Impacts of L1 Promoter Variation and L2 Clavulanate Susceptibility on Ticarcillin-Clavulanate Susceptibility of Stenotrophomonas maltophilia

Inducible expression of L1 and L2 -lactamases is the principal mechanism responsible for -lactam resistance in Stenotrophomonas maltophilia. Ticarcillin-clavulanate (TIM) is one of the few effective -lactams for S. maltophilia treatment. Clavulanate (CA) is a -lactamase inhibitor that specifically targets class A, C, and D -lactamases. In view of the presence of class B L1 -lactamase, it is of interest to elucidate why TIM is valid for S. maltophilia treatment. The L1-L2 allelic variation and TIM susceptibilities of 22 clinical isolates were established. Based on L1 and L2 protein sequences and TIM susceptibility, three L1-based phylogenetic clusters (L1-A, L1-B, and L1-C) and three L2-based phylogenetic clusters (L2-A, L2-B1, and L2-B2) were classified. The contribution of each L1- and L2-based phylogenetic cluster to ticarcillin (TIC) and TIM susceptibility was investigated. All the L1s and L2s tested contributed to TIC resistance. The L1s tested were inert to CA; nevertheless, the sensitivities of L2s to CA were widely different. In addition, the genetic organizations upstream of the L1 gene varied greatly in these isolates. At least three different L1 promoter structures (K279a type, D457 type, and none) were found among the 22 isolates assayed. The differences in the L1 promoter structure had a great impact on TIC-induced L1 -lactamase activities. Collectively, the L1 promoter activity in response to TIC challenge and L2 susceptibility to CA are critical factors determining TIM susceptibility in S. maltophilia.