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Microbiological Contamination
Definition
Microbiological contamination refers to the non-intended or accidental
introduction of infectious material like bacteria, yeast, mould, fungi, virus,
prions, protozoa or their toxins and by-products.[1,2]
“A nosocomial infection — also called “hospital-acquired infection” is
defined as:
An infection occurring in a patient in a hospital or other healthcare facility in
whom the infection was not present or incubating at the time of admission.
This includes infections acquired in the hospital but appearing after discharge,
and also occupational infections among staff of the facility.” 3
Types of microbiological pathogens
There is a broad range of microbiological pathogens, which can cause
contamination and thus infections. Within these groups, several different types
of pathogens exist:
Fig. 1:
MeningococcusBacteria under the
scanning electron
microscope (SEM)
1. Bacteria: are microorganisms with a size of up to 5 µm and represent the
most important group of pathogens when discussing microbiological
contamination. According to the constitution of their cell wall, bacteria can
be distinguished into Gram-positive and Gram-negative bacteria (see Figure 1,
MeningococcusBacteria).
Bacteria can be further distinguished as follows:
1.1 “Commensal” bacteria: belong to the normal flora of healthy humans. They
are usually harmless to healthy people or even have a significant protective
role by preventing colonization by pathogenic microorganisms. Some
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commensal bacteria may however cause infection, if the natural host is
compromised or if they are brought into the host’s tissue.
1.2 Pathogenic bacteria: have greater virulence and cause infections regardless
of the host’s status.
Fig. 2: HI-Virus
2. Viruses: subcellular biological objects with a size of 20-200 nm. They exist
with and without envelopes (shells mostly derived from host membranes
covering the virus) and can cause serious infections (see Figure 2, HI-Virus).
3. Prions: infectious protein particles. They are the smallest pathogens, which
are below 5 nm in size.
Both prions and viruses are particles without own metabolism and are thus not
regarded as living organisms. For reproduction, they depend on the
metabolism of a host organism.
Fig. 3:
Photomicrograph of
Penicillium digitatum
mold showing
conidiophores and
spores
4. Fungi, yeasts and protozoa with up to 200 µm in diameter are three further
groups of infection sources.3 A Mycelium, the vegetative part of a fungus, is
shown in Figure 3, Penicillium digitatum.
Toxic by-products of microorganisms
Endotoxin:
The most common example for endotoxins are the lipopolysaccharides (LPS)
found in the outer membrane of the group of Gram-negative bacteria. If this
membrane degenerates, e.g. when the bacteria dies, LPS’s are released. LPS
are heat stable and cause serious fever, chills, sepsis and irreversible shock.
Exotoxin:
Exotoxins are toxic substances, which are actively excreted or released by a
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microorganism, like bacteria, fungi, algae, and protozoa. They can cause major
damage to the host by destroying cells or disrupting normal metabolism, but
they are mostly destroyed by heat. For example, Clostridium tetani produces
the tetanospasmin which leads to the symptoms of tetanus; Vibrio cholerae
produces the choleratoxin and leads to the symptoms of cholera.
Under normal circumstances, one single bacterium will not cause any harm.
However, even one bacterium can quickly replicate itself into millions: Under
optimal conditions, bacteria like Escherischia coli can double their population
every 20 minutes.
Time
20min.
40min.
1h
2h
3h
4h
5h
6h
6h 40min.
