Transfer of microorganisms - General infection theory

From a contamination source, micro-organisms can move through suitable contamination routes. Various types of cohabitation can develop between the micro-organism and the host, hopefully in favor of the micro-organisms, so that the survival of the species is ensured. These cohabitation forms are not always favorable from the viewpoint of the host and vary from innocent to pathogenic.

Transfer of microorganisms - General infection theory
From a contamination source, micro-organisms can move through suitable contamination routes.

Transfer of microorganisms


From a contamination source, micro-organisms can move through suitable contamination routes. Various types of cohabitation can develop between the micro-organism and the host, hopefully in favor of the micro-organisms, so that the survival of the species is ensured. These cohabitation forms are not always favorable from the viewpoint of the host and vary from innocent to pathogenic.


Contamination sources

The site from which the micro-organisms spread is the source of infection. These can be dirty instruments, spoiled food, people, animals or insects that carry living germs. The presence of germs in humans and animals is not always clear. It is known that in some diseases, even before the outbreak of the symptoms (disease symptoms), during the so-called incubation period, so many living germs are present in, for example, saliva or other body fluids, that someone is a source of infection without knowing it. In tuberculosis, this phase can last for many years! Furthermore, individuals who are carriers of a particular disease are a source of infection, depending on the viral load.


In mouth care practices, the sources of infection can in principle be localized properly. It is assumed that all patients are a potential source of infection and that dirty instruments and contaminated surfaces are also sources of infection. In addition, unit water may still be of insufficient quality or the presence of textile towels is a traceable source of infection. These last two sources of infection should normally not occur in a dental practice.


Contamination routes

Micro-organisms will have to realize an actual and effective distribution to a new host via a contamination route from a source of infection. This is a complex undertaking for micro-organisms. For a 'successful' crossing to a next patient, not only the feasibility of the distance to be covered plays a role for a micro-organism, but also that the route can be covered under the right conditions (conditioned transport).


Requirements for contamination route

Various requirements are imposed on a contamination route:

  • Living conditions must be sufficient (enough) to survive the trip. Factors such as temperature, humidity and duration must ensure that the germs reach their destination alive (or in any case viable).
  • The capacity of the transport route must be sufficiently large to transport large numbers of micro-organisms. This increases the chance of survival of the micro-organisms. To give an impression of the numbers, it can be said that for the development of most infections a contamination with millions of microorganisms is required. Infection with 'only' several hundred or thousands of microorganisms does not usually cause infection or disease, because the host's defense will be able to destroy it. The germs will then fail to maintain their own species at the expense of the next 'victim'.
  • The 'carrier' must be suitable, because micro-organisms, with the exception of protozoa, can not actively move themselves! Passively, they can piggyback on hand (shoes) and instruments. They can also be transported in water via the principle of dilution and finally they can be transported through the air, for example in the spray droplets that form during a cough or sneeze or droplets of the aerosol. These means of transport are also called vectors;
  • A 'stopover' can be built-in. In case of insufficient hygiene, a micro-organism can also end up somewhere on a surface before it is brought to the final destination at a later time via a next hand (shoe). This interrupted transport route via surfaces is called lubrication contamination;
  • Finally, a good final destination is still a worthwhile aspect of a good transport route. Preferably, this is at a suitable location on or in the body of the next host, so that the microorganism can maintain and multiply under optimal conditions. The best 'landing site' for microorganisms that thrive in the oral environment is therefore the mouth of the next patient. Damaged skin or the ocular mucosa is generally also a good location for microorganisms supplied from a mouth to multiply. A location where the microorganism can easily penetrate is called porte d'entrée.


Apart from the passive 'swimming' in unit water pipes and the 'getting sunbathing' in the aerosol, hand hygiene of the team members is the decisive factor for the spread of microorganisms in oral care practice. From this is the enormous importance of good hand hygiene to make as it applies not only to oral care but to the entire health care! (Only in the second place comes the concern for putting the used instruments in order.)


