Domestic and Indoor Microbiology
Charles P Gerba and Ian L Pepper
1. Determine the time in hours for influenza A and hepatitis A virus to decrease
in titer by 99% on a fomite.
Time for a 99% decrease = 2 log10 reduction
Thus time in hours for 2 log10 reduction = 2 log10/ inactivation rate per hour
From Table 27.9 Influenza is inactivated at a rate of 0.0278/hour
Thus: time for 99% inactivation = 2 log10/ 0.0278 log10/hour = 71.9 hours
From Table 27.10 Hepatitis A virus is inactivated at a rate of 0.00278/hour
Thus: time for 99% inactivation = 2 log10/ 0.00278 log10/ hour = 719 hours
2. Why would influenza virus be more likely to be transmitted in the winter by
fomites than poliovirus?
Influenza virus survives better under low humidity conditions, which occur in
temperate climates because of indoor heating.
3. What conditions would favor the growth of bacteria in or on fomites?
High relative humidity, standing water on the surface, warm temperatures, and
biodegradable organic matter.
4. Determine the number of Salmonella a person will ingest if a person touches a
cutting board contaminated with 100,000 Salmonella per square centimeter.
Assume that a fingertip has an area of 1 cm2 and that one finger touches the
surface. Using the risk model presented in Chapter 29, determine the
probability of the individual becoming infected.
Since a number of assumptions have to be made to answer this problem there is
no exact numeric answer. It is more an exercise of setting up the problem.
Number of Salmonella bacteria transferred to finger (1 cm2) = 1 cm2 = 100,000
Number of Salmonella transferred from cutting board to hand is 34% of that on
the cutting board. This is assumed to be the same as transfer from mouth to lip
(see page 562) = 100,000 X 0.34 = 34,000
Number of Salmonella transferred from hand to lip is also assumed to be 34%.
Thus, 34,000 X 0.34 = 11,560
Thus, it is assumed that 11,560 Salmonella are ingested.
Risk of becoming infected (Table 29.9 and Eq. 29.2) = P = 1 – exp(-rN)
For Salmonella r = 0.00752 (from Table 29.9)
P = 1 – exp[(- 00752)(11,560)] = 1 – exp( -86.93) = 1
5. Look at the classroom you are sitting in and list three objects that would most
likely become contaminated by a person infected with norovirus. Give your
reasons why. Which object would have the greatest efficiency for transfer of a
virus onto your hand?
Objects might include: desk tops, pencil sharpener, pens and pencils, light
switch, computer key board, computer mouse, cell phone, door knob. These
are high touch areas. Stainless steel has high transfer efficiency, so perhaps
the door knob.