphylococcus in Food

The percentage of compliance between Petrifilm STX and the reference method was calculated for 77 out of the 82 samples considered in this study. The results of the 5 discharged samples could not be used because of technical problems. Of the 77 results, 62 were equivalents and the percentage of compliance was 80.5 percent. This level is considered excellent for a new method. Amongst the discordant results, 9 samples (11.7 percent) presented higher counts of coagulase-positive S. aureus when tested with Petrifilm STX and 6 (7.8 percent) when using BP method.

There was no significant difference (p >0.05) between the results of the counts obtained by both methods, being the correlation coefficient (r) equal to 0.950, slope equal to 1.00 and the intercept equal to -0.1545, as shown in the Figure 4.

Using paired T- Test, the average count was 4.16 log using BP method and 4.32 log using Petrifilm STX, getting a difference of -0.154571 between the averages. The standard deviation was 2.26512 for BP method and 2.15304 for Petrifilm STX. The T-value (-1.01) and p-value (0.326), related to the test, also indicate that the methods did not present statistically significant difference, allowing the acceptance of the hypothesis that, BP method and Petrifilm STX methods are equivalent. (Continued On Page 59)

Conclusion

The results obtained in this study indicate that the 3M Petrifilm Staph Express System may be an alternative method to the FDA-BAM protocol for enumeration of coagulase-positive Staphylococcus.

References:

• Lancette, G.A., Bennett, R.W. Staphylococcus aureus and staphylococcal enterotoxins. In: Downes, F. P.; Ito, K. “Compendium of methods for the microbiological examination of foods.” 4 ed. APHA, 2001, p. 387-403.
• Panalimentos. “Sirveta Sistema de informação para a Vigilância das Enfermidades Transmitidas por Alimentos.” Disponível em: http://www.panalimentos.org/sirveta. Consultado em 05/04/04.
• 3M Do Brasil LTDA. “3M Petrifilm Staph Express Count System Interpretation Guide.” USA, 2002.

B.D. Gombossy, P.K. Noda, G.M. Franco, M. Landgraf and M.T. Destro are members of the of Pharmaceutical Sciences Faculty at the, University of Sao Paulo, Brazil Faculty, while A.R. Tassinari is from 3M do Brasil Ltda. For more information, contact Vicki Moss, sr. communications administrator, 3M Microbiology, 651-733-1213.