The Basics

Propionibacterium acnes, the bacteria behind acne vulgaris, is highly susceptible to some antibiotics and resistant to others. Some countries are experiencing an increase in the frequency of antibiotic resistant P. acnes.

For the last forty years, physicians and researchers have been screening the susceptibility of Propionibacterium acnes to commonly used types of antibiotics. The results from these studies clearly demonstrate that over time P. acnes bacteria has become increasingly resistant to certain classes of antibiotics. Particularly important are observations that a significant percentage of the bacteria isolated from acne patients are now resistant to the most common antibiotics used in acne treatment: Clindamycin, Erythromycin, Tetracycline, Doxycycline and Minocycline.

In this section we overview the results from antibiotic susceptibility and resistance testing in P. acnes.

What Does “Antibiotic Resistance and Susceptibility” Mean?

Most Antibiotic Susceptibility Testing is Done on Petri Dishes in the Laboratory

Not all antibiotics are created equal. The same is true for bacteria. Some types of antibiotics are highly effective against certain types of bacteria, while essentially worthless against others. Moreover, antibioitic 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 it is not a uniform process.

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 that will inhibit the visible growth of a microorganism after overnight incubation.”  Generally speaking, a bacteria is considered to be susceptible to an antibiotic when the MIC of that antibiotic is significantly less than the concentration of that antibiotic in the body after a standard clinical dosage.

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.

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. The studies are listed on the right side of the chart, in chronological order. The value “NT” indicates that the antibiotic was not tested in that particular study.

The Limitations of Antibiotic Resistance Testing

Propionibacterium acnes growing in pores produces a fluorescent orange glow when illuminated by ultraviolet light

The primary problem with standard, laboratory-based 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 proliferating in the body. This is because bacteria are not static organisms, they adapt to their environment. A P. acnes bacteria growing in a follicle and feeding on sebum has a different metabolic profile than one growing on a petri dish and feeding on a bacterial nutrition supplement. Furthermore, bacteria modulate expression of surface proteins, cell wall structures and antibiotic resistance genesdepending on their environment, and these changes can have a profound effect on their susceptibility to a particular antibiotic.

The second major limitation is that antibiotics are not evenly dispersed throughout the different tissues in the body. Many antibiotics do not effectively accumulate in the follicle and/or sebaceous glands, and therefore do not effectively reach the bacteria responsible for acne. Even if a bacteria is highly susceptible to a particular antibiotic in lab-based 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.  As a result there can be major differences in the effectiveness of oral antibiotics and topical antibiotics used in acne treatments.

What Causes Antibiotic Resistance?

Hospitals are a Major Source of Antibiotic Resistant Bacteria

The most commonly held belief is that doctor’s over-prescribing antibiotics and patients failing 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 instiutional settings (hospitals, nursing homes, prisons) and HIV/AIDS, among others. 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 Propionibacterium 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 in the past.  In particular, they found that most of the bacteria was resistant to both erithromycin and azithromycin.  Additionally, antibiotic resistance was common to clindamycin and tetracycline.  In the chart from this study, the numbers on the left indicate more susceptibility, while numbers on the right indicate more resistance.  In their second figure, they analyze the incidence of erythromycin and tetracycline resistant P. acnes from different countries around the world.

Antibiotic Resistance in P. acnes Bacteria Isolated from Acne Patients (Ross, et al. 2001)

Tetracycline and Erythromycin Resistance in P. acnes Bacteria from Different Countries (Ross, et al. 2001)

A 2005 research study also found that higher percentages of P. acnes bacteria isolated from acne patients were resistant to some commonly used antibiotics.

Percent of P. acnes Bacteria Resistant to Common Antibiotics (Oprica, et al. 2005)

Summary of Antibiotic Resistance in Propionibacterium acnes Bacteria

Propionibacterium acnes Bacteria

Antibiotic resistance testing clearly indicates that acne causing P. acnes bacteria is becoming increasingly resistant to the antibiotics commonly used to treat acne vulgaris.  Particularly in places like Europe and the United States, where antibitiotic treatment of acne is fairly common, a large percentage of bacteria isolated from acne patients are now resistant.  The data indicates that in Europe, resistance to erythromycin and clindamycin is very high, and resistance to tetracycline is also elevated.  The situation is similar in the US, but tetracycline resistance appears to be more common.

The scientific research 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 or non-antibiotic treatment alternatives (like retinoids, light therapy and naturopathic medicine) to improve their acne symptoms.

References and Sources

Hoeffler, et al. 1976.  Antimicrobial Susceptibility of Propionibacterium acnes and Related Microbial Species.
Wang, et al.  1977.   Susceptibility of Propionibacterium acnes to Seventeen Antibiotics.
Ross, et al.  2001.  Phenotypic and genotypic characterization of antibiotic-resistant Propionibacterium acnes isolated from acne patients attending dermatology clinics in Europe, the U.S.A., Japan and Australia.
Oprica, et al.  2005.   European surveillance study on the antibiotic susceptibility of Propionibacterium acnes.

Related Posts @ The Science of Acne

In Depth: What is Propionibacterium acnes?
In Depth: What causes antibiotic resistance?
Overview: Oral antibiotics for Acne
Overview: Topical antibiotics for Acne

Additional Online Resources

Antibiotic Resistance of Propionibacterium acnes in Acne Vulgaris @ eMedicine
Antibiotic resistance @ Wikipedia