142 GENERAL MICROBIOLOGY 



Age. Dilutions. 



Fresh milk. . 1 : 1,000, 1 : 10,000 and 1 : 100,000 



One day old 1 : 10,000, 1 : 100,000 and 1 : 1 M * 



Four days old 1 : 10 M, 1 : 100 M and 1 : 1,000 M 



Eight days old 1 : 10 M, 1 : 100 M and 1 : 1,000 M 



Ten days old 1 : 1 M, 1 : 10 M and 1 : 100 M 



Keep the plates at room temperature. 



Sterile pipettes are to be used always in making dilu- 

 tions, plating and titrating. 



After the milk curdles it is advised to make the first 

 dilution 1 : 10 to give a more uniform sample, from which 

 further dilutions are made. Use a 10 c.c. pipette having 

 a large opening in the delivery end to prevent clogging. 



4. Titrate the milk sample every day. After the milk 

 curdles, shake well before titrating and choose a 5 c.c. 

 pipette having a large aperture for delivery for obtaining 

 the sample for titration. 



5. Record the reaction in degrees of Fuller's scale. 

 After using pipettes, dilution flasks, etc., clean, refill 



and sterilize them at once for future use. 



6. Note the macroscopical changes in the milk sample 

 (due to microbial growth), e.g., kind and consistency of 

 curd, extrusion of whey, gas formation, peptonization; also 

 note odor from time to time. 



7. Note the macroscopical evidences of microbial growth 

 such as molds, etc., and the time of appearance. Identify 

 the group to which these organisms belong, giving genus 

 and species if possible. 



8. Determine the changes in the numbers of micro- 

 organisms by counting the colonies of the different sets of 

 plates after they have developed seven days at room tem- 

 perature (see p. 56, Exercise 14, for method). 



9. Estimate the number of colonies of each type (see 

 Exercise 14, p. 56). 



*M = Million. 



