Staphylococcus Epidermidis: Biofilms and Antibiotic Resistance
A Potentially Harmful Bacterium
Staphylococcus epidermidis is a normal component of our skin and mucous membrane flora and often causes no problems. It may even help us. Under certain conditions, however, it produces harmful and sometimes deadly infections. Researchers have recently discovered that some strains of the species are resistant to multiple antibiotics and are hard to treat. The bacterium has long been considered a nuisance in hospitals and care facilities because it contaminates patient specimens, but it may have entered a new and dangerous phase of its existence.
S. epidermidis is a strange bacterium with respect to its effects on our lives. Depending on the situation, the bacterium may be possibly helpful, apparently neutral, mildly harmful, or potentially deadly.
Staphylococcus epidermidis is unicellular and is classified as a coccus, which means its body consists of a spherical cell. The cocci are arranged in grape-like clusters. The word “staphylo” is derived from an Ancient Greek word meaning “bunch of grapes”. The species name refers to the fact that the bacterium is often found on the outer layer of the skin, or the epidermis. The species is non-motile, gram-positive (appearing purple after the gram staining technique), and coagulase-negative (unable to make an enzyme called coagulase).
The bacterium is found inside the body on mucous membranes. These membranes line cavities and tubes that lead to the outside world. They’re found in the respiratory, digestive, and urinary tract, for example. Mucous membranes often secrete mucus, to a greater or lesser extent. Mucus is a protective and lubricating substance.
When it’s on the skin, the bacterium is thought to help us by hindering harmful bacteria. Even when it’s located on the mucous membranes inside our body, it's often harmless and may even be helpful. It’s an opportunistic pathogen, however. A pathogen is a microbe that causes disease. When conditions are suitable—which generally occurs when the host and the immune system are weakened in some way—the bacterium makes us ill. The infection is sometimes fatal. The bacterium is a special problem in hospitals and other care facilities.
Staphylococcus epidermidis lives in the epidermis or the upper layer of the skin. The epidermis contains no blood vessels. If the bacterium reaches the blood vessels in the underlying dermis, it may spread to other parts of the body and cause disease.
Like some other bacteria, S. epidermidis has the ability to form biofilms in the body. This is one reason why it can be dangerous. In a biofilm, bacteria are surrounded by a slimy and sticky material that is attached to a surface. The film protects the bacterial cells and makes them very hard to attack with antibiotics. It also makes it hard for the body’s immune system to attack them. The film is technically known as an extracellular matrix and is produced by the bacteria that it contains. It consists of polysaccharides, proteins, and extracellular DNA, or deoxyribonucleic acid.
Dental plaque is a form of biofilm. It can be removed mechanically by brushing. Other films in the body can be much harder to remove. Biofilms may form on hip replacements and on medical catheters and valves implanted in the body, for example. Some bacteria produce biofilms in wounds on the skin and in tissues within the body.
The bacteria inside a film are active, releasing chemicals into their immediate environment that enable them to communicate with one another and coordinate their activity. They are also able to feed on nutrients that enter the film. Narrow channels that allow nutrient chemicals to enter have been found in some biofilms. These channels may also release wastes and toxins made by the bacteria.
Some bacterial cells are released from the mature colony. These travel through the body and eventually establish new colonies inside biofilms.
Biofilms in the body and the behaviour of the bacteria inside them are fascinating biologically, but they can be harmful for our health. The films also form outside the body and may be found on food preparation surfaces and industrial materials.
How Antibiotic Resistance Develops
Even when it's not in a biofilm, Staphylococcus epidermidis is resistant to many antibiotics. Researchers have discovered that strains with a high tendency to form biofilms have a higher degree of antibiotic resistance than those with a low tendency, however. The development of antibiotic resistance in bacteria is a natural process. Humans are speeding up the process by the overuse and misuse of the medications.
A gene is a segment of a DNA molecule that codes for a particular protein and controls a specific feature in the body. The code is "written" in a sequence of chemicals called nitrogenous bases. A bacterium (and a human) has many genes. Each gene exists in variant forms known as alleles.
The genetic composition of a bacterium changes over time due to mutations and/or obtaining DNA from other bacteria. Mutations are genetic changes that are produced by mistakes made during DNA replication or by factors such as radiation or the influence of specific chemicals. DNA replication takes place before a bacterium divides to produce offspring. As a result of mutations and the arrival of new DNA, a bacterium may develop or pick up an allele that is able to give it resistance to a particular antibiotic.
If a group of bacteria are exposed to the antibiotic, individuals that are sensitive to the medication's effects die. If any of the bacteria have the allele that gives them resistance to the antibiotic, they will survive the treatment. The survivors reproduce and pass a copy of the beneficial allele to their offspring. The offspring then repeat the process. The allele gradually spreads through the bacterial population. Eventually the majority of the population will be resistant to the effects of the antibiotic.
