the present invention will now be described by way of example only with reference to the figures which show : fig1 illustrates the histidine biosynthesis pathway consisting of 10 enzymatic reactions ; fig2 illustrates acanthamoeba growth inhibition in the absence of histidine , wherein acanthamoeba growth is inhibited in a dose dependent manner — the ic 50 is between 0 . 015 mm and 0 . 031 mm — the x axis represents the dose of 3 - at in mm and the y - axis is the percentage of alamarblue reduction , which is directly proportional to mitochondrial activity ; fig3 illustrates acanthamoeba growth inhibition is ablated in the presence of histidine wherein the addition of 10 mm histidine can allow acanthamoeba to grow in the presence of 0 . 98 mm ( ic 20 ) 3 - at — the x - axis represents the quantity of histidine , whereas the y - axis represents the percentage of alamarblue reduction and 100 mm histidine was toxic to acanthamoeba , but at 10 mm histidine significantly ( p & lt ; 0 . 05 ) ablated the effect of 3at ; fig4 illustrates a reduction of alamarblue over a range of concentrations of 3at wherein 3at (▪) significantly ( p & lt ; 0 . 05 ) inhibited a . castellanii growth , compared with untreated control cultures , in a dose dependent manner with concentrations of 0 . 00095 mm or above . in the presence of 1 mm histidine (♦) this was reduced and 0 . 06 mm 3at or above was required in order to significantly ( p & lt ; 0 . 05 ) inhibited a . castellanii growth ; fig6 illustrates the cobalamin - independent methionine biosynthesis pathway and its enzymatic reactions . the inventors have cloned and sequenced the sixth enzyme in the histidine biosynthetic pathway , imidazole glycerol - phosphate dehydratase ( igpd ) ( ec 4 . 2 . 1 . 19 ), from a . castellanii ( see fig1 ). this enzyme is the molecular target for 3 - amino - 1 , 2 , 4 - triazole ( 3at ), a commonly used commercially available herbicide the imidazoleglycerol - phosphate dehydratase ( igpd ) gene ( hisb ) of acanthamoeba can be obtained according to the following protocol . in a total volume of 25 μl , 1 μl , cdna ( derived from acanthamoeba grown in the absence of histidine ) was added to 1 μl 10 mm dntp mixture , 2 mm mgso4 , 25 pmol of forward primer ( 5 ′ gaaaagagggaggcacaggt 3 ′) ( seq id no 1 ) and 25 pmol of reverse primer ( 5 ′ atcactcgagtacgcccttgg 3 ′) ( seq id no 2 ) in the presence of 1 unit of platinum taq high fidelity and 10 × high fidelity buffer . the thermo cycling conditions consist of an initial denaturation at 95 ° c . for 3 minutes , followed by 35 cycles of denaturation at 95 ° c . for 30 seconds , annealing at 62 ° c . and extension at 72 ° c . a final extension step is at 72 ° c . for 10 minutes . these thermocycling conditions were also used to obtain the fragments of the other genes of enzymes belonging to the histidine biosynthesis pathway . as illustrated in fig2 and 3 , the inventors have determined that acanthamoeba can grow in the absence of histidine and that 3 - amino triazole , an inhibitor of igpd , the sixth enzyme of the histidine biosynthetic pathway inhibits growth of acanthamoeba . furthermore , inhibition was ablated in a dose dependent manner by the addition of histidine to the medium . this demonstrates that 3at acts specifically to inhibit igpd and that the histidine biosynthesis pathway in a . castellanii . this provides the first evidence of the histidine biosynthesis pathway in a protozoan and demonstrates its potential as a target for therapeutic intervention during acanthamoeba infection . the inventors have further identified portions of genes that transcribe enzymes belonging to the histidine pathway providing further support that the histidine biosynthetic pathway is present in acanthamoeba . the inventors have also determined portions of genes that transcribe enzymes belonging to the methionine pathway which can synthesise methionine from cysteine in a cobalamin - independent manner ( see fig6 ). the inventors have further established that acanthamoeba castellanii possesses this metabolic pathway as they have identified that that it expresses two genes that transcribe enzymes involved in methionine biosynthesis , namely , methionine synthase and cystathione gamma synthase . currently , there are no known inhibitors for cobalamin - independent methionine synthase but studies concentrating on cobalamin - independent methionine synthases from fungal species are expected to identify plausible candidates . inhibitors of cystathionine gamma synthase include dl - e - 2 - amino - 5 - phosphono - 3 - pentenoic acid , 3 -( phosphonomethyl ) pyridine - 2 - carboxylic acid and 5 - carboxymethylthio - 3 -( 3 ′- chlorophenyl )- 1 , 2 , 4 - oxadiazol . using the protocol and thermocycling conditions of example 1 , fragments of the genes of methionine synthase and cystathione gamma synthase , belonging to the methionine biosyntheis pathway , were obtained . the reverse and forward primers for use therein are a shown in the table 1 below . using the above protocol , different sections of the methionine synthase gene were determined as described below . the inventors have found that inhibitors of the histidine and / or methionine pathways are useful in eye care products including artificial tears and rewetting drops . in such preparations the inhibitors can generally function as preservative ( s ) and so prolong the product shelf life by preventing acanthamoeba contamination . a typical eye care product formulation may include water and salts with one or more of : carboxymethyl cellulose , hydroxypropyl methycellulose , hydroxypropyl cellulose , glycerin , propylene glycol , polyethylene glycol ( peg 400 ) and hp - guar . important aspects of such formulations are lubrication and osmoprotection and the formulations must be compatible with contact lens wear as they would be considered as in eye contact lens solutions . although the invention has been particularly shown and described with reference to particular examples , it will be understood by those skilled in the art that various changes in the form and details may be made therein without departing from the scope of the present invention