The antimicrobial preservative efficacy of the 
eye-drops challenged with E. coli, S. aureus and 
P. aeruginosa is shown in Table 1.Viable 
microbial count, as recommended in both 
Pharmacopeia, was determined by plate count 
method using 1 ml of 1:10 dilution of product in 
neutralizer and no bacterial growth means that the 
number of challenging bacteria was reduced to 
lower than 10 CFU per ml of the product instead 
of NR. Therefore, it seems that the term of NR 
should define an acceptable range.The 
preservative employed in phenylephrine zinc eyedrop did not possess adequate antimicrobial 
activity against P. aeruginosa to be able to bring 
about acceptable low levels of microbial 
contamination as demanded by regulatory bodies.Other eye-drops showed appropriate reductions in 
bacterial viability after 6 hrs, 24 hrs and 7 days,
except a very low bacterial recovery after 28 days
(10-50 CFU ml-1
) which didn't comply with the 
no recovery (NR) term of BP 'A' criteria.After a contact 
time of 6 hrs all the eye-drops except 
phenylephrine zinc which showed only 1.7 logs
reduction in P. aeruginosa initial count, reduced
at least 2 logs of all bacterial counts.The 
number of P. aeruginosa in phenylephrine zinc 
was reduced 2 logs after 24 hrs of inoculation and
was increased to about the initial count after 7
days.After 14 days, all the eye-drops except 
phenylephrine zinc which showed 1 log reduction 
in P. aeruginosa count appeared well preserved 
against all the challenging organisms (?3 logs
reduction).As shown in Table 3, more than 2 logs reduction 
in bacterial counts (after 30 min) and more than 3
logs reduction in fungal counts (after 24 hrs) were 
observed for all eye-drops except phenylephrin 
zinc.Most of the eye-drops eradicated the inoculated 
microorganisms more than 3 logs in 24 hrs and 
also 7 days, except phenylephrine zinc.After 28 days, there was no bacterial recovery 
from betamethasone eye-drop, while the number 
of P. aeruginosa in phenylephrine zinc increased.The results of this study showed that eight out of 
the nine products met the BP 'B' criteria and USP
while all of them except artificial tear were highly 
contaminated during hospital uses (Table 4).Other eye-drops showed no increase in bacterial 
counts after 28 days which were about 10-50 CFU
ml-1
of the products. In all cases the number of fungi after 7 and 14
days were acceptable and those after 28 days were 
at least 2 logs lower than the initial counts (Table 
2).Therefore another effective antimicrobial 
preservative system for this formulation should be 
employed.