ROSCO DIAGNOSTICA With the introduction of the new European Standards (EUCAST) in many Europeans countries now, we often get the question – how is it possible that ROSCO Neo-Sensitabs 9 mm can obtain the same zone sizes as a paper disk 6 mm. We have tried to explain how this is possible in the below chapter. Explanation on how it is possible to develop a 9 mm tablet that gives the same zone of inhibition as 6 mm paper disk?
PAPER DISK The structure of antibiotics in paper disks is always the amorphous state for two reasons: The paper disks are impregnated with a solution of the antibiotic and the presence of water molecules in the finished product. This means that the structure is without order and therefore the molecules in an unfavorable thermodynamic structure. This in turn means that the molecules are much more prone to degradation – the activation energy for hydrolysis is low. This is why the paper disks are inherently unstable and must be handled with great care. The only small advantage is that the amorphous antibiotics are easily soluble and along with the short thickness of paper, gives an instant release of antibiotic.
ROSCO NEO-SENSITABSTM Neo-Sensitabs™ are produced by a unique dry process using only dry crystalline antibiotics. With crystalline or morphous antibiotics, the molecules are in a long-range ordered form – the most thermodynamically favorable structure for a given molecule. This very stable form of the antibiotic means much higher activation energy of hydrolysis and thus a far more stable molecule. This means that Neo-Sensitabs™ are very resistant to degradation and has a shelf life at room temperature of 52 weeks. In contrast the paper disks have a shelf life of 1 week, stored cold, with loaded desiccants´, in sealed containers that must acclimatize before work. All in all the paper disk method has too much room for error.
CONCLUSION Because NeoSensitabs™ are manufactured with crystalline antibiotics that are encapsulated in inert dry granules there is a delay in release of antibiotics as compared with the paper disks. The formulation, thickness and diameter of Neo-Sensitabs™ have been carefully calibrated to match the release of antibiotics from the 6 mm paper disks – work that has been ongoing since the 50th and now is a technology completely mastered by ROSCO Diagnostica. MIC (µg/ml)
: The Thickness, Diameter and formulation of Neo-SensitabsTM have been carefully calibrated to give the exactly same zones of inhibition as the 6 mm paper disks. This accomplishes a controlled delay in release of antibiotic from the tablet – a technology developed and refined by ROSCO over the last 50 years.
: 6 mm Paper disk : 9 mm Paper disk : 9 mm Neo-SensitabsTM
Zone Size (mm)
THE REAL STORY of Neo-Sensitabs™ In the nineteen fifties one pneumococcus strain from an African country was found resistant to penicillin. The paper disc method had been used for testing and by re-testing it was found that the pneumococcus strain was penicillin susceptible, categorized as false resistant due to penicillin inactivation in the paper disc. From this time, researchers from ROSCO in collaboration with Dr. Erna Lund from the Pneumococcus Department of State Serum Institute in Copenhagen, studied the possibility of obtaining more stable diffusion tests. It was achieved by using crystalline antimicrobials included in a disc/tablet. Crystalline antimicrobials are much more stable to temperature and humidity changes than amorphous antimicrobials contained in all paper discs. In fact most penicillin, cephalosporin and carbapenem antibiotics are stable for several years at room temperature in the form of Neo-Sensitabs™ tablets (discs), while in the form of paper discs must be stored in the refrigerator or in the freezer. After opening the cartridges, the difference in stability between paper discs and Neo-Sensitabs™ is even bigger. Neo-Sensitabs is the only option on the market with this crystalline antimicrobials, which ensures uniform tablets, homogeneous content of the active substance and long shelf-life at room temperature. The few Neo-Sensitabs™ that should be stored in the refrigerator at 2-8 degrees can be stored at room temperature for up to 2 months. Laboratories that choose Neo-Sensitabs™ are therefore always ready for testing. One would ask what have the paper disc manufacturers done to improve the stability of their paper discs? The instability problem has been known since 1950. Why have they not improved their products? Today, 60 years later, paper discs containing beta-lactam antibiotics are as instable as they were in 1950. There is no product development (technology) in this field. Due to the use of primary susceptibility testing in Denmark, the first manufactured Neo-Sensitabs™ had in general larger antimicrobial content than the corresponding paper discs. They produced larger zones of inhibition, permitting the recognition of different types of colonies inside the zones of inhibition. Later, Neo-Sensitabs™ were also exported to other European countries, where they used secondary or pure culture susceptibility testing. Approximately 5 years ago, ROSCO Diagnostica developed CLSI/EUCAST potency Neo-Sensitabs™ in order to comply 100 % with the recommendations of CLSI and EUCAST. CLSI/EUCAST potency Neo-Sensitabs™ has been developed to produce exactly the same zones of inhibition as the corresponding paper discs of the same potency. This has been possible, in spite of Neo-Sensitabs™ being 9 mm and paper discs 6 mm in diameter - by a special technology from ROSCO Diagnostica.
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