Small-Molecule Type III Secretion System Inhibitors Block Assembly of the Shigella Type III Secreton

Abstract

Type III secretion systems (T3SSs) are essential virulence devices for many gram-negative bacteria that are pathogenic for plants, animals, and humans. They serve to translocate virulence effector proteins directly into eukaryotic host cells. T3SSs are composed of a large cytoplasmic bulb and a transmembrane region into which a needle is embedded, protruding above the bacterial surface. The emerging antibiotic resistance of bacterial pathogens urges the development of novel strategies to fight bacterial infections. Therapeutics that rather than kill bacteria only attenuate their virulence may reduce the frequency or progress of resistance emergence. Recently, a group of salicylidene acylhydrazides were identified as inhibitors of T3SSs in Yersinia, Chlamydia, and Salmonella species. Here we show that these are also effective on the T3SS of Shigella flexneri, where they block all related forms of protein secretion so far known, as well as the epithelial cell invasion and induction of macrophage apoptosis usually demonstrated by this bacterium. Furthermore, we show the first evidence for the detrimental effect of these compounds on T3SS needle assembly, as demonstrated by increased numbers of T3S apparatuses without needles or with shorter needles. Therefore, the compounds generate a phenocopy of T3SS export apparatus mutants but with incomplete penetrance. We discuss why this would be sufficient to almost completely block the later secretion of effector proteins and how this begins to narrow the search for the molecular target of these compounds.

Mutations in hemG Mediate Resistance to Salicylidene Acylhydrazides, Demonstrating a Novel Link between Protoporphyrinogen Oxidase (HemG) and Chlamydia trachomatis Infectivity

“…Thus, an extension of this procedure can complement genetic approaches such as chemical mutagenesis. Selected SAH compounds have been used to study T3S of Chlamydia and other Gram-negative bacteria (41,(53)(54)(55). These compounds inhibit chlamydial growth, development, and secretion, suggesting a role for the T3S throughout the intracellular developmental cycle (17, 18, 19-21, 23, 24, 25, 26, 56).…”

Small Molecule Inhibitors of LcrF, a Yersinia pseudotuberculosis Transcription Factor, Attenuate Virulence and Limit Infection in a Murine Pneumonia Model

“…Some of the T3S inhibitors block the expression of the T3SS, possibly by inhibition of LcrF, while others appear to interfere with the secretion mechanism directly. The broad-spectrum potential of the "direct" T3S inhibitors is being established, with some inhibitors showing cross-activity against Salmonella enterica serovar Typhimurium, Shigella flexneri, and Chlamydia trachomatis (20,31,40,42). Other antivirulence approaches include inhibition of individual Yop cytotoxins delivered by the T3SS or production of iron-scavenging siderophores (3).…”

Characterization of the Effects of Salicylidene Acylhydrazide Compounds on Type III Secretion in Escherichia coli O157:H7

“…The repression of T3S in a diverse range of pathogens suggests that there is a common, conserved target. The most conserved region of the T3SS is the basal apparatus proteins, and this region has recently been proposed to be the target of related inhibitors in Shigella flexneri (38). We noted that the results presented in the study of Veenendaal et al do not contradict our findings as these authors showed that treatment with an inhibitor decreased the amount of the basal apparatus in the membrane, suggesting that inhibition occurs at a very early stage of assembly and insertion or at the level of transcription.…”