Every time that an antibiotic is used, there is a risk that resistant bacteria will survive. If an individual's immune system can't destroy these, the person may get sicker. If they are transmitted to other people, resistance may spread. Antibiotics are an essential treatment for some illnesses, but they must be prescribed and used correctly to reduce the development of resistance.
The white circles in the dishes above contain antibiotics. The bacteria in the dish on the left are sensitive to all of the antibiotics. The bacteria in the dish on the right are resistant to some of the antibiotics and sensitive to the others.
Distribution and Resistance to Antibiotics
The recent research on antibiotic resistance in S. epidermidis was done by Australian scientists. Researchers from the University of Melbourne examined hundreds of Staphylococcus epidermidis specimens from seventh-eight hospitals around the world. They found multi-drug resistant forms of the bacterium in samples from ten countries. They also discovered three strains of the bacterium in Europe that “cannot be reliably tamed” by any antibiotic that is currently on the market.
Strains of a bacterial species have minor genetic differences from one another. These differences are not significant enough to cause the division of the bacterium into different species, but they may be significant with respect to their effect on humans.
At the moment, S. epidermidis primarily harms elderly patients, people who have an implanted urinary catheter, valve, or joint replacement, and people with a malfunctioning immune system. It's certainly not inevitable that people in these categories become infected, that they experience serious symptoms from the infection if it does develop, or that any unpleasant symptoms must be due to an S. epidermidis infection. If unexplained symptoms do appear, however, a doctor should be consulted.
Patients are generally seriously weakened by another factor before succumbing to the effects of the bacteria at the moment. The fact that some people die from the infection is sad news. The situation may become even worse if antibiotic resistance in the species increases or if resistant strains spread.
It can be deadly, but it's usually in patients who already are very sick in hospital... it can be quite hard to eradicate and the infections can be severe.— Ben Howden, University of Melbourne, via Medical Xpress
As might be expected, doctors and scientists are trying to minimize the possibility that a bacterial biofilm will form on a joint replacement such as an artificial hip.
An Unusual Genetic Change
The University of Melbourne researchers say that one surprising discovery related to S. epidermidis was that a small change in the DNA of some strains gives resistance or reduced susceptibility to two types of antibiotics commonly used to treat the infection. These antibiotics are not chemically similar to one another, so it’s strange that such a small change in the genetic material gives resistance to both types of medications.
The antibiotics that were explored with respect to the above observation were rifampin (also known as rifampicin), vancomycin, and teicoplanin. The last two antibiotics belong to a different chemical family from rifampin and are known as glycopeptide antibiotics. The researchers say that the strains of S. epidermidis that they investigated were resistant to rifampin and had reduced susceptibility to vancomycin and teicoplanin. Until quite recently, vancomycin has been thought of as a powerful and effective medication that works when other antibiotics don't. Unfortunately, multiple species of bacteria are becoming resistant to it.
The existence of multiple drugs to attack bacteria is important. A patient with an S. epidermidis infection may be given two different types of antibiotics at the same time. The general idea behind giving two antibiotics at once is that if one fails to work or is only partially successful, the bacterium may still be killed by the other medication. If the bacterium is successfully killed, the patient is cured. In addition, the chance of antibiotic resistance developing in the bacterial population may be decreased. Even though the bacterium may have been resistant to one of the two drugs, it was destroyed by the other one and therefore can't reproduce or transfer DNA to other bacteria. The failure of this strategy and the drugs could have serious effects.
Some bacteria obtain isolated bits of DNA from their environment. An intact or living bacterium is not always required for genetic transfer.
Fighting Staphylococcus epidermidis
The resistance of bacteria to substances designed to destroy them is a worrying development. We could be heading for a serious situation with regards to disease prevention and treatment.
Removing a medical device coated with a biofilm or surgically removing tissue covered by a film might cure a Staphylococcus epidermidis infection, but this may not be the case if bacteria have left the film and travelled to other areas of the body. Antibiotics may still be needed in order to kill the bacteria left in the body.
Researchers are searching for new substances to kill bacteria that cause disease. There are some signs of hope, but much more research is needed. Time is of the essence. We need effective drugs as soon as possible, not only to destroy S. epidermidis but also to kill other microbes that make us sick.
Host Response to Staphylococcus epidermidis Colonization and Infections from Frontiers in Cellular and Infection Microbiology
Understanding the Mechanism of Bacterial Biofilm Resistance to Antibacterial Agents from The Open Microbiology Journal
Drug-resistant superbug spreading in hospitals from the Medical Xpress news service
Global spread of three multi-drug resistant lineages of Staphylococcus epidermidis (abstract) from Nature Microbiology
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© 2018 Linda Crampton