Contradiction Of Result By Drug Susceptibility Test

Post date:2018.01.09

When veterinarians see animals with infectious diseases, they perform various bacteriological examinations to diagnose the disease, while they often rely on clinical testing companies, sometimes they examine the diseases by themselves. After separating and identifying bacteria, a drug susceptibility test is performed, and antibiotics which have both high transfer activity and effect are selected and administered for animals. However, sometimes antibiotics judged “susceptible” show ineffectiveness clinically, on the contrary, sometimes antibiotics judged “resistant” show effectiveness clinically. That is when veterinarians have the opportunity to show their skills, depending on their knowledge and clinical experience of antibiotics. Now, let me tell a story of a drug which didn’t have antimicrobial activity but was available clinically.

Atrophic rhinitis; AR is bacterial infection in pigs, characterized by atrophy of the turbinate bones of growing pigs caused by toxins produced by the bacteria. AR rarely leads to death yet prevents growth by insufficient breathing and leads to large economic losses. One bacteria implicated as a major cause is Bordetella bronchiseptica. This bacterium has a capsule with a polysaccharide layer outside the cell envelope of the bacteria. The type of B. bronchiseptica which possesses a capsule is called phase Ⅰ organism, while phase Ⅲ organism lacks a capsule and phase Ⅱ organism is an intermediate type. Bacteria with a capsule have the ability to proliferate after the adhesion to turbinate bones and to prevent the formation of bones by the production of toxins. Therefore, a phase Ⅰ organism is a strongly toxic strain, while a phase Ⅲ organism doesn’t cause atrophy due to the lack of adhesive activity.

Sulfamonomethoxine (SMMX) is sulfa drug for treatment of AR, however, most strains of B. bronchiseptica isolated from AR show resistance for SMMX in a drug susceptibility test. Therefore, SMMX cannot be selected for treatment regarded as the lack of antimicrobial activity, whereas SMMX has been sold as an AR treatment drug even now. Then, Dr. K, a researcher in a manufacturing and selling company, stood up to investigate, and finally clarified that SMMX leads phase Ⅰorganisms to become phase Ⅲ organisms. Thus, a drug susceptibility test is the most useful test for the selection of antibiotics, whereas it may not reflect the clinical effect. After that, Dr. K clarified which part of the sulfa drug structure induces the mutation, and then explained it in a paper. This research was performed by a researcher who just belongs to the manufacturing and selling company of SMMX where there might be limitations on research. I want to express my respect for his great result. In later years, he received the president’s award.

Kuwano A: Inhibitory effect of sulfamonomethoxine on capsule formation of Bordetella bronchiseptica. Zentrabl Veterinarmed B., 1991 Nov, 38(9)685-688.