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Microbiological Contamination


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:

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 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.

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.


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

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.  

Exotoxins are toxic substances, which are actively excreted or released by a 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 Quantity of Escherichea coli
20 min. 2
40 min. 4
1 h 8
2 h 64
3 h 512
4 h 4,096
5 h 32,768
6 h 262,144
6 h 40 min. 1,048,576

 “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” 4

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 a tunneled catheter in the absence of concomitant blood stream infection is required.5

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 CR-BSI, 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

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 Gram-negatives
  • Imipenem resistant Acinetobacter baumannii
  • Imipenem resistant Pseudomonas aerginosa
  • Multidrug resistant Mycobacterium tuberculosis (MDR-TB) and extremely drug resistant Mycobacterium tuberculosis (XDR-TB)


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1 Ghiglione JF, Martin-Laurent F, Pesce S. (2015) Microbial ecotoxicology: an emerging discipline facing contemporary environmental threats. Environ Sci Pollut Res; DOI 10.1007/s11356-015-5763-1

2 Gabriel J. (2008) Infusion therapy. Part two: Prevention and management of complications.
Nurs Stand; 22(32): 41-8

3 World Health Organization 2002

4 Ducel G, Beraud C, Benassouli 1995

5 Guembe M, Martín-Rabadán P, Echenagusia A, Camúñez F, Rodríguez-Rosales G, Simó G, Echenagusia M, Bouza E. (2012) How should long-term tunneled central venous catheters be managed in microbiology laboratories in order to provide an accurate diagnosis of colonization? J Clin Microbiol;50(3):1003-7

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, Pollock DA, Fridkin SK; National Healthcare Safety Network Team; Participating National Healthcare Safety Network Facilities. (2008) NHSN annual update: antimicrobial-resistant pathogens associated with healthcare-associated infections: annual summary of data reported to the National Healthcare Safety Network at the Centers for Disease Control and Prevention, 2006-2007. Infect Control Hosp Epidemiol. 2008 Nov;29(11):996-1011

9 Centers for Disease Control and Prevention (CDC)

To read the abstract go to the literature page.

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