Smaller Can Be Tougher: Small Colony Variants of Staphylococcus aureus
Synopsis: Small colony variants of S. aureus may be responsible for some cases of relapsed infection.
A 45-year-old male with a history of nephrolithiasis and bacteremia due to methicillin-resistant Staphylococcus aureus (MRSA) presented with fever and left flank pain. Urine culture grew multiple organisms, including gram-negative rods and gram-positive cocci, which were, however, not identified by the laboratory. Blood cultures grew MRSA, a large proportion of which were detected as tiny nonpigmented colonies. The patient was treated with vancomycin and a fluoroquinolone, but fever and flank pain persisted. CT scan demonstrated left hydronephrosis and a staghorn calculus. Drainage by percutaneous nephrostomy yielded pus from which was grown Pseudomonas aeruginosa and MRSA with the same characteristics as that recovered from blood culture. The patient became afebrile after the drainage procedure and antibiotic therapy was continued.
COMMENT BY STAN DERESINSKI, MD, FACP
Small colony variants (SCV) of S. aureus are readily missed or misidentified.1,2 They are often part of a mixed population of S. aureus of normal phenotype. The latter replicate 3-6 times more rapidly than the SCV and thus may make their SCV detection difficult. After 18 hours of incubation, SCV colonies are approximately one-tenth the size of usual S. aureus colonies and are nonpigmented and nonhemolytic. Furthermore, evidence of coagulase activity is often delayed, and SCVs use glucose and fructose, but not mannitol or other sugars. The phenotype may be unstable in vitro, reverting back to normal colonial morphology and biochemical characteristics.
The slow growth of SCVs is the result of auxotrophy, most commonly for menadione, hemin, or thiamine. As with other auxotrophs, supplementation with the necessary compound alters the phenotype to that of the parent. These three compounds are necessary for ATP production and slow growth of SCVs is the result of defective respiratory activity and energy starvation. Reduced ATP synthesis results from the critical role of these compounds in the respiratory chain: thiamine is required for menadione synthesis; menadione in turn is isoprenylated to menaquinone, which is the final acceptor of electrons from FADH2; and hemin is required for synthesis of cytochromes, which accept electrons from menaquinone. The consequent reduced ATP synthesis results in slowed metabolic activity, reduced cell wall synthesis, and reduced synthesis of the caretenoid pigments that give S. aureus its characteristic golden hue.
S. aureus SCVs are reported to be present in approximately 1% of blood cultures and more than 25% of sputum cultures from patients with cystic fibrosis.4 They can readily be produced in vitro by exposure of broth cultures of S. aureus to gentamicin or other aminoglycosides. Other antibiotics, including fluoroquinolones, may also induce SCVs.5 Studies of infection of endothelial cells demonstrate that the intracellular milieu is even more potent in inducing or selecting S. aureus SCV.6,7
The need for identification of SCVs is not just a matter of microbiologic curiosity. SCVs tend to be more resistant to a number of antibiotics and, furthermore, antibiotic susceptibility testing by disk diffusion or by automated overnight methods may give invalid results. Resistance is further increased when SCVs grow attached to a surface.8 Furthermore, the organisms are able to persist intracellularly, possibly because of their reduced production of cellular toxins, particularly a-toxin.6 During their intracellular residence, they are markedly resistant to clarithromycin, clindamycin, ciprofloxacin, and rifampin, antibiotics which are ordinarily concentrated within eukaryotic cells.
Thus, the major importance of S. aureus SCVs is in their resistance to antibiotic exposure and their ability to persist.8,9 These characteristics result in the potential for relapsing infection and may account for instances of relapsed S. aureus infection that occur decades after the initial event. Other organisms can be induced to produce SCVs in vitro. Recently, a SCV of Escherichia coli, with a mutation in the heme biosynthetic pathway, was isolated from the hip joint of a patient with a chronic prosthetic hip joint infection.11