The purpose of this study was to investigate bacterial recovery and transfer from three biometric sensors and the survivability of bacteria on the devices. The modalities tested were fingerprint, hand geometry and hand vein recognition, all of which require sensor contact with the hand or fingers to collect the biometric. Each sensor was tested separately with two species of bacteria, Staphylococcus aureus and Escherichia coli. Survivability was investigated by sterilizing the sensor surface, applying a known volume of diluted bacterial culture to the sensor and allowing it to dry. Bacteria were recovered at 5, 20, 40 and 60 minutes after drying by touching the contaminated device with a sterile finger cot. The finger cot was re-suspended in 5 mL of saline solution, and plated dilutions to obtain live cells counts from the bacterial recovery. The transferability of bacteria from each device surface was investigated by touching the contaminated device and then touching a plate to transfer the bacteria to growth medium to obtain live cell counts. The time lapse between consecutive touches was one minute, with the number of touches was n = 50. Again, S. aureus and E. coli were used separately as detection organisms. This paper will describe the results of the study in terms of survival curves and transfer curves of each bacterial strain for each device.

1. BACTERIAL SURVIVABILITY AND TRANSFERABILITY
ON BIOMETRIC DEVICES
Christine R. Blomeke
Researcher
Biometrics Standards,
Performance, & Assurance
Laboratory
Purdue University
401 N. Grant Street
West Lafayette, IN 47906
USA
Stephen J. Elliott
Associate Professor
Biometric Standards
Performance, & Assurance
Laboratory
Purdue University
401 N. Grant Street
West Lafayette, IN 47906
USA
Thomas M. Walter
Continuing Lecturer
Department. Of
Biological Sciences
Purdue University
915 W. State Street
West Lafayette, IN 47906
USA
Abstract - The purpose of this study was to
investigate bacterial recovery and transfer from three
biometric sensors and the survivability of bacteria on
the devices. The modalities tested were fingerprint,
hand geometry and hand vein recognition, all of which
require sensor contact with the hand or fingers to
collect the biometric. Each sensor was tested
separately with two species of bacteria,
Staphylococcus aureus and Escherichia coli.
Survivability was investigated by sterilizing the
sensor surface, applying a known volume of diluted
bacterial culture to the sensor and allowing it to dry.
Bacteria were recovered at 5, 20, 40 and 60 minutes
after drying by touching the contaminated device with
a sterile finger cot. The finger cot was re-suspended
in 5 mL of saline solution, and plated dilutions to
obtain live cells counts from the bacterial recovery.
The transferability of bacteria from each device
surface was investigated by touching the
contaminated device and then touching a plate to
transfer the bacteria to growth medium to obtain live
cell counts. The time lapse between consecutive
touches was one minute, with the number oftouches
was n = 50. Again, S. aureus and E. coli were used
separately as detection organisms. This paper will
descrbe the results of the study in terms of survival
curves and transfer curves of each bacterial strain for
each device.
Index Terms - biometrics, hygiene, bacterial
transmission
1. Introduction
As the concerns over infectious diseases grow, so
do the concems over the hygiene of surfaces in public
areas. This is of interest to the biometric community
as efforts are made to implement the use of biometric
technologies in the public arena, specifically in
airports, retail stores, and various financial
institutions.
The motivation to conduct the current study arises
from antidotal evidence from previous studying in the
Biometrics Standards, Performance and Assurance
Laboratory. During previous studies, specifically with
fingerpfint and hand geometry, subjects would ask
questons about the cleanliness ofthe sensor. Latent
prints left on the platen glass of a device by the
deposition of finger moisture, sweat, or oils, can make
it appear to be unclean [1]. In 2006, s survey yielded
responses about the perception of the cleanliness of
biometric devices. Seventy-five percent of surveyed
subjects responded that they felt that biometric
devices were somewhat sanitary, neutral, or
somewhat unsanitary. This indicates that the subject
population does not view biometric devices as being
overwhelmingly sanitary nor unsanitary [2].
In addition, the development of touchless sensors
has prompted the question whether or not bacteria
and other disease causing organisms are a potential
hazard that can be transferred from person to person
through touching a common device. Studies have
shown that human pathogens can be transmitted
between nonliving objects by direct hand contact [31.
Contactless sensors eliminate the possibility of latent
prints and decrease the hygiene concerns because
the fingertip never comes into contact with the device
[4. Although this lessens concems with the
cleanliness of using common devices, there is a need
to investigate whether using a common biometric
device is any different that touching common surfaces
in public areas. For instance, in many workplaces,
individuals touch common items without a second
thought. Doorknobs, elevator buttons, pens on a
countertop, are a few items that are touched
throughout the day by various people. However,
recent media attention regarding the implementation
of biometric devices in the work place has cited that
employees have concerns about the hygiene of
touching the devices, and has prompted the
installation of hand sanitizer stations next to the
biometric devices [51.
