Biomedical Engineering Reference
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whereas the concentration of
C. difficile
, VRE and MRSA in stool ranges from 10
3
to 10
9
CFU per gram [
34
,
57
,
58
]. The concentration of norovirus in stool can
be
10
7
norovirus particles
assuming a vomit volume of 20-30 ml and the fact that 10
6
particles/ml need to be
present for detection by electron microscopy [
60
]. In contrast, the concentration of
nosocomial pathogens on surfaces is generally in the range of
10
12
particles per gram [
59
] and patients can vomit
>
>
1 to 100 CFU/cm
2
[
61
,
62
] and is often detected only by broth enrichment [
16
,
17
]. Reports of
higher concentrations of surface contamination do occur and include total aerobic
counts of 10
4
CFU per cm
2
on some intensive care units (ICU) surfaces [
63
],
>
200 CFU/cm
2
both before and after cleaning [
64
] and
>
15 to
>
100 MRSA
colonies from 23 % of sites positive by direct plating in the rooms of MRSA-
positive patients with diarrhea [
34
].
The presence of a pathogen on a surface does not necessarily represent a
transmission risk [
10
]. However, the infectious dose for most environmentally-
associated nosocomial pathogens appears to be low. For example, less than
15
S. aureus
cells were sufficient to cause infection in experimental lesions [
65
],
<
<
1 CFU/cm
2
was sufficient to cause
C. difficile
disease in mice [
66
] and a single
norovirus particle is thought to have the capacity to cause infection [
67
].
Importantly, despite the comparatively low concentration of contamination on
surfaces compared with patients, touching a contaminated surface carries approx-
imately the same risk for the acquisition of MRSA, VRE and
C. difficile
on hands as
touching an affected patient [
22
,
30
,
54
,
55
]. Therefore, the presence of a pathogen
on a surface in any concentration may be a risk for transmission, and this is reflected
in proposed guidelines for microbiological hygiene standards [
68
].
3.4 Nosocomial Pathogens Can Survive
on Surfaces for Long Periods
Studies investigating the survival of nosocomial pathogens on surfaces have
recently been reviewed by Kramer et al. (Table
3.1
and Chap.
2
)[
73
]. Under certain
conditions,
C. difficile
spores, VRE, MRSA and
Acinetobacter
spp. can survive for
4-5 months or more on dry surfaces and norovirus can survive for a week or more.
Table 3.1 Survival of hospital pathogens on dry hospital surfaces
Organism
Survival time
Clostridium difficile
(spores)
5 months
>
Acinetobacter
spp.
3 days to 11 months [
69
]
Enterococcus
spp. including VRE
5 days to
46 months [
70
]
>
Pseudomonas aeruginosa
6 h to 16 months
Klebsiella
spp.
2 h to
30 months
>
Staphylococcus aureus
, including MRSA
7 days to
12 months [
71
]
>
Norovirus (and feline calicivirus)
8 h to
2 weeks [
72
]
>
Adapted from Kramer et al. [
73
].
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