SHORT INFORMATION ON BACTERIA

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"The total cost for the 3 500 - 4 000 postoperative wound infections after total hip and knee replacements that occur annually in the USA is about 200 millions $$". (Spangehl 1999)

The deep postoperative wound infection (PWI)

When the patients speak about "infection in the total hip or knee" they usually do not realize that this infection is a postoperative deep wound infection that developed around these joints. The absolute majority of these infections is caused by bacteria that landed in the operation wound during the total joint surgery. These bacteria are living with all of us in a close and innocent partnership, they cannot exist without us. Yet, when in contact with an artificial  total joint, these bacteria change their character from a partner to an enemy.

All of us produce bacteria that cause our postoperative wound infections

• For the start it is good to realize that every healthy person has millions of bacteria living on the skin surface and on the mucous membranes of his / her nose, mouth, and bowels. We spread continually these bacteria in the environment around us. These bacteria do not cause any problems to us in our everyday lives. Yet, these bacteria cause the majority of our postoperative infections.

• We spread  bacteria from the mouth and nose. These bacteria travel on small, relatively heavy droplets of saliva and nose secret. These particles cannot sail longer distances in the air as the light skin scales. They settle to the ground within a radius of 1 meter.

• The most important contamination of our environment is by bacteria that travel on small skin scales (the mean size is 12 microns = thousandths of millimeter)  in the air of the operation room and sediment as an invisible dust on all surfaces in the operation room as long as people are staying there.

• On work-free nights, when there are no people to spread the bacteria, the air on modern operation rooms, ventilated with clean and filtrated outdoor air,  is devoid of bacteria.

• When the first patient and his operation team arrive on the morning, they start spreading their own bacteria in the clean operation room environment. In spite of sophisticated ventilation techniques and other prerequisites, some of these bacteria will find eventually ways into the operation wound.

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Bacteria are everywhere around us, also in our clean operation wounds

• The presence of bacteria in the operation wound is called contamination and it happens also on the cleanest operation rooms.  The body's defense takes care of the majority of these bacteria and destroys them .
• It  depends on at least four factors whether this contamination develops into a full-blown postoperative infection.

In order of importance, these factors are:

The body’s capability to destroy the bacteria, (some diseases, such as  rheumatoid arthritis or HIV infection diminish the defense capability of the body)

how many bacteria are present in the operation wound (the more clean the air, the less bacteria will settle in the operation wound)  

the character of the operation wound :clean or dirty

(clean wound is a wound where there were no bacteria from the start, such as the operation wound done at the first time total joint replacement)

dirty wound is a wound where there are many bacteria already before the start of the operation, such as the operation through an already infected total joint)

the virulence of the bacteria. (Virulence= the capability of bacteria to cause infection by production of bacterial poisons.)

Bacteria living in the hospital and not sensitive to ordinary antibiotic (synthetic penicillins) are especially virulent.

  

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Bacteria living in hospitals are dangerous

• The bacteria living in the hospitals are special. The majority of them developed resistance against the antibiotics used in that hospital. After some days, these resistant bacteria occupy (contaminate ) the body surface of every new visitor of the hospital, replacing his / her original bacteria which once were sensitive to antibiotics.
• A special bacterial strain that often appears in hospitals is Staphylococcus aureus strain resistant to methicillin antibiotics. It is called Methicillin Resistant Staphylococcus Aureus (MRSA).  See website: http://www.mayoclinic.com/health/mrsa/DS00735
  • Methicillin is a synthetic penicillin that is active against bacterial producing enzyme penicillinase. These penicillinase producing bacteria are resistant against the "usual" penicilline

The patients coming to the hospital for their first total joint replacement operation have only 20 % of their skin bacteria resistant to antibiotics. The patients coming for a second or third operation, (the patients who were in the hospital previously), have 60% of their skin bacteria resistant to antibiotics!

Conclusion: If you wish to escape the contamination with bacteria resistant to antibiotics, limit your stay in the hospital to the shortest possible.

 

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In patients not protected by prophylactic antibiotics there is a close correlation between the numbers of bacterial colonies in the operation room air and the rate of postoperative wound infections.

The bacteria-carrying particles (mainly skin scales, but also droplets coming from mouths and noses) are generated  by the operation room staff members & the patient. (Friberg 1998)

The bacteriologists use different techniques to grow the bacteria.

However, bacterial causing chronic total joint infections are slow growers and must be identified by other methods. This is a very modern part of bacteriology. See the chapter: Bacteria  Identification for details.

