The Antibiotic Susceptibility of Propionibacterium acnes


Antibiotics are medications that are used to treat bacterial infections, including acne. The acne-causing Propionibacterium acnes (P. acnes) bacterium is naturally resistant to some antibiotics, and naturally susceptible to others. For the past 50 years, physicians and researchers have been screening the susceptibility of P. acnes bacteria to different antibiotics. The results from these studies clearly demonstrate that in many places, P. acnes bacteria are becoming more resistant to certain classes of antibiotics.

The Rise of Antibiotic Resistance in P. acnes Bacteria

In many countries, a significant percentage of the P. acnes bacteria isolated from acne patients are now resistant to the some of the antibiotics that are commonly used in acne treatment (eg. Clindamycin, Erythromycin, Tetracycline, Doxycycline and Minocycline). The patterns of antibiotic resistance among acne-causing P. acnes bacteria tend to vary between countries. These variations are influenced by many factors. Not all of these factors are directly associated with acne vulgaris.

What Does Antibiotic Resistance and Susceptibility Mean?

Not all antibiotics are created equal. The same is true for bacteria. Some types of antibiotics are highly effective against certain types of bacteria, but useless against other types of bacteria. Antibiotic susceptibility and resistance is a dynamic process that is constantly changing. Over time, certain types of bacteria may gain or lose resistance to particular antibiotics. The general trend is that over time, bacterial resistance to commonly-used antibiotics increases, but this is not a uniform process.

How Does Antibiotic Susceptibility Testing Work?

Antibiotic susceptibility testing is usually conducted by growing bacteria in special petri dishes with small disks that contain known amount of antibiotics. When scientists test the susceptibility of bacteria to different antibiotics, they generally focus on the Minimum Inhibitory Concentration (MIC) of an antibiotic. The MIC is defined as the lowest concentration of an antimicrobial compound that will inhibit the visible growth of a microorganism after overnight incubation.

The Limitations of Antibiotic Resistance Testing

The primary limitation of conventional antibiotic resistance testing is that the susceptibility of a bacteria to an antibiotic is often different when it is growing on a petri dish versus when it is growing in the body. There are 2 main reasons for this:

The first reason for these differences are because bacteria adapt to their environment. P. acnes bacteria that is growing in a hair follicle and feeding on sebum from the sebaceous glands has a different metabolic profile than one growing on a petri dish and feeding on a bacterial nutrition supplement. In addition, bacteria can modulate expression of surface proteins, cell wall structures and antibiotic resistance genes in response to changes in their environment. The adaptation of a bacteria to its specific environment can have a profound effect on its susceptibility to a particular antibiotic.

The second major limitation with antibiotic susceptibility testing is that antibiotics are not evenly dispersed throughout the different tissues in the body. Different types of antibiotics tend to accumulate in different tissues. Many antibiotics do not effectively accumulate in the skin, which means that they may not inhibit acne-causing bacteria growing deep inside follicles. Even if a bacteria is highly susceptible to a particular antibiotic in laboratory testing, if that antibiotic does not make it to the site of infection at a sufficient concentration, it is not going to be an effective treatment.

What Causes Antibiotic Resistance?

A commonly held belief is that the over-use of antibiotics in an outpatient setting and patients that fail to complete their prescribed antibiotic treatments are the primary causes of emerging antibiotic resistance. While these two factors do contribute to the growing incidence of antibiotic resistant infections, they are far from the only causes. Other sources of antibiotic-resistant bacteria include antibiotic use in commercial livestock farming, unsatisfactory hygiene in institutional settings (hospitals, nursing homes, prisons) and HIV/AIDS. For an in-depth discussion of both the mechanisms and causes of antibiotic resistance read – How Do Bacteria Become Resistant to Antibiotics?

The Emergence of Antibiotic Resistant Strains of P. acnes

Starting in the 1990’s some popular antibiotics started becoming less effective for the treatment of acne. This change was particularly pronounced in places where acne vulgaris was routinely treated with antibiotics (eg. North America and Europe). A 2001 study by Ross, et al examined P. acnes isolated from acne patients and found that the bacteria was much more likely to be resistant to commonly used anti-acne antibiotics than they had been in the past. In particular, they found that most of the bacteria was resistant to Macrolide Family (eg. Erythromycin, Azithromycin, Clindamycin) and Tetracycline Family (Minocycline, Doxycycline) antibiotics. It is unlikely to be a coincidence that these two families of antibiotics include the most commonly used anti-acne antibiotics.

Antibiotic resistance testing clearly indicates that acne-causing P. acnes bacteria are becoming increasingly resistant to the antibiotics commonly used as acne treatments. Particularly in places like Europe and the United States, a significant percent of bacteria isolated from acne patients are now show an elevated level of antibiotic resistance. Generally speaking, the data indicates that in Europe, resistance to Macrolide antibiotics is very high, and resistance to Tetracycline antibiotics is also elevated. The situation is similar in the US, but Tetracycline resistance appears to be more common.

The scientific research also clearly shows that some of the antibiotic treatments that have been the mainstay of dermatologists in the fight against acne, are now becoming ineffective. As a result, for patients who have P. acnes infections that are resistant to these common treatments, it may be helpful to explore alternative types of anti-acne medications, such as Retinoids, Hormonal Treatments, Naturopathic Treatments and Light & Laser Therapies.

Antibiotic Resistance and Susceptibility Test Results for Propionibacterium acnes

Scientists have been testing antibiotics against P. acnes bacteria for over forty years. To summarize this history of testing into a single document, we have compiled a composite chart that includes the results of many of these research studies on the antibiotic susceptibility screens of P. acnes bacteria.

How To Read Our Composite Antibiotic Susceptibility Chart

Many studies use different standards and measurements. We have translated these various results into a simple 1 (Worst) to 5 (Best) scale. The lower the value the LESS effective the antibiotic was in testing. The higher the value the MORE effective the antibiotic was. The average score for each medication is listed on the left hand side of the chart and is color coded (red = least effective, yellow = moderately effective, green = most effective). The average score for each family of antibiotic is also listed next to the name of that family. On the chart itself, a box that is highlighted in red indicates that scientists detected P. acnes bacteria that were highly resistant to that particular antibiotic.