the term &# 34 ; antimicrobial &# 34 ; means an agent which delays onset or retards rate of growth to less than 1 logarithmic increase over a 24 hour period as compared to an unpreserved formulation . the composition of the present invention comprises an oil - in - water emulsion in which the 2 , 6 - diisopropylphenol , either alone or dissolved in a water immiscible solvent , for example a vegetable oil , is emulsified with water by means of surfactant . typically the solvent is an oil such as soybean oil , safflower oil , cottonseed oil , corn oil , sunflower oil , arachis oil , castor oil , or olive oil . preferably the oil is soybean oil . alternatively the solvent can be an ester of a medium or long chain fatty acid , for example a mono -, di -, or triglyceride ; or a compound such as ethyl oleate , isopropyl myristate , isopropyl palmitate , a glycerol ester or a polyoxyl hydrogenated castor oil . other suitable solvents may be marine oils , for example cod liver or other fish derived oils , or fractionated oils , such as fractionated coconut oil or modified soybean oil . the composition of present invention may also comprise a mixture of two or more of the above water immiscible solvents . the 2 , 6 - diisopropylphenol , either alone or dissolved in the water immiscible solvent , is emulsified in an aqueous medium with the aid of a surfactant . suitable surfactants include synthetic non - ionic surfactants , for example ethoxylated ethers and ethoxylated esters , polypropylene polyethylene block copolymers , and phosphatides , as for example egg and soy phosphatides . preferably , the surfactant is egg phosphatide . preferred compositions of the present invention comprise from 0 . 1 to 5 . 0 % by weight , preferably 1 to 2 % by weight , and most preferably 1 % by weight of 2 , 6 - diisopropylphenol ; from to 1 to 30 % by weight , preferably 10 to 20 % by weight of a water immiscible solvent ; and from about 0 . 2 to 2 . 0 % by weight , preferably 1 . 2 % by weight of a surfactant . the compositions of the present invention can also contain ph adjusting agents such as sodium hydroxide or hydrochloric acid so they can be formulated at a physiologically neutral ph . the compositions of the present invention may also be made isotonic by the incorporation of a suitable additive such as glycerol . the antimicrobial systems utilized in the compositions of the present invention are selected from the group consisting of benzyl alcohol ; benzyl alcohol and disodium ethylenediamine tetraacetate ; benzethonium chloride ; and benzyl alcohol and sodium benzoate . the concentration of the antimicrobial agents in the final composition will vary depending on the particular agent or agents selected . for instance in a preferred composition of the invention the amount of benzyl alcohol is in the range of about 0 . 0175 % to 0 . 9 % ( w / v ), more preferably about 0 . 07 % to about 0 . 45 %, most preferred in the range of 0 . 15 %. in an alternate preferred composition of the invention , the amount of benzyl alcohol is about 0 . 07 % to about 0 . 9 %, optionally including an amount of disodium edta of about 0 . 005 %. yet another embodiment provides a composition including an amount of benzethonium chloride of about 0 . 01 % to about 0 . 1 %. optionally , the composition of the present invention includes 0 . 07 % sodium benzoate . the most preferred compositions of the present invention include benzyl alcohol and sodium benzoate . the compositions of the present invention may be prepared by conventional processes as for example that disclosed in u . s . pat . no . 5 , 714 , 520 . a particularly preferred composition of the present invention is as set forth below . table 1______________________________________ ( weight percent ) broad preferred particularly preferred component range range amount______________________________________2 , 6 - 0 . 1 - 5 . 0 1 . 0 - 2 . 0 1 . 0 diisopropylphenol soybean oil 1 . 0 - 30 . 0 10 . 0 - 30 . 0 10 . 0 egg phosphatide 0 . 2 - 2 . 0 0 . 7 - 2 . 