Skin bacteria:

Staphylococcus albus (the white Staphylococcus) is spherical bacteria forming clusters. From that characteristic comes the name ( Staphyle - Greek for a bunch of grapes). S albus is the most frequent skin bacteria. It has been considered an innocent partner living on the skin surface of all of us and it has been found in about 30 % of all clean orthopedic operation wounds- all of these wounds healed without complications. This friendly coexistence between the people and the white Staphylococcus changed with the advent of total hip operations.

When Staphylococcus albus colonizes the surfaces of total joints, it changes from doctor Jekyll to doctor Hyde. Once fixed to the surface of the total joint prosthesis, S. albus develops a slime cover and changes its characteristics: it develops resistance to antibiotics and causes slow, indolent infections around total joints.

Other skin bacteria, some with long names, have been found only rarely as a cause of infections around total joints. It may seem curious that some of the bacteria living on our skin don't stand oxygen in the air we breathe. These bacteria are living deep in the sweat glands. Occasionally, also these   anaerobe bacteria may cause infection around total  joints.

Staphylococcus aureus is a dangerous cousin of S. albus. His name "aureus" (Latin for golden) relates to the fact that the colonies of this bacterium have nice golden color. These bacteria cause serious infections, for example skin boils or dangerous infection of bones. These bacteria   live occasionally on  skin surface and in noses of certain persons, without causing them any troubles. Carriers of this bacterium may cause small epidemics of wound infection if they are shedding this bacterium in the hospital environment.

The classification of bacteria:

Gram staining:

Bacteriologists classify usually the bacteria into Gram- positive and Gram-negative. The Gram -positive bacteria  are those that can be stained by a method developed by the Danish bacteriologist HC Gram. In many reports it is "gram-positive".

This distinction is important because the Gram positive bacteria have been sensitive to common antibiotics (such as Penicillins or Cephalosporins).

Gram-positive bacteria,

such as Staphylococcus albus and aureus are spreading through air route, but also through direct contact with objects covered by the invisible dust of skin scales containing these bacteria.   Staphylococcus albus and aureus  are still causing the majority of infections around total joints.

Gram-negative bacteria

do not stain with the Gram stain method. Many of these bacteria need wet environment for thriving. These bacteria do not travel through air, but are transported through direct contacts with (mostly wet) objects.

Gram negative bacteria are often called gut bacteria, because many of them live in our bowels. The infections caused by these bacteria need treatment with special antibiotics, the results of treatment are  usually less good.

 

Living without the air

Another classification is based on the fact that some bacteria do not endure air atmosphere. The bacteriologists thus distinguish between

aerobe bacteria i.e. bacteria living in oxygen containing atmosphere and

Anaerobe bacteria, i.e. bacteria that die in the oxygen containing atmosphere.

Some of the most dreaded infections (tetanus or gas gangrene , e.g.) are caused by anaerobe bacteria.

One of the main principles of all surgery is to care for a good circulation in all tissues of the operative wound because tissues with bad circulation are good nourishing ground for anaerobe bacteria.

Anaerobe skin bacteria may cause indolent infections around the total joints.

BACTERIA  CHARACTERISTICS	PER CENT OF ALL FINDINGS IN INFECTED TOTAL JOINTS
Aerobe bacteria	88 %
Anaerobe  bacteria	12 %

(Garvin 1995)

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From friend to foe - why?

The bacteria living as innocent partners on the surfaces of our bodies can cause fearful infection when they come in contact with the total joint prosthesis. Why?

Race for surface theory

The total joint prosthesis is a non-living object with no   protection of its own against the colonization by bacteria. When a total joint prosthesis is placed into the human body, the body's cells hurry to get hold on the surface of the prosthesis.

Once the body cells colonize the surface of the total joint prosthesis they can protect this non living  surface against the colonization attempts of  the bacteria. 

If any bacteria are present in the operation wound, and most often these bacteria are the skin bacteria, they hurry to get hold on the prosthesis surface too. Thus there develops a race - who will be first to colonize the surface, bacteria or body's cells.

By the long biological development, the bacteria evolved the capability to adhere to surfaces for their  survival, not only on the human body, but everywhere in the nature. Bacteria have thus biological advantage against the body's cells in this race.

Once the bacteria attached themselves to the surface of the prosthesis they form a slime envelope that protects them against the action of antibiotics and the immune defense of the body. Moreover, adherence to a surface also changes the characteristics of the adhering bacteria, among others their resistance to antibiotics increases 100 to 1000 times.

References:

Friberg B.   AORN J 1998;67: 841 - 51

Garvin et al.  J Bone Joint Surg-Am 1995; 77-A:1576 - 88

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