Quantity of Escherichea coli
2
4
8
64
512
4,096
32,768
262,144
1,048,576
Fig. 4: Prevalence of
MRSA in Europe 2008
[7]
“Nosocomial infections are widespread. They are important contributors to
morbidity and mortality. They will become even more important as a public
health problem with increasing economic and human impact because of:
▪Increasing numbers and crowding of people
▪More frequent impaired immunity
(age, illness and treatments)
▪New microorganims
▪Increased bacterial resistance to antibiotics” 5
Definition of Catheter-Related Blood Stream Infection (CR-BSI)
The definition of CR-BSI helps with the decision whether a catheter is the
primary source of bacteremia in a patient. They include exit site or tunnel
infections and are defined as:
▪Erythema or induration within 2 cm of the catheter exit site, in the
absence of concomitant bloodstream infection and without
concomitant purulence
▪For tunnel infections, presence of tenderness, erythema, or site
induration >2 cm from the catheter site along the subcutaneous tract of
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a tunneled catheter in the absence of concomitant blood stream
infection is required.7
Microbiological contamination is most dangerous for patients when it affects
parenteral therapy and the intravenous catheters used. In this case, pathogens
can directly reach the systemic circulation and cause catheter-related blood
stream infection
(CR-BSI) or travel to various organs and induce organ failure.
Therefore, prevention of CR-BSI is crucial. In the mid-90s the Centers for
Disease Control and Prevention (CDC) published a standard definition for CRBSI, which is the most widely accepted definition for CR-BSI. 6
Bacterial infections can mostly be treated with antibiotic drugs. However,
there are cases where this is extremely difficult or even impossible because the
bacteria have become multidrug resistant. Against most viruses and all prion
diseases, there are also no effective drugs available. Thus, prevention of such
infections is crucial.
Incidence and Prevalence of MRSA
Methicillin resistant Staphylococcus aureus (MRSA) infection is a serious
worldwide health concern. MRSA is dened as any strain of Staphylococcus
aureus that has developed resistance to beta-lactam antibiotics which include
the penicillins (methicillin, dicloxacillin, nafcillin, oxacillin, etc.) and the
cephalosporins.
According to the Centers for Disease Control and Prevention (CDC), MRSA
currently causes about 1 % of all staphylococcus infections and more than 50
% of health-care associated staphylococcus infections. After Staphylococcus
epidermidis, Staphylococcus aureus is the second most common pathogen
causing health care-associated infections in the United States, and 49 % of
those infections are caused by the highly antibiotic resistant bacteria MRSA.
A strain called USA100 is the most common type of MRSA involved in health
care-associated infections in U.S. hospitals.8 MRSA is especially troublesome
in hospitals and nursing homes where patients with open wounds, invasive
devices and weakened immune systems are at greater risk of infection than the
general public. Each year in the United States, more than 290,000 hospitalized
patients are infected with Staphylococcus aureus. Of these staphylococcal
infections, approximately 126,000 are related to MRSA.9
Definition of multidrug resistant bacteria
Multidrug resistance is a condition enabling a disease-causing organism to
resist distinct drugs or chemicals of a wide variety of structure and function
targeted at eradicating the organism.10
Important multidrug resistant organisms are
▪Methicillin resistant Staphylococcus aureus (MRSA)
▪Vancomycin resistant Enterococci (VRE)
▪Extended spectrum β-lactamase (ESBLs) producing Gram-negative
bacteria
▪Klebsiella pneumoniae carbapenemase (KPC) producing Gramnegatives
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▪Imipenem resistant Acinetobacter baumannii
▪Imipenem resistant Pseudomonas aerginosa
▪Multidrug resistant Mycobacterium tuberculosis (MDR-TB) and
extremely drug resistant Mycobacterium tuberculosis (XDR-TB)
References:
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1 Gabriel J. 2008
2 Dougherty L. 2006
3 World Health Organization 2002
4 Schmidt 2003
5 Ducel G, Beraud C, Benassouli 1995
6 O‘Grady NP, Alexander M, Dellinger EP, Gerberding JL, Heard SO, Maki
DG, Masur H, McCormick RD, Mermel LA, Pearson ML, Raad II, Randolph
A, Weinstein RA. 2002
7 European Centre for Disease Prevention and Control.Prevalence of MRSA in
Europe 2008
8 Hidron AI, Edwards JR, Patel J, Horan TC, Sievert DM, PollockDA, Fridkin
SK. 2006–2007
9 Centers for Disease Control and Prevention (CDC)
10 Hebert C, Weber SG 2011
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