General Overview

Pseudomonas aeruginosa is a Gram-negative, aerobic rod belonging to the bacterial family Pseudomonadaceae. Like other members of the genus, Pseudomonas aeruginosa is a free-living bacterium, commonly found in soil, water and biofilms. However, it occurs regularly on the surfaces of plants and occasionally on the surfaces of animals. Pseudomonas aeruginosa is an opportunistic pathogen, meaning that it exploits some break in the host defenses to initiate an infection. (CDC 1990, Todar 2012) .

Pseudomonas aeruginosa generally invades the body or organs lacking natural defenses, and usually the infection are chronic rather than acute, evoking little systemic response. When introduced into the cornea, however, as in penetration by a foreign body or in contaminated medicines, it acts with extreme virulence, in many cases causing blindness and even necessitating enucleation (Spencer, 1953) . It is the organism most frequently implicated in contact lens related keratitis (Lawin-Brüssel et al., 1993) .

http://www.cdc.gov/mmwr/preview/mmwrhtml/00001546.htm

Summary Data

Lawin-Brussel et al. (1993) studied the effect of Pseudomonas aeruginosa in experimental contact lens related microbial keratitis. Forty white New Zealand rabbits were used in the experiment. New and worn soft contact lenses were soaked in 5 ml bacterial suspension of different concentrations for an hour, then fitted into the eyes of the rabbit (one lense per rabbit). The clinical findings after seven days of contact lens wear and additional 48 hours of P. aeruginosa contaminated lens wear were recorded. The experimental dose unit was CFU/ml. However, Lawin-Brussel et al. (1993)  had observed quantification of bacterial attachment on soft contact lens after an hour of soaking in different concentration of P. aeruginosa. Based on the figure, the bacterial attachment on a lens was calculated as the dose.

Dose Conversion Table
CFU/ml CFU (per lens)
100 63.22
10000 2220.49
100000 13159.77
1000000 77991.7
10000000 86799.77
100000000 462219,6

Similarly, Hazlett et al (1978)  studied the susceptibility of newborn and infant mice to eye infection by P. aeruginosa. 15-16-day old mice were infected by topical application of P. aeruginosa of various dilutions onto wounded and unwounded corneas. None of the infected mice died of bacteremia and only those mice receiving corneal wounding before infection showed keratitis and eye shrinkage. Moreover, no cataract formation was detected in the mice.

 

Recommended Model

 

The pooled model (experiment number 297 and 298) was recommended as both the data could statistically pooled and more data in the model means improvement in fits. The recommended model is as the marker of severe eye infection , it could be either corneal ulceration or stromal ulceration.

Exponential and betapoisson model.jpg

 

ID Exposure Route # of Doses Agent Strain Dose Units Host type Μodel LD50/ID50 Optimized parameters Response type Reference
286 contact lens 5.00 ATCC 19660 CFU Swiss webster mice exponential 7.88E+06 k = 8.8E-08 infection (Keratitis)
Hazlett, L. D., Rosen, D. D., & Berk, R. S. (1978). Age-Related Susceptibility to Pseudomonas aeruginosa Ocular Infections in Mice. Infection and Immunity, 20, 1.
297 contact lens 5.00 CFU white rabbit beta-Poisson 6.57E+03 a = 3.55E-01 N50 = 6.57E+03 corneal ulceration
Lawin-Brüssel, C. A., Refojo, M. F., Leong, F. L., Hanninen, L. ., & Kenyon, K. R. (1993). Effect of Pseudomonas aeruginosa concentration in experimental contact lens-related microbial keratitis. Cornea, 12, 1.
297 & 298 contact lens 10.00 CFU white rabbit beta-Poisson 1.85E+04 a = 1.9E-01 N50 = 1.85E+04 corneal ulceration
Lawin-Brüssel, C. A., Refojo, M. F., Leong, F. L., Hanninen, L. ., & Kenyon, K. R. (1993). Effect of Pseudomonas aeruginosa concentration in experimental contact lens-related microbial keratitis. Cornea, 12, 1.
298 contact lens 5.00 CFU white rabbit beta-Poisson 1.52E+05 a = 1.09E-01 N50 = 1.52E+05 Severe stromal ulceration
Lawin-Brüssel, C. A., Refojo, M. F., Leong, F. L., Hanninen, L. ., & Kenyon, K. R. (1993). Effect of Pseudomonas aeruginosa concentration in experimental contact lens-related microbial keratitis. Cornea, 12, 1.
Exposure Route:
contact lens
# of Doses:
5.00
Agent Strain:
ATCC 19660
Dose Units:
CFU
Host type:
Swiss webster mice
Μodel:
exponential
LD50/ID50:
7.88E+06
Optimized parameters: k = 8.8E-08
Response type:
infection (Keratitis)

Swiss webster mice /  Pseudomonas aeruginosa 
Dose Infection (keratitis)

Not Infection (keratitis)

Total
1E+04 0 8 8
1E+05 1 9 10
1E+06 1 7 8
1E+07 5 5 10
1E+08 10 0 10

 

Goodness of fit and model selection
Model Deviance Δ Degrees 
of freedom
χ20.95,1 
p-value
χ20.95,m-k 
p-value
Exponential

3.6

0.134 4 3.84 
0.715
9.49 
0.464
Beta Poisson

3.46

3 7.81 
0.326
Exponential is preferred to beta-Poisson; cannot reject good fit for exponential.