0 1 . 2 benzyl alcohol 0 . 0175 - 0 . 9 0 . 07 - 0 . 45 0 . 15 sodium benzoate 0 - 0 . 07 0 . 07 0 . 07 glycerol 2 . 0 - 3 . 0 2 . 35 - 2 . 75 2 . 25 sodium hydroxide q . s . q . s . q . s . water for injection to 100 to 100 to 100______________________________________ the compositions of the present invention may be used as is conventional in the art , e . g ., for the induction of anesthesia prior to maintenance with a conventional inhalation anesthetic ; as a sole anesthetic agent for short duration , by repeated administration , or by continuous infusion . the compositions of the invention may be used as a sole anesthetic agent of longer duration . the compositions of the present invention may be formulated following procedures well known to those skilled in the art . specific reference is made to u . s . pat . no . 5 , 714 , 520 which is hereby incorporated by reference . propofol oil - in - water emulsions containing 0 . 45 % benzyl alcohol / 0 . 005 % disoduim edta , 0 . 035 % benzyl alcohol / 0 . 005 % disodium edta , 0 . 45 % benzyl alcohol / 0 . 07 % sodium benzoate , and 0 . 035 % benzyl alcohol / 0 . 07 % sodium benzoate were tested by the usp 23 preservative effectiveness test as described in united states pharmacopoeia 23 - nf 18 , 1995 ed ., chapter 51 , which is incorporated herein by reference . briefly , this involves inoculating the test solution with 10 5 to 10 6 test organisms per milliliter and then determining the number of surviving organisms after 7 , 14 , 21 , and 28 days incubation at 20 - 25 ° c . using standard microbiological methods . day 0 data is not required by usp 23 but was included in this study . a filtration and buffer wash method was used to remove the inactivating agents for purposes of recovering the microorganisms , but other equivalent methods can also be validated for use . the usp test organisms include the bacteria staphylococcus aureus , escherichia coli , and pseudomonoas aeruginosa , a yeast ( candida albicans ), and a mold ( aspergillus niger ). in order to meet the criteria of the usp 23 preservative effectiveness test , the bacteria must demonstrate a 90 % ( 1 logarithmic ) reduction at day 7 and a 99 . 9 % reduction ( 3 logarithmic ) reduction at day 14 from the initial inoculum level . the initial inoculum level can either be calculated knowing the stock culture concentration or by using a buffer control instead of the test solution . the results , using formulations which are 10 % fat emulsions , are given below in tables 2 through 5 where the number reported in the number of organisms pre milliliter . nd means not detected , i . e ., below the levels of detection by the assay . although same formulations did not meet the criteria of the usp test , in most cases they met the definition of &# 34 ; antimicrobial &# 34 ;. table 2______________________________________ ( 0 . 45 % benzyl alcohol / 0 . 005 % disodium edta ) organism p . time a . niger c . albicans e . coli aeruginosa s . aureus______________________________________inoculum 390 , 000 440 , 000 750 , 000 460 , 000 610 , 000 per ml product 0 hr . 300 , 000 360 , 000 380 , 000 310 , 000 380 , 000 day 7 210 , 000 340 , 000 680 3 , 000 47 , 000 day 14 210 , 000 350 , 000 190 200 7 , 200 day 21 3 , 400 270 , 000 40 60 4 , 300 day 28 130 105 , 000 10 10 1 , 020______________________________________ table 3______________________________________ ( 0 . 035 % benzyl alcohol / 0 . 005 % disodium edta ) organism p . time a . niger c . albicans e . coli aeruginosa s . aureus______________________________________inoculum 390 , 000 440 , 000 750 , 000 460 , 000 610 , 000 per ml product 0 hr . 300 , 000 310 , 000 360 , 000 310 , 000 430 , 000 day 7 300 , 000 330 , 000 40 105 , 000 36 , 000 day 14 210 , 000 310 , 000 & lt ; 10 68 , 000 3 , 500nd day 21 120 , 000 320 , 000 & lt ; 10 67 , 000 740nd day 28 29 , 000 110 , 000 & gt ; 10 38 , 000 170nd______________________________________ table 4______________________________________ ( 0 . 45 benzyl alcohol / 0 . 