Method of transfer

The transfer method can take place as follows:

  • aerogenic: germs are supplied through the air and infect the mucous membrane of the airways. This is the case when coughing or breathing in contaminated air or aerosol.
  • oral or enteral: the pathogen is brought into the body through the mouth, after which an infection can also develop in the gastrointestinal tract. This can occur in the case of direct contact with the oral mucosa of the partner or indirect transmission of saliva during 'speaking with consumption' or during spraying.
  • hematogen: germs are transmitted by contact of blood residues with the bloodstream of the recipient; by using dirty injection needles or by needle-stick injuries in health care.
  • dermal: normally the skin functions as a barrier against intruders, but at the site of an injury the skin has been lost as a barrier and micro-organisms can easily enter the body. An injury then counts as a porte d'entrée.
  • genital: the mucous membranes of the genitals can become contaminated by direct contact with sperm or infected mucous membrane of the partner (STI). Some micro-organisms penetrate intact mucous membranes, others are dependent on (minor) damage to the skin or mucous membranes to cause an infection.
  • congenital: the germs are transferred from the mother to the unborn child.


Toddlers who touch each other's toys or even put them in their mouths can easily develop an infection via this oral route of infection. The frequent hand-mouth contact of the toddlers ensures the correct route of movement of, for example, colds or childhood diseases. When inadequate hygiene measures are taken in oral care practices for these young patients, because they 'put each other's toys in the mouth at the daycare', this is in conflict with the professional obligation to guarantee patient safety in a care setting.


Indirect contamination routes

An indirect infection can take place via:

  • aerosol: this is the fine (invisible!) mist created by the use of spray cooling with rotating instruments or an ultrasonic scaler. The aerosol can remain in the treatment room for a long time and cause respiratory infections in subsequent patients and the treatment team. In addition to (contaminated?) Cooling water, the aerosol also contains blood, saliva and solid particles, such as pieces of tartar or restorative material.
  • surfaces (and controls) of equipment, countertops or textile towels. This contamination route is indicated by the term lubrication contamination.
  • medical instruments that have not been adequately cleaned: a contamination route known as cross-infection in health care.
  • unwashed hands contain a variety of bacteria in relation to washed hands and together with inadequate personal hygiene in different situations lead to the development of infections. Contamination is lurking not only in the medical sphere, but also in daily life. Fortunately, the average person has sufficient resistance, so that this type of daily infection usually does not get a chance. In people with reduced resistance, however, this is much more sensitive and sometimes extra hygiene measures are required.


Bacterial culture of unwashed hands


Bacterial culture of washed hands


Cohabitation forms

The way in which the micro-organism will coexist with the host after infection will not always be a potential reason for developing a disease. The host can sometimes also benefit from an infection. Hereafter, five different forms of society (symbiosis) are described in which micro-organisms reach their goal and can maintain and multiply.

  1. Mutualism: this is a relationship in which host and micro-organism need each other. On the skin, for example, many micro-organisms that feed on excretory products of the skin and which by their presence simultaneously protect our body against diseases.
  2. Commensalism: microorganisms reside in the body of a host, without causing harmful effects. This applies to the period that they stay at the location where they 'belong'. Large numbers of so-called commensals occur in the intestines, for example. However, when they end up in a different location in the body, they sometimes cause illness. For example, a harmless colon infection can cause bladder inflammation on contact with the urinary tract.
  3. Pathogen: here society is basically pathogenic. The host can defend himself against this pathogen, the pathogen, with a local or general immune response.
  4. Opportunistic: in a healthy host the opportunistic microorganism (also known as an opportunist for short) appears innocent and is often present as a commensal. However, as soon as the host weakens, the pathogenic properties of the opportunist come forward and infections occur. The fungus Candida albicans and various intestinal bacteria are notorious opportunists.
  5. Parasitic: here lives a small organism, the parasite, not only in but also at the expense of the host, without it being able to defend itself (for example in a tapeworm infection).