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Significance of Case Style in the Transfer of Pseudomonas
aeruginosa to Contact Lenses During Storage
Linda M. Ellis
Department of Biology, York College of Pennsylvania
ABSTRACT
The leading cause of human keratitis in the United States is a
Pseudomonas aeruginosa eye infection related to contact lens use.
Though potentially blinding, keratitis is prevented by proper hygiene.
This study’s objective was to compare the amount of bacteria
transferred to lenses from infected cases of different styles. Three
types of cases were inoculated with P. aeruginosa and incubated with a
sterile contact lens. First, lens disinfection by commercial saline was
evaluated as a negative control. Second, the level of bacterial transfer
was assessed with varying incubation times and concentrations of salt
solution. Finally, Ciba, Alcon and Bausch & Lomb brand cases were
compared by incubating disposable contact lenses in nutrient broth and
swirl-plating the broth in nutrient agar. Colonies were quantified by
standard plate count and the results analyzed by nonparametric
ANOVA (n = 10). The difference between the three case styles was
not significant (p = 0.3138).
There are more people in the world today
who need visual correction than people with
naturally perfect vision (Dart 1991).
The most serious complication of contact
lens wear is corneal inflammation, or
keratitis. Bacterial keratitis, particularly
when caused by the species Pseudomonas
aeruginosa, can lead to blindness
(Rosenfeld 1990).
Manufacturers continuously test the
disinfecting capacity of their saline solutions
to prevent eye infections, such as keratitis.
Yet, even sterile contacts can transmit
bacteria to the eye when stored in an
infected case.
This study tests the ability of saline to
prevent lens contamination from an unclean
case. And, the probability of bacterial
transfer from case to lens is compared
between case styles.
Questions Asked
1. Does commercial saline kill
bacteria in storage cases?
2. What is the limit to disinfection?
3. Which case style is most prone to
transfer bacteria to lenses?
Bacteria was not diluted or plated in
experiment I because there was no
growth from the lens placed in nutrient
broth after 24-hours of incubation at 37°
C. Figure one shows the growth on the
plates from experiment II. There was a
lawn on every 100 and 102 dilution plate.
The most clearly differential colonies were
obtained on plate L, which was incubated
for 24-hours in a seven-percent salt
solution and plated at a 106 dilution.
The mean number of colonies grown
from Ciba broth was 91.2, while the mean
for Alcon was 72.5, and 43.7 for Bausch &
Lomb. Each of the three categories of
case style had a sample size of ten from
five separate dilutions.
The median
number of colonies on the Ciba plates was
79, Alcon’s median was 75, and the
median of colonies from Bausch & Lomb
brand cases was 47. The Kruskal-Wallis
nonparametric ANOVA test indicated that
the variation among the medians was not
significantly greater than expected by
chance with a p value of 0.3138.
P. aeruginosa grown
In nutrient broth
Add broth culture
and lens to cases
Ex. III
Varied
Case Style
Ex. II
Varied Time &
Salt Solution with
Serial Dilution
Incubate lenses
in nutrient broth
Swirl-plate
and count colonies
Table 1. Percent salt concentrations and
incubation times of cases in experiment II.
Figure 1. Bacterial colonies
growth from contacts stored
in different brands of cases in
experiment III.
Experiment III Results
120
Incubation Time (hrs.)
% salt
0
1
2
A
D
8
B
E
24
C
F
5
G
H
I
7
J
K
L
Average # of
colonies/plate
INTRODUCTION
RESULTS
METHODS
Ex. I
Commercial
Saline
DISCUSSION
*
100
The total lack of growth in experiment I
clearly exemplified the effectiveness of Alcon
brand commercial saline. If lenses are
properly disinfected there is virtually no risk
of P. aeruginosa infection from contaminated
storage cases as long as commercial saline is
properly used.
However, the risk of bacterial transfer
increased proportionally with incubation time
and inversely to salt solution concentration.
Perhaps a greater sample size would
indicate Bausch & Lomb as having the lowest
risk of the three case style. While lens sit
directly on the bottom of the flat cases, it
suspends the lens above the infected case.
Nonetheless, there is no significant
difference in the risk of lens contamination
from the three case types.
Conclusions
1. Alcon™ commercial saline
completely disinfects cases
2. Salt solutions are not an
effective means of storage
case disinfection
3. There is no significant
difference between case styles
in risk of bacterial transfer
Literature Cited
Dart, J.K., Stapelton, F. and Minassian, D. 1991.
Contact lenses and other risk factors in
microbial keratitis. The Lancet 338:650- 654.
Rosenfeld, S.I. et al. 1990. Granular epithelial
keratopathy as an unusual manifestation of
pseudomonas keratitis associated with
extended-wear soft contact lenses. American
Journal of Ophthalmology 109:17- 23.
80
60
40
20
0
Ciba
Alcon
Case Brand
* standard error of the mean
B&L
Acknowledgements:
Barbara Taylor, lab instructor
Ellis Eyes, optical supplies
Dr. Jeffrey Thompson, PhD, senior thesis advisor