It is important to note that the skin surface serves as
the habitat of a flora of microorganisms,
predominately consisting of gram-positive bacteria.
These are naturally occurring organisms living on the
skin, and normally do not cause disease or infection.
Temperature, humidity, and skin physiology all play a
role in maintaining the skin microflora [6].
1-4244-1129-7/07/$25.00 ©2007 IEEE 80

2. Each bacterial species has a different infectivity
level. As a result, different numbers of each
bacterium must enter the body for that particular
species to cause an infection or disease. They can
enter the body through the thin tissues in the eyes,
nose, and mouth, or at injury sites where the skin is
broken.
The literature focuses on studies conducted on the
survivability of bacteria on non-porous inanimate
surfaces are limited to health care and domestic
environments, but do not consider public surfaces.
Thus, this paper examines bacterial survival and
transfer on biometric devices that could be used in
public environments.
II. Methodology
The three biometric devices tested in this study were
the Recognition Systems HandKey Ill, Crossmatch
Venfier 300 LC, and TechSphere VP-Il S. Each
device was tested separately with two strains of
bacteria. Each bacterial strain was tested
independently of the other. The test organisms
utilized were Staphylococcus aureus and Escherichia
coli. The two organisms were common teaching
laboratory strains of bacteria that contain genetic
mutations so not to cause infection, but were used as
tracer organisms. Staphylococcus aureus is a
representafive of gram positive bacteria, and in
contrast, Eschenchia coli represent gram negative
species.
A. Survivability of Bacteria on a Biometric Device
The survivability of the bacteria on the device was
determined by contaminating each device with a
known concentration of bacteria and recovering
organisms over a period of time. Just prior to
contamination, the device was sterilized with a 70
percent ethanol solution and control samples were
collected. The control samples ensured that the
sterilization was complete, and only bacteria on the
device surface were the intentional contaminates.
Sterile finger cots were worn over sterilized laboratory
latex gloves. This ensured that the inside of the finger
cot remained sterile, and prevented the naturally
occurring bacteria on the skin's surface from being
introduced into the experiment.
After the collection ofthe control samples, 50
microliters of the bacterial suspension were applied to
the device surface. The solution was allowed to dry.
Bacteria were recovered by a single touch to the
device surface with a sterile finger cot after five,
twenty, forty and sixty minutes of dry time. The finger
cots were resuspended in 5 milliliters of saline
solution. Systematic dilutions of the solution were
plated onto Tryptic Soy Agar (TSA) agar plates and
allowed to grow for 24 hours at 37 degrees Celsius.
Tryptic soy agar is a growth medium that contains all
essential nutrents for bacterial growth. The colony
growth on the plate allowed for the quantification of
the number of surviving cells recovered at each time
point.
B. Transfer of Bacteria from Biometric Device
The transferability of bacteria over tme from
biometric devices was investigated by intentionally
contaminating the device surface with one species of
bacteria at a time. Prior to contamination, the device
surface was sterilized with a 70 percent ethanol
solution. Sterilized gloves were worn, and the
device surface was touched by a sterile finger, and
then touched to a TSA agar plate. The sterilized
surface was touched ten times with a different sterile
glove fingertip, and touched to the TSA agar plate as
the control samples. The agar plates were labeled
to indicate the touch number, with an average
number oftouches to a plate being 12, see Figure 1
below. Bacterial colonies will only grow where they
have been placed, and do not migrate over the
surface; therefore the touches to the plate were non-
overlapping to ensure that the colonies could be
quantified for each touch. Immediately pror to touch
11 on the device, the device surface was
contaminated with one species of bacteria, and
allowed to dry. The remaining 40 touches were
collected after contamination of the device. A sterile
glove fingertip was used to collect each successive
touch to the device. Successive touches lasted for
10 seconds were collected and plated at 20 second
intervals. Touches 20, 30, 40, and 50 were collected
with a sterile fingercot worn over the sterile glove,
and was placed into 5 milliliters of saline solution.
Serial dilutions ofthe saline solution were plated and
grown over night to quantify the number of live cells
that were recovered from touching the contaminated
device.
Figure 1: Transferability collection of bacteria on the
Techsphere VP-Il S Vein Reader.
111. Conclusions
81

3. The survivals of S. aureus and E. coli over time
were measured by quantifying the number of cells
recovered from the biometric devices over time.
