Current Concepts In Treating Diabetic Foot Osteomyelitis
What You Should Know About Biofilms And Small Colony Variants
Neut and colleagues discussed the importance of biofilms and small colony variants (SCVs) in clinical infections in a 2007 study.5 Infections related to the biomaterials used for joint replacements are the second-leading cause of implant failure.5 Once microorganisms adhere to these materials, they have adapted several powerful mechanisms of evading host defenses and most treatment options including antibiotics. Much of the success of these organisms can be attributed to the production of biofilms, a protective layer made of polysaccharides and protein, and another lesser known phenomenon known as small colony variants.
Bacteria inside biofilms can enter a dormant state and remain that way until a situation such as depressed immune function awakens them, allowing them to cause a clinical infection. While in a biofilm, bacteria are 10 to 1,000 times more resistant to antibiotic treatment.13,14 Researchers have attributed this resistance to an inability of the antibiotic to penetrate the biofilm along with a slower metabolic rate of organisms residing deep in the biofilm. Researchers have also suggested that the biofilm slows the penetration of certain antibiotics such as vancomycin so the organisms in the biofilm are exposed to gradually increasing doses of the antibiotic instead of one strong dose.15
Small colony variants are an alternate form of the bacteria produced by some microorganisms, which have a slower growth rate, higher antibiotic resistance and possibly a greater ability to persist intracellularly than their normal counterparts. With some species such as Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa, researchers have theorized that when bacteria cannot grow exponentially due to either an unfavorable environment or antibiotic treatment, they switch phenotype to a persister (survival cell) that neither grows nor dies under the current conditions.16-19 Once conditions become more favorable, they can switch their phenotype back and continue biofilm growth.
Persister cells and SCVs share many characteristics.5 Researchers have suggested that the low nutrient and oxygen levels deep in the biofilm cause the phenotype to switch these variants. Small colony variants grow much slower and form colonies that are 10 times smaller when grown on standard media.20 These colonies also produce little pigment, are non-hemolytic, show less coagulase production, and demonstrate greater resistance to aminoglycosides and cell wall antibiotics.
Since the growth and metabolism of these SCVs are so slow, they are easy to miss on routine laboratory cultures which are only grown for 48 hours. Researchers have demonstrated that SCVs in Staphylococcus aureus take six times longer to grow and it is advised that one should incubate cultures for at least six days.20,21
Small colony variants also produce less virulence factors. This subsequently limits cytokine production and leads to corresponding rises in C-reactive protein (CRP) and the erythrocyte sedimentation rate (ESR). Accordingly, these tests are unreliable in diagnosing a postoperative infection caused by SCVs. Fewer virulence factors lead to less cytokine production, which leads to no corresponding rise in CRP and ESR.
While SCVs are more resistant to certain antibiotics, when they are appropriately identified, they are still very susceptible to antibiotics that are effective against slow growing bacteria. These antibiotics include tetracycline, erythromycin and especially rifampin when one uses this in combination with a quinolone.
It remains to be seen what constitutes a gold standard of treatment of osteomyelitis. At present, there is no one antibiotic that has been proven to be far and away better than another. Evidence-based studies do not support the use of parenteral only therapy but rather give credence to the use of oral only therapy with highly bioavailable agents directed against biopsy recovered pathogens. The idea that small colony variants of S. aureus could evade host defenses, culture and antibiotics by remaining dormant in bone cells is an intriguing area of study, which will no doubt evolve over the years to come.