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aPhenol coefficient number for S. typhi (PCN S. typhi) = 350/90 = 3.89. 350, Highest dilution of disinfectant that kills test organism in 10 min but not in 5. 90, Highest dilution of phenol that kills test organism in 10 min but not in 5. 3.89, Dilution of disinfectant capable of killing S. typhi in 10 min but not in 5. b0, organism not present; +, organism present.

aPhenol coefficient number for S. typhi (PCN S. typhi) = 350/90 = 3.89. 350, Highest dilution of disinfectant that kills test organism in 10 min but not in 5. 90, Highest dilution of phenol that kills test organism in 10 min but not in 5. 3.89, Dilution of disinfectant capable of killing S. typhi in 10 min but not in 5. b0, organism not present; +, organism present.

11. Thoroughly agitate individual subculture tubes before incubation. Macroscopic examination is usually sufficient. Occasionally a 3-d incubation period, agar streak, microscopic examination, or agglutination with specific antiserum may be necessary to determine feeble growth or suspected contamination.

3.2. Calculation

Express results in terms of phenol coefficient number, or highest dilution that kills the test organism in 10 min but not in 15 min, whichever most accurately reflects the germicidal value of the disinfectant. The phenol coefficient is the number obtained by dividing the numerical value of the greatest dilution (denominator of fraction expressing dilution) of disinfectant capable of killing the microorganism in 10 min but not in 5 min by the greatest dilution of phenol showing the same results. Examples of how to calculate the phenol coefficient number are given in (Tables 3 and 4).

If none of the dilutions of disinfectant shows growth in 5 min and killing in 10 min, estimate a hypothetical dilution only when any three consecutive dilutions show the following results: first, no growth in 5 min; second, growth in 5 and 10 min but not in 15 min; and third, growth in 5, 10, and 15 min (Table 5).

4. Notes

1. All methods used to evaluate antimicrobial efficacy are influenced by factors that can affect the activity of a disinfectant, such as (5):

a. Efficacy under test conditions.

b. Activity in the target food process.

c. Factors inherent in the microorganism (phenotypes and genotypes from the type test satrain) (11).

d. The antimicrobial agent.

e. The test medium.

Table 4

Satisfactory Readings for Phenol Controla

Table 4

Satisfactory Readings for Phenol Controla

Phenol dilution

5 min

10 min

15 min

1:90

+ or 0

+ or 0

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