Below in Figure 2, the survival of S. aureus is
graphically represented in terms of the percentage
of cells surviving over time for all three of the
biometric devices. At five minutes past the dry
time, the survival ofthe bacteria on the devices had
decreased to 40 percent, 20 percent, and 15
percent for the hand geometry reader, vein reader,
and fingerprint sensor respectively. After 20
minutes, the survival rate had approached zero for
all three devices, yet even at 60 minutes a small
quantity of bacteria were still recoverable.
Survival of S. aureus
F'-
k . . . _, . . _ _. . , 4 ... .. _ _ . ..... .I
0 5 10 15 20 25 30 35 40 45 50 55 60
Time (mlnutMs)
Figure 2: Survival of S. aureus, presented as the
percentage of survival over time in minutes.
In contrast to S. aureus, the survival of E. coli, the
drop off in survival rate is not nearly as steep in the
first five minutes. The percent survival at five minutes
is 50, 40, and 28 percent for the vein reader, hand
geometry reader, and fingerprint reader respectively.
Similar to the test with S. aureus, the survival rate
approached zero for all three devices at 20 minutes
for E. coli. Figure 3 depicts the survival curves over a
one hour time period.
Survival of E. coli
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The results of the survivability experiment can be
reported as a rate of the log of the survival. The
survival rate of the bacteria decreases over tme, and
hence can be considered a death curve. Table 1
illustrates the rate at which the survival decreases
over time by device and bacterial species.
TABLE 1
RATE OF LOG SURVIVAL OF BACTERIA
BY DEVICE
S. aureus E. coi l
Hand Geometry0.904
Reader T 09004 ----
Fingerprint 0.10
Sensor ___ 0.14_T___I
Vein Recognition 0.14 0.05
Device__ _ _ _ _ _ __ _ _ _ _ _
The result of this comparison between S. aureus and
E. coli is that neither bacterial species survived for a
long time on the device surface. Bacteria generally
prefer warm, moist environments, but this study was
conducted room temperature with low humidity, in an
open-air environment, as would be implemented in
most public environments.
The result of the transfer of bacteria varies by the
hardness of the device surface. As seen in Figure 4,
the vein reader exhibits a more consistent level of
transfer throughout the time frame, as it plateaus off
before either the hand geometry reader or the
fingerprint sensor. The vein reader device surface
tested was the pliable plastic cup that is exposed to
the back ofthe hand. This is the largest surface
area that makes contact with a person's hand. The
pliability of the surface being tested may have
allowed bacteria to work itself into the microcrevices
ofthe plastic surface, in which it could protect itself,
and prolong the transfer to the gloved hand. In
contrast, the fingerprint sensor and the hand
geometry reader surfaces were hard surfaces,
similar to that of a doorknob.
The trend lines indicate a smoothing of the data to
reveal the shape of the transferred bacteria over the
consecutive touches to the surface. The fingerprint
sensor rapidly decreased the amount of cells
transferred with each successive touch. Essentially,
the number of cells transferred over time decreases,
as there are fewer cells on the surface to be
transferred to the gloved hand and the agar plate.
The hand geometry reader exhibited a similar curve
to the fingerprint sensor, although not as extreme.
0 5 10 15 20 25 30 35 40 45 50 55 60
Time (minutes)
Figure 3: Survival of E. coli, presented as the
percentage of survival over time in minutes.
82
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23 25 30
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-FP.V - . V3 -Lol. (H% -LcZ (F
Figure 4: Transfer of S. aureus cells is presented as
the log ofthe percentage of the cells
transferred over the touch number.
Figure 5 represents the transfer curve for E. coli. In
addition to testing the three biometric devices, a metal
doorknob was also tested using the same method.
The doorknob and hand geometry reader exhibited
very similar transferability from the surface to the
gloved hand. The majority of the bacteria were
transferred from the surface for all four apparatuses
within the first ten touches. It should be noted that
the concentration of bacteria was not the same for
each device. The concentration of E. coli on the
fingerprint sensor was less than the concentration on
the other three test surfaces. Therefore, there were a
smaller number of bacteria to be transferred offthe
device sensor. However, the fingerprint sensor and
the vein reader followed the same general shape of
the curve for transfer. Again, the pliable material that
was tested on the vein reader may have influenced
the transfer rate by providing microcrevices where
bacteria could embed themselves, and thereby
transferring fewer bacteria with each consecutive
touch to the surface.
E odco transfer
10 15 20 25 30 35 40 4p
Touch rmu,r
# mP* VR . HS . _ LZ (F
Figure 5: Transfer of E. coli is presented as the log
of the percentage of the cells transferred
over the touch number.
In summary, E. coli and S. aureus both exhibited a
similar survival curve that drops off drastically after
20 minutes. E. coli did exhibit a slower death curve
than S. aureus within the first 20 minutes, but the
differences were negligible after 20 minutes. The
transfer curves for S. aureus and E. coli were very
similar in terms of most of the bacteria being
transferred offthe device by touch 10.
