Biomedical Engineering Reference
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on the patients'/healthcare workers' skin or immediate environment. The concentra-
tion of the nosocomial pathogen can vary from as few as 1 to over 10
6
CFU per cm
2
.
Subsequently, the microbe must be transferred to the healthcare worker. Simple acts
such as lifting a patient, obtaining a blood pressure, pulse, or assessing a temperature
can easily result in the transfer of between 100 and 1,000 CFU of a common Gram-
negative pathogen
Klebsiella
spp. [
19
]. In fact these authors learned that 17 % of the
staff of an intensive care unit were found to have
Klebsiella
contaminating their hands
when screened and that the serotypes were related to those isolated from infected or
colonized patients within the ICU on the same day [
19
]. Further advancing the
importance of hand hygiene was a study that found healthcare workers were as likely
to contaminate their hands or gloves from commonly-touched environmental surfaces
as from direct contact with colonized patients [
85
].
The third aspect of the model is dependent upon the biology of the microbe.
Some microbes can survive for longer periods of time on hands than others.
Epidemic and non-epidemic strains of
E. coli
and
Klebsiella
spp. were found to
have significantly different survival times [
30
] supporting the argument that bacte-
rial properties other than the survival of a typed strain under defined conditions may
contribute to the ability of a microbe to be easily transmitted and retained within
healthcare setting. In other studies workers found that bacterial colonization of the
hands of healthcare workers progressively increased with time [
60
,
62
]. In these
two studies they found that the concentration of commensal and pathogenic flora
increased as a consequence of patient care. Additionally, the authors reported that
the dynamics of hand contamination were independent of whether or not the
healthcare worker was working while gloved or ungloved [
60
,
62
].
Such an establishment of causality in the development of HAI, and an intrinsic
ability to survive on the hands of the healthcare workers, provides strong support for a
role for hand hygiene for limiting the incidence and controlling the spread of HAI. The
fourth and fifth aspects of the model advanced by Pittet and colleagues addresses the
issue of defective and/or absent hand cleansing and how it can lead to the cross
transmission of the microbes [
61
]. Here they have raised the issue of the need to
microbiologically validate proper hand cleansing in order to control the spread of
microbes regardless of their source. In citing a study by Sala and colleagues, they
describe how an outbreak of Norovirus was traced to an infected food handler within a
hospital cafeteria. Here the implicated foodstuffs consumed during the outbreak were
handmade by the infected worker [
69
]. Independently, it has been shown that
Norovirus contaminated fingers can sequentially transfer this virus to up to seven
surfaces [
7
]. Sequential transfer is not only confined to human to surface transfer. In
the same study, the virus was found to move from contaminated cleaning cloths to
clean hands and surfaces [
7
]. Recently, Snitkin and colleagues used whole genome
sequencing to track an outbreak of Carbapenem-Resistant
Klebsiella pneumonia
that occurred at the U.S. National Institutes of Health Clinical Center where they
learned that despite early implementation of infection control procedures, including
aggressive hand hygiene controls, the microbe persisted in the environment [
83
].
Consequently, the built environment can serve as a reservoir from which clean
hands can serve as a source of HAI.
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