07 sodium benzoate ) organism p . time a . niger c . albicans e . coli aeruginosa s . aureus______________________________________inoculum 390 , 000 440 , 000 750 , 000 460 , 000 610 , 000 per ml product 0 hr . 290 , 000 340 , 000 380 , 000 440 , 000 390 , 000 day 7 260 , 000 390 , 000 86 , 000 101 , 000 30 , 000 day 14 29 , 000 350 , 000 62 , 000 14 , 900 1 , 350 day 21 22 , 000 203 , 000 80 , 000 2 , 800 100 day 28 290 87 , 000 76 , 000 150 10______________________________________ table 5______________________________________ ( 0 . 035 benzyl alcohol / 0 . 07 % sodium benzoate ) organism p . time a . niger c . albicans e . coli aeruginosa s . aureus______________________________________inoculum 390 , 000 440 , 000 750 , 000 460 , 000 610 , 000 per ml product 0 hr . 370 , 000 450 , 000 420 , 000 550 , 000 520 , 000 day 7 250 , 000 530 , 000 8 , 800 , 000 3 , 500 , 000 310 , 000 day 14 130 , 000 410 , 000 7 , 100 , 000 3 , 400 , 000 92 , 000 day 21 41 , 000 440 , 000 5 , 800 , 000 30 , 000 49 , 000 day 28 13 , 000 300 , 000 2 , 180 , 000 7 , 000 22 , 100______________________________________ the antimicrobial properties of various propofol formulations were determined by a spiked hold time study . briefly , a propofol formulation is inoculated to achieve approximately 100 organisms per 10 ml sample . the organisms used include staphylococcus aureus , pseudomonas aeruginosa , escherichia coli , candida albicans , bacillus subtilis , aspergillus niger , and clostridium sporogenes . the inoculated samples are held for various times and then filtered in duplicate . the filters are washed with buffer and then placed on appropriate agar growth media . the antimicrobial agents should reduce the growth rate such that there is less than a 1 logarithm increase within a 24 hour period . c . albicans presented the most resistance to the preservative system . the c . albicans results from the spiked hold time study are shown in table 6 for various propofol formulations . e . coli , p . aeruginosa , and b . subtilis also demonstrated some resistance to the antimicrobial agents ( data not shown ). the remaining test organisms increased less than 2 - fold over the 7 day test period . the results are shown in organisms per ml of test solution . a value of & gt ; 300 indicates too numerous to count ; these data points may still meet the acceptance criteria of less than 1 logarithmic increase . in the tables , ba = benzyl alcohol , nb = sodium benzoate . the percent of benzyl alcohol is indicated in w / v for each formulation . the percent sodium benzoate , when present , is 0 . 07 % ( w / v ). table 6______________________________________formulation 0 hours 24 hours 48 hours 7 days______________________________________0 . 45 ba / nb 37 35 28 56 0 . 15 ba only 59 102 & gt ; 300 & gt ; 300 0 . 15 ba / nb 59 45 & gt ; 300 & gt ; 300 0 . 13 ba / nb 66 57 & gt ; 300 & gt ; 300 0 . 10 ba / nb 53 59 & gt ; 300 & gt ; 300 0 . 07 ba / nb 34 120 & gt ; 300 & gt ; 300 0 . 035 ba / nb 33 125 & gt ; 300 & gt ; 300 0 . 0175 ba / nb 38 185 & gt ; 300 & gt ; 300______________________________________ propofol oil - in - water emulsions containing 0 . 45 % benzyl alcohol / 0 . 07 % sodium benzoate , 0 . 035 % benzyl alcohol / 0 . 07 % sodium benzoate , 0 . 035 % benzyl alcohol / 0 . 005 % disodium edta , and 0 . 045 % benzyl alcohol / 0 . 005 % disodium edta were tested by the usp 23 preservative effectiveness test as described in example 2 . the results , using formulations which are 1 % fat emulsions , are given below in tables 7 through 10 where the number reported in the number of organisms per milliliter . lowering the percent oil , increased the effectiveness of the preservative ( s ) compound in example 2 . table 7______________________________________ ( 0 . 45 % benzyl alcohol / 0 . 07 % sodium benzoate ) organism p . time a . niger c . albicans e . coli aeruginosa s . aureus______________________________________inoculum 390 , 000 440 , 000 750 , 000 460 , 000 610 , 000 per ml product 0 hr . 