Biomedical Engineering Reference
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Fig. 4.2 Transmission dynamic of microbes resident in the clinical environment. the ubiquitous
distribution of microbes, coupled with the stochastic nature of care, facilitates a continuous risk of
the patient, healthcare worker or high touch object introducing, acquiring or spreading unwanted
microorganisms
remain viable on these inanimate surfaces for weeks to months (Chap. 2 and ref [ 42 ]).
Further, the distribution and dispersal of the microbes from healthcare workers, visitors
and patients can contribute to the resident microbial flora of the built environment.
Humans shed a minimum of ten million of their 100 million skin cells per day. Routine
activities such as walking can result in the loss of approximately 10 4 skin particles per
minute with a complete layer of skin cells being lost and replaced from healthy
individuals on average approximately every 4 days [ 51 ]. The displaced skin cells are
covered with the endogenous flora of the individual. Not all individuals shed skin
equally. In one study of microbial dispersal by skin in a hospital ward, Noble defined a
' Staph aureus disperser' as a patient who contributed greater than six S. aureus per
cubic meter of air [ 52 ]. Given that the mean concentration of bacteria within the ward
was the equivalent of 800 viable bacteria per cubic meter, the concentration of
S. aureus observed was thought to represent 1 % of the total flora [ 52 ]. Over the
years the number has been revised to suggest that an individual is a ' Staph aureus
disperser' when they are able to disseminate more than four viable particles per
microbe per cubic meter of air [ 9 ]. Causality, or the linkage of an environmental
isolate to that organism responsible for disease in individuals has been demonstrated as
early as 1945 when deForest and Kerr reported cases of eczema which occurred
amongst nurses that were caused by streptococci that were shed [ 26 ]. With the advent
of molecular techniques, such as Pulsed Field Gel Electrophoresis (PFGE) and whole
genome sequencing, the ability to demonstrate casualty has now become much more
straightforward but is still nevertheless time intensive and cost prohibitive.
Once established within the built environment the microbe must then be able to
resist the perturbations introduced as a consequence of cleaning and other infection
control measures. To that end, some pathogens have become resistant
to
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