malT knockout mutation invokes a stringent type gene-expression profile in Actinobacillus pleuropneumoniae in bronchoalveolar fluid

In reverse transcription PCR differential display (RT-PCR DD) experiments, A. pleuropneumoniae CM5 exposed to BALF up-regulated, among other genes, a gene predicted to encode LamB, an outer-membrane transport protein of the maltose regulon. To determine the role of the lamB and other genes of the maltose regulon in the pathogenesis of A. pleuropneumoniae, knockout mutations were created in the lamB and malT genes, the latter being the positive transcriptional regulator of the maltose regulon. Relative to the lamB mutant and the wild type, the malT mutant had a significant (P < 0.05) decrease in growth rate and an increased sensitivity to fresh porcine serum and high concentrations (more than 0.5 M) of sodium chloride. In DNA microarray experiments, the BALF-exposed malT mutant exhibited a gene-expression profile resembling that of a stringent type gene-expression profile seen in bacteria facing amino acid or carbon starvation. Genes encoding proteins for protein synthesis, energy metabolism, and DNA replication were down-regulated, while genes involved in stringent response (e.g., relA), amino acid and nucleotide biosynthesis, biofilm formation, DNA transformation, and stress response were up-regulated.These results suggest that MalT may be involved in protection against some stressors and in the transport of one or more essential nutrients in BALF. Moreover, if MalT is directly or indirectly linked to the stringent response, an important global mechanism of bacterial persistence and virulence in many bacterial pathogens, it might play a role in A. pleuropneumoniae pathogenesis.A. pleuropneumoniae causes contagious pleuropneumonia in pigs. The disease can occur in acute, sub-acute, or chronic form [1]. The acute form is characterized by fibrinohemorrhagic pneumonia and the sub-acute and chronic forms by pleuritis with localized necrotizing lesions. The severity and the spread of the disease depend upon the serovar and dose of the strain, and in large measure, upon t