THE PRESERVATION OF OPHTHALMIC PREPARATIONS 383 Ps.aeruginosa increased progressively from an initial 4 •g/ml to 64 •g/ml after 8 subcultures in the presence of chlorhexidine diacetate. Eronopol (2-bromo-2-nitropropane-1, 3-diol) is a new antimicrobial agent shown to be active against numerous strains of several gram-positive and gram-negative species including 22 strains of Ps.aeruginosa (52). Pre- liminary experiments showed that bactericidal concentrations are not irritant to the skin or mucous membranes. The authors failed to increase the resistance of Ps.aeruginosa and Staph.aureus to Bronopol by repeated subculture in its presence. SUMMARY AND CONCLUSIONS Consideration of the available published evidence reveals that at present there is no single ideal preservative for ophthalmic preparations. The capacity of strains of Ps.aeruginosa or closely related species to become resistant to quaternary ammonium compounds (11), chlorbutol (39), phenylethyl alcohol (36) chlorhexidine (51) and even to decompose phenol (53) and phydroxy-benzoates (9) would suggest caution in accepting any new agent supposed not to have this defect against this enigmatic organism. Many of the results in the literature are conflicting. Some of the discrepancies may be accounted for by reason of variation in such important factors as reaction temperature, composition of reaction mixture, and recovery medium. Importance must be attached to the results of those workers who have clearly defined their experimental conditions, and who have related the efficiency of the inactivators used in their recovery media to the results of in vivo tests. The capacity or not to cause disease would appear to be the ultimate, objective assessment, particularly when activity is measured in a final ophthalmic formulation. An appraisal of the scientifically acceptable data on the phydroxy- benzoates would suggest that nearly saturated solutions are capable of sterilizing heavy inocula (108) of Ps.aeruginosa in several hours. Such concentrations are too irritant for normal ophthalmic use. A concentra- tion of about 0.1% mixture may be useful in some eye preparations excluding, in particular, fluorescein. The capacity of Ps.aeruginosa to utilise (9) these compounds as a source of carbon would seem to preclude their recommendation for widespread use. The available evidence suggests that the concentration used in Solution for Eye Drops B.P.C. (0.0229% methyl, 0.0114% propyl) is unlikely to sterilize, and may possibly allow growth of Ps.aeruginosa.

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The small amount of available evidence supports the use of chlorocresoI 0.05% for use with intact, but not damaged eyes. The considerable volume of work on chlorbutol has shown that almost all workers have found it to be consistently an effective, but relatively slow, sterilizing agent. It would seem that formulation aspects of this agent have been adequately investigated (36,54). The mercurials have been shown to be effective sterilizing agents by nearly all workers. Riegelman et al (11) found 0.01% P.M.N. required longer than a week to sterilize a heavy inoculum of Ps.aeruginosa, but Kohn et al (24), using several strains and similar procedures to those of Riegelman and his co-workers, found that sterilization was effected in 6 hr. It has been asserted that these mercurials may give rise to mercurial sensitization ("mercurialentis"), and that mercury may deposit on the lens capsule (6,26). There is little published evidence to support these assertions. Ridley (16) found very few cases of dermatitis medicamentosa with P.M.N. preservation. These few cases were with drops used for a long period of time, and in every case the reaction was slight. Work is in progress in our laboratory to elucidate this situation. Rabbits' eyes have been irrigated with mercurial solution, subsequently sectioned, and examined with an electron probe microanalyser. It is hoped that these procedures will provide useful information about local concentrations of mercury. The available evidence offers little support for the use of phenylethyl alcohol as an ophthalmic preservative. The existence of resistant Ps.aerug- inosa strains would not appear to be the result of inadvertent training due to faulty procedures. The evidence about quaternary ammonium compounds is clouded by numerous quotations of opinions, assertions unsupported by data, and even errors in quoting the literature. A literature search has revealed very few instances where resistance to benzalkonium chloride has been demonstrated. Murphy et al (36) and Riegelman et al (11) have shown the existence of highly resistant strains of Ps.aeruginosa. On the other hand, it has been shown that faulty procedures may enable contaminants to acquire resis- tance, particularly to quaternary ammonium compounds (55). The available evidence supports the conclusion of Riegelman and Vaughan (1) "with all its limitations, benzalkonium chloride is among the most effective and rapidly acting preservatives when the conditions of its use are properly controlled."