Wednesday, May 15, 2013

Back Pain: An Infectious Process?

I wrote a piece for Big Think the other day about the recent finding that many cases of back pain are septic in nature: the pain comes from propionic acid (and other acids) released by anaerobic bacteria that have found their way into spinal-disc tissues. The principal offender is something called Propionibacterium acnes, a common mouth and skin germ that also often can be found in lung tissue.

Propionibacterium acnes can take on
an intracellular lifestyle.
Propionibacterium is, for most of us, a harmless stowaway. It is characterized as a "low virulence" organism, meaning it doesn't aggressively pathologize the host by default, the way (for example) a tuberculosis bacterium does. But for certain individuals, under certain conditions, Propionibacterium can be a major hazard. In addition to causing acne (and in severe cases, an accompanying arthritis), P. acnes is also seen in post-operative infections, prosthesis failure, breast-implant infection, corneal infection, sarcoidosis, bacteremia, and inflammation of lumbar nerves. Its involvement in sarcoidosis is controversial. What seems to be happening is that a special protein (a "trigger factor"), secreted by P. acnes, stimulates a cellular immune response in sensitive individuals. The macrophages that arrive to attack P. acnes become overwhelmed by the bacteria as they go into intracellular-parasite mode. Granulomas then form as P. acnes takes up residence in the aggregated macrophages. (More about P. acnes's role in sarcoidosis can be found in the March 2013 paper by Eishi in Respiratory Investigation.)

The immunological response triggered by P. acnes can be far-reaching. In the 1980s, back when P. acnes was called Corynebacterium parvum, researchers found that killed suspensions of the bacteria, injected into mice, caused 80% to 100% suppression of tumor growth. The dead bacteria stimulated the murine immune system to the point where mice could fight off cancer. Why this technique has not been used for human cancer treatment, I don't know. (It might be because it's too cheap and too easy. What do you think?)

In recent years, researchers have been finding P. acnes in the lumbar discs of back patients, typically at the rate of 40% to 50%. (About half of patients don't have the bacterium.) Simply finding the bacterium in discs doesn't prove a causal role for P. acnes in back pain, of course, but in a double-blind randomized controlled trial involving back patients who got either placebo or amoxicillin for 100 days, the amoxicillin-treated patients did better (both over the 100 days and a year later), which tends to suggest that P. acnes might well be playing a causal role in back pain.

People hurt their backs (to a greater or lesser degree) all the time without experiencing huge pain or lasting damage, but in a certain proportion of cases, disc herniation leads to Type 1 Modic Change (so-called bone edema) in nearby vertebrae, and at that point you're almost guaranteed to be in excruciating pain. But antibiotics might obviate the need for surgery, in at least some cases.

The nuclear material of intervertebral discs is an ideal place for P. acnes (an anaerobe) to grow, because it's warm, nutrient-rich, and (with no vascular content) oxygen-depleted. The question of how P. acnes finds its way into a disc in the first place is an interesting one (which I discuss in my post at Big Think). The short answer is, there's a ton of P. acnes in your mouth, especially if you happen to be (how shall we say?) not very attentive to oral hygiene, and bacteria can enter the bloodstream directly via the gums when you brush your teeth or have them professionally cleaned (or when a dentist picks and pokes at your teeth with one of those sharp pointy thingies). Almost any dental event, even vigorous brushing, can lead to a transient bactermia. Your spleen and white blood cells will clear bacterial cells from your blood very quickly, of course, and there are factors in your blood that are chemotoxic to most bacteria, but if a few P. acnes cells happen to stay in your blood long enough to find an inflammation zone in your body (where they can take up residency), you could be in trouble. By "inflammation zone," I mean an inflamed joint, a catheter or shunt, an implant of any kind, or any irritated tissue, really. Did you recently hurt your back? That counts.

Because even tooth brushing poses a significant risk of bacteremia, you may want to consider investing in a stock of mouthwash and using it before every brushing, to cut the live-bacteria count down and thus reduce your risk of lumbar disc infection, endocarditis, sarcoidosis, acne, and other bacteremic sequelae involving P. acnes. If you think I'm being alarmist, fine; you're entitled to your opinion. For me, it's mouthwash five times a day.