The Antibiotic Susceptibility of Propionibacterium acnes

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.

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 growing on your 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 genes depending 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).

Several studies that have examined P. acnes isolated from acne patients have found that these 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, P. acnes bacteria were commonly resistant to clindamycin and tetracycline.

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.

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, the editors of The Science of Acne have compiled the results from over 50 different research articles that reported results of P. acnes antibiotic susceptibility to various antibiotics.

How To Read Our Propionibacterium Acnes Antibiotic Susceptibility Chart: All of the results from each study were translated into a simple 1 to 5 scale.  These results were then averaged to get the final scores you see below.  The susceptibility score indicates how effective that particular antibiotic was in killing P. acnes bacteria (1=Not Effective, 5=Very Effective).  The resistance score indicates how often scientists found P. acnes bacteria that were highly resistant to that antibiotic (1=No Resistance, 5=Frequent Resistance).   To help understand these scores, the chart is also color coded (red = bad, yellow = OK, green = good).