This initial investigaton of the survivability and
transferability of bactera on biometric devices
brings up several important points to remember, as
well providing new questions for further study.
From the bacterial survival curves, it is understood
that these species could survive on an infrequently
touched surface. However, a frequently touched
surface, if contaminated, such as in the transfer
experiments, is essentially cleaned within five to ten
touches, as the bacteria are moved to the hand.
As mentioned in the beginning of this paper, there
are naturally occurring bacteria on the hand. The
infectivity of each bacterial species is different.
Some species would require hundreds if not
thousands of cells to be ingested, enter the body
through a cut in the skin, or mucus tissues for any
sort of infection to occur. Likewise, if bacteria had
been deposited on the device surface by an
individual, most likely this would not be enough to
infect a subsequent person using the device. The
protocol in this study required the bacteria to be
placed on the surface in liquid form and allowed to
dry. In the event that a device is contaminated
simply by hand to surface contact, the
concentration of bactera on the surface would be
far less than the concentration utilized in this study.
In the event that hygiene is still a concern, it is
best for those touching common surfaces to wash
their hands with soap and warm water on a regular
basis. This will remove the majority of bacteria
from the surface ofthe hand. However, the
naturally occurring bacteria on the hand will still be
on the underlying layers of skin, and will rise to the
epidermis when the oils, moisture, and sweat come
to the surface.
Further work is necessary to investigate and
compare these results with results other species of
bacteria. In particular, it is important to take a
closer look at species of bacteria that only require a
low infectivity number, as this may be the most
important when regarding biosecurity measures.
Additionally, other species of bactera that have a
short survivability in dry conditions should be
examined to determine how well they transfer.
Additionally, further examination ofthe different
surface types, hard and soft, could prove to be
beneficial to alleviate concerns of bacterial
transmission.
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5. IV. References
[1] Sano, E., Maeda, T., Nakamura, T., Shikai,
M., Sakata, K., Matsushita, M., et al. (2006).
Fingerprint authenticaton device based on
optical characteristics inside a finger. In
2006 Conference on Computer Vision and
Pattem Recognition (CVPRW06).
[2] Elliott, S., Massie, S., and Sutton, M. (2007).
Perspective of Biometric Technologies: A
Survey. In Proceedings of IEEE Workshop
on Automatic Identification Technologies,
June 7 - 8 2007, (259-264).
31 Reynolds, K.A., Watt, P.M., Boone, S.A., &
Gerba, C.P., (2005). Occurrence of bacteria
and biochemical markers on public surfaces
[Electronic version]. Intemational Journal of
Environmental Health Research, 15(3), 225-
234.
relate to biometric technologies, where he is
responsible for the Biometrics Standards,
Performance, & Assurance Laboratory as well as two
classes related to biometric technologies. Dr. Elliott is
also involved in educational initiatives for the
American National Standards Institute, is a member of
Purdue University's e-Enterprise, Learning and
Center for Educational Research In Information
Assurance Security (CERIAS) Centers.
Dr. Thomas Walter is a Continuing Lecture in the
Department of Biological Sciences at Purdue
University. Dr. Walter is involved with teaching
various microbiology and molecular biology lecture
and laboratory courses at the undergraduate and
graduate level.
[4] Lee, C., Lee, S. and Kim, J.. (2006).A study
of Touchless Fingerprint Recognition
System. Republic of Korea: Yonsei
University, Department of Electrical and
Electronic Engineering.
[5] Chan, S. (2007, January 23). Scanners for
Tracking City Workers. New York Times, p.
1B.
[6] McBrde, M., Duncan, W., and Knox J.
(1977). The Environment and the Microbial
Ecology of Human Skin. Applied and
Environmental Microbiology, 33(3), 603-608.
V. VITA
Christy Blomeke is a member ofthe Biometrics,
Standards, Performance, & Assurance Laboratory in
the Department of Industrial Technology at Purdue
University. She is currently pursuing a Ph.D. in
Technology at Purdue University. Christy holds a
Masters degree in Agricultural and Extension
Education and a Bachelors degree in Biology with a
specialization in Genetics. Christy's experience with
biometric technologies has been in the area of animal
biometrics.
Dr. Stephen Elliott is an Associate Professor in the
Department of Industrial Technology at Purdue
University. Dr. Elliott is involved in a variety of
activities relating to biometrics and security. He is
actively involved in biometric standards, acting as
Secretary on INCITS Ml Biometric Committee. Dr.
Elliott has given numerous lectures on biometric
technologies, the latest conference presentations
being specifically aimed at the banking industry. Dr.
Elliott is also involved in educational initatives as they
84