 

Optimized k parameter for the exponential model, from 10000 bootstrap iterations
Parameter MLE estimate Percentiles
0.5% 2.5% 5% 95% 97.5% 99.5%
k 8.8E-08 3.31E-08 3.89E-08 4.53E-08 1.58E-07 1.94E-07 2.47E-07
ID50/LD50/ETC* 7.88E+06 2.81E+06 3.57E+06 4.38E+06 1.53E+07 1.78E+07 2.09E+07
*Not a parameter of the exponential model; however, it facilitates comparison with other models.

 

Parameter histogram for exponential model (uncertainty of the parameter)

Exponential model plot, with confidence bounds around optimized model

Exposure Route:
contact lens
# of Doses:
5.00
Agent Strain:
Dose Units:
CFU
Host type:
white rabbit
Μodel:
beta-Poisson
LD50/ID50:
6.57E+03
Optimized parameters: a = 3.55E-01 N50 = 6.57E+03
Response type:
corneal ulceration

White rabbit /  Pseudomonas aeruginosa 
Dose Corneal Ulceration

Not Corneal Ulceration

Total
63.2 0 5 5
2220 2 3 5
13200 3 2 5
78000 3 2 5
462000 5 0 5

 

Goodness of fit and model selection
Model Deviance Δ Degrees 
of freedom
χ20.95,1 
p-value
χ20.95,m-k 
p-value
Exponential

9.41

7.02 4 3.84 
0.00807 
9.49 
0.0516
Beta Poisson

2.39

3 7.81 
0.495
Beta-Poisson fits better than exponential; cannot reject good fit for beta-Poisson.

 

Optimized parameters for the beta-Poisson model, from 10000 bootstrap iterations
Parameter MLE estimate Percentiles
0.5% 2.5% 5% 95% 97.5% 99.5%
α 3.55E-01 1.53E-01 1.76E-01 1.91E-01 4.52E+00 1.73E+07 1.35E+11
N50 6.57E+03 7.71E+02 1.44E+03 1.70E+03 2.55E+04 4.30E+04 8.32E+04

 

Parameter histogram for exponential model (uncertainty of the parameter)

Exponential model plot, with confidence bounds around optimized model

Highest quality
Exposure Route:
contact lens
# of Doses:
10.00
Agent Strain:
Dose Units:
CFU
Host type:
white rabbit
Μodel:
beta-Poisson
LD50/ID50:
1.85E+04
Optimized parameters: a = 1.9E-01 N50 = 1.85E+04
Response type:
corneal ulceration

Pooled data of White rabbit /  Pseudomonas aeruginosa 
Dose Corneal Ulceration

Not Corneal Ulceration

Total
63.2 0 5 5
63.2 0 5 5
2220 2 3 5
2220 2 3 5
13200 3 2 5
13200 1 4 5
78000 3 2 5
78000 1 4 5
462000 5 0 5
462000 4 1 5

 

Goodness of fit and model selection
Model Deviance Δ Degrees 
of freedom
χ20.95,1 
p-value
χ20.95,m-k 
p-value
Exponential

30.9

20.8 9 3.84 
5.11e-06 
16.9 
0.000312
Beta Poisson

10.1

8 15.5 
0.26
Beta-Poisson fits better than exponential; cannot reject good fit for beta-Poisson.

 

Optimized parameters for the beta-Poisson model, from 10000 bootstrap iterations
Parameter MLE estimate Percentiles
0.5% 2.5% 5% 95% 97.5% 99.5%
α 1.9E-01 9.63E-02 1.15E-01 1.24E-01 3.86E-01 5.51E-01 2.50E+00
N50 1.85E+04 3.18E+03 4.73E+03 6.01E+03 7.05E+04 8.98E+04 1.97E+05

 

Parameter histogram for exponential model (uncertainty of the parameter)

Exponential model plot, with confidence bounds around optimized model

Exposure Route:
contact lens
# of Doses:
5.00
Agent Strain:
Dose Units:
CFU
Host type:
white rabbit
Μodel:
beta-Poisson
LD50/ID50:
1.52E+05
Optimized parameters: a = 1.09E-01 N50 = 1.52E+05
Response type:
Severe stromal ulceration

White rabbit /  Pseudomonas aeruginosa 
Dose Severe Stromal Ulceration

Not Corneal Ulceration

Total
63.2 0 5 5
2220 2 3 5
13200 1 4 5
78000 1 4 5
462000 4 1 5

 

Goodness of fit and model selection
Model Deviance Δ Degrees 
of freedom
χ20.95,1 
p-value
χ20.95,m-k 
p-value
Exponential

13.2

8.73 4 3.84 
0.00313 
9.49 
0.0105
Beta Poisson

4.43

3 7.81 
0.219
Beta-Poisson fits better than exponential; cannot reject good fit for beta-Poisson.

 

Optimized parameters for the beta-Poisson model, from 10000 bootstrap iterations
Parameter MLE estimate Percentiles
0.5% 2.5% 5% 95% 97.5% 99.5%
α 1.09E-01 4.05E-02 5.23E-02 5.72E-02 9.05E+06 4.82E+07 2.53E+12
N50 1.52E+05 4.66E+03 1.13E+04 1.55E+04 1.55E+07 6.25E+07 1.96E+09

 

Parameter histogram for exponential model (uncertainty of the parameter)

Exponential model plot, with confidence bounds around optimized model

References