470 , 000 1 , 000 & lt ; 10 1 , 200 & lt ; 100nd day 7 33 , 000 & lt ; 100 & lt ; 10 & lt ; 10 & lt ; 100 day 14 & lt ; 10 & lt ; 100 & lt ; 10 & lt ; 10 & lt ; 100 nd nd nd nd nd day 21 & lt ; 10 & lt ; 100 & lt ; 10 & lt ; 10 & lt ; 100 nd nd nd nd nd day 28 & lt ; 10 & lt ; 100 & gt ; 10 & lt ; 10 & lt ; 100 nd nd nd nd nd______________________________________ table 8______________________________________ ( 0 . 035 % benzyl alcohol / 0 . 07 % sodium benzoate ) organism p . time a . niger c . albicans e . coli aeruginosa s . aureus______________________________________inoculum 390 , 000 440 , 000 750 , 000 460 , 000 610 , 000 per ml product 0 hr 420 , 000 1 , 050 & lt ; 10 5 , 500 20nd day 7 200 , 000 & lt ; 10 & lt ; 10 5 , 200 & lt ; 10 nd nd nd day 14 14 , 000 & lt ; 10 & lt ; 10 3 , 700 & lt ; 10 nd nd nd day 21 14 , 000 & lt ; 10 & lt ; 10 4 , 100 & lt ; 10 nd nd nd day 28 17 , 000 & lt ; 10 & gt ; 10 5 , 000 & lt ; 10 nd nd nd______________________________________ table 9______________________________________ ( 0 . 035 % benzyl alcohol / 0 . 005 % disodium edta ) organism p . time a . niger c . albicans e . coli aeruginosa s . aureus______________________________________inoculum 390 , 000 440 , 000 750 , 000 460 , 000 610 , 000 per ml product 0 hr . 460 , 000 112 , 000 & lt ; 10 680 100nd day 7 290 , 000 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 100 nd nd nd nd day 14 4 , 100 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 100 nd nd nd nd day 21 2 , 900 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 100 nd nd nd nd day 28 2 , 700 & lt ; 10 & gt ; 10 & lt ; 10 & lt ; 100 nd nd nd nd______________________________________ table 10______________________________________ ( 0 . 45 % benzyl alcohol / 0 . 005 % disodium edta ) organism p . time a . niger c . albicans e . coli aeruginosa s . aureus______________________________________inoculum 390 , 000 440 , 000 750 , 000 460 , 000 610 , 000 per ml product 0 hr . 320 , 000 300 & lt ; 10 & lt ; 10 & lt ; 10nd nd nd day 7 260 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd day 14 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd nd day 21 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd nd day 28 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd nd______________________________________ propofol oil - in - water emulsions containing 0 . 45 %/ benzyl alcohol / 0 . 005 % disodium edta and 0 . 45 % benzyl alcohol / 0 . 07 % sodium benzoate were tested by the usp 23 preservative effectiveness test as described in example 2 . the results , using formulations which are 1 % fat emulsions of a medium chain triglyceride oil , are given below in tables 11 and 12 where the number reported is the number of organisms per milliliter . table 11______________________________________ ( 0 . 45 % benzyl alcohol / 0 . 005 % disodium edta ) organism p . time a . niger c . albicans e . coli aeruginosa s . aureus______________________________________inoculum 390 , 000 440 , 000 750 , 000 460 , 000 610 , 000 per ml product 0 hr . 490 , 000 180 & lt ; 10 & lt ; 10 & lt ; 10nd nd nd day 7 28 , 000 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd day 14 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd nd day 21 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd nd day 28 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd nd______________________________________ table 12______________________________________ ( 0 . 45 % benzyl alcohol / 0 . 07 % sodium benzoate ) organism p . time a . niger c . albicans e . coli aeruginosa s . aureus______________________________________inoculum 390 , 000 440 , 000 750 , 000 460 , 000 610 , 000 per ml product 0 hr . 420 , 000 260 & lt ; 10 1 , 780 & lt ; 10nd nd day 7 20 , 000 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd day 14 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd nd day 21 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd nd day 28 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 & lt ; 10 nd nd nd nd nd______________________________________