ASTM_D_3870_-_91.pdf

Designation:D 3870 91Standard Practice forEstablishing Performance Characteristics for ColonyCounting Methods in Microbiology1This standard is issued under the fixed designation D 3870;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon(e)indicates an editorial change since the last revision or reapproval.1.Scope1.1 This practice deals with the performance characteristicsof enumeration methods for microorganisms of health andsanitary significance.The performance characteristics covermembrane filter,pour plate,and spread-plate colony countingtechniques.A performance characteristic is a quantitative,experimentally determined value that is used to assess thesuitability of an analytical method for a given purpose.Theperformance characteristics dealt with here are specificity,including selectivity,recovery,upper counting range,andprecision and lower counting range.1.2 The purpose of establishing performance characteristicsis to provide a set of uniform properties to describe bacterialenumeration techniques and selective media.1.3 This standard does not purport to address the safetyproblems,if any,associated with its use.It is the responsibilityof the user of this standard to establish appropriate safety andhealth practices and determine the applicability of regulatorylimitations prior to use.2.Referenced Documents2.1 ASTM Standards:D 1129 Terminology Relating to Water23.Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 lower limit of counting rangethat count belowwhich the anticipated error becomes unacceptably large inrelation to the count itself.3.1.2 precisionthe degree of agreement of repeated mea-surements of the same sample.The usual index of precision isthe standard deviation.3.1.3 recoverythe degree of agreement between the den-sity of microorganisms obtained with a test method and thedensity obtained with an acceptable reference method.3.1.4 selectivitythe ability of a method to encouragegrowth of the target organism while retarding development onnontarget organisms.In this way,overcrowding problems canbe minimized.3.1.5 specificitythe ability of a method to select anddistinguish the microorganism under consideration from allothers in the same environment.3.1.6 upper limit of counting rangethat point above whichthe reliability of the colony count on a single plate ormembrane from a specified volume is affected by uncontrol-lable factors.3.2 DefinitionsFor definitions of other terms used in thispractice,refer to Terminology D 1129.4.Significance and Use4.1 Data on the performance characteristics are required todescribe the acceptability of microbiological counting methodsto the user.4.2 Such data are used to determine the applicability ofcounting methods for research,monitoring,and regulatorypurposes in order to assure uniformity and comparability ofmethod results.4.3 Living microorganisms are inherently more variable innumbers and in responses to test conditions,than chemicalanalytes.Hence,there is a need to establish criteria to assurethat different microbiological methods are evaluated and char-acterized against a standard set of performance characteristics.These are herein established.5.Statistical Procedures5.1 Specificity and Selectivity:5.1.1 Specificity is evaluated by selecting a representativenumber of target and nontarget colonies recovered fromvarious aquatic environments.Multiple dilutions of a watersample are plated or filtered in triplicate from a sample orsample dilution that will provide noncrowded colonies.Incu-bate as directed.Examine all the colonies from no less thantwo plates or filters.Each plate must contain at least 30presumptive target organisms.Perform sufficient biochemicaltests on each colony to identify it as the target organism.Designate as false positives all colonies that do not verify astarget types.Similarly,designate as undetected target allpresumptive nontarget colonies that verify as target types.5.1.2 The results of specificity testing are expressed as twoindividual terms;the error introduced by false positive coloniesand the error resulting from undetected target colonies.Calcu-late the first term by dividing the number of false positivetarget colonies by the total presumptive target colony count.Ifthere are no false positive colonies,this term will equal zero.1This practice is under the jurisdiction of ASTM Committee D-19 on Water andis the direct responsibility of Subcommittee D19.24 on Water Microbiology.Current edition approved June 15,1991.Published October 1991.Originallypublished as D 3870 79.Last previous edition D 3870 79(1984)e1.2Annual Book of ASTM Standards,Vol 11.01.1AMERICAN SOCIETY FOR TESTING AND MATERIALS100 Barr Harbor Dr.,West Conshohocken,PA 19428Reprinted from the Annual Book of ASTM Standards.Copyright ASTMCalculate